Kawasaki Computer Hardware 80C152 User Manual

KS152JB Universal Communications Controller

Technical Specifications

1.0 INTRODUCTION

The 80C152 Universal Communications Controller is an 8-bit microcontroller designed for the

intelligent management of peripheral systems or components. The 80C152 is a derivative of the

80C51 and retains the same functionality. These enhancements include: a high speed multi-proto-

col serial communication interface, two channels for DMA transfers, HOLD/HLDA bus control, a

ﬁfth I/O port, expanded data memory, and expanded program memory.

In addition to a standard UART, referred to here as Local Serial Channel (LSC), the 80C152 has

an on-board multi-protocol communication controller called the Global Serial Channel (GSC).

The GSC interface supports SDLC, CSMA/CD, user deﬁnable protocols, and a subset of HDLC

protocols. The GSC capabilities include: address recognition, collision resolution, CRC genera-

tion, ﬂag generation, automatic retransmission, and a hardware based acknowledge feature. This

high speed serial channel is capable of implementing the Data Link Layer and the Physical Link

Layer as shown in the OSI open systems communication model. This model can be found in the

document “Reference Model for Open Systems Interconnection Architecture”, ISO/TC97/SC16

N309.

The DMA circuitry consists of two 8-bit DMA channels with 16-bit addressability. The control

signals; Read (RD), Write (WR), hold and hold acknowledge (HOLD/HLDA) are used to access

external memory. The DMA channels are capable of addressing up to 64K bytes (16 bits). The

destination or source address can be automatically incremented. The lower 8 bits of the address

can be automatically incremented. The lower 8 bits of the address are multiplexed on the data bus

Port 0 and the upper eight bits of address will be on Port 2. Data is transmitted over an 8-bit

address/data bus. Up to 64k bytes of data may be transmitted for each DMA activation.

The new I/O port (P4) function the same as Ports 1-3 found on the 80C51.

Internal memory has been doubled in the 80C152. Data memory has been expanded to 256 bytes,

and internal program memory has been expanded to 8 bytes.

There are also some speciﬁc differences between the 80C152 and the 80C51. The ﬁrst difference

is that RESET is active low in the 80C152 and active high in the 80C51H. The second difference

is that GF0 and GF1, general purpose ﬂags in PCON, have been renamed GFIEN and XRCLK.

GFIEN enables idle ﬂags to be generated in SDLC mode, and XRCLK enables the receiver to be

externally clocked. All of the previously unused bits are now being used and interrupt vectors

have been added to support the new enhancements.

Throughout the rest of this manual the 80C152 will be referred to generically as the “C152”. The

C152 is based on the 80C51 architecture and utilizes the same 80C51 instruction set. There have

been no new instructions added. All the new features and peripherals are supported by an exten-

sion of the Special Function Registers (SFRs). A brief information on cpu functions as: the

instruction set, port operation, timer/counters, etc., is included in this document.

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2.1 Pin Description

Table 1: PIN DESCRIPTION

Name

Port 0

Description

Port 0 is an 8-bit open drain bi-directional I/O Port. As an output port each pin can

sink 8 LS TTL inputs. Port 0 pins that have 1s written to them ﬂoat, and in that

state can be used as high-impedance inputs.

external program memory if EBEN is pulled low. During accesses to external

Data Memory, Port 0 always emits the low-order address byte and serves as the

multiplexed data bus. In these applications it uses strong internal pullups when

emitting 1s.

Port 0 also outputs the code bytes during program veriﬁcation. External pullups

are required during program veriﬁcation.

Port 1

Port 1 is an 8 -bit bidirectional I/O port with internal pullups. Port 1 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 1 pins that are externally being pulled low will

source current (I , on the data sheet) because of the internal pullups.

Port 1 also serves the functions of various special features of the 8XC152, as listed

below:

Name

GRXD

GTXD

DEN

TXC

RXC

Alternate Function

GSC data input pin

GSC data output pin

GSC enable signal for an external driver

GSC input pin for external transmit clock

GSC input pin for external receive clock

DMA hold input/output

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

HLD

HLDA

DMA hold acknowledge input/output

Port 2

Port 2 is an 8-bit bi-directional I/O port with internal pullups. Port 2 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 2 pins that are externally being pulled low will

source current (I , on the data sheet) because of the internal pullups. port 2 emits

the high-order address byte during fetches from external program memory if

EBEN is pulled low. During accesses to external Data Memory that use 16-bit

addresses (MOVX @ DPTR and DMA operations), Port 2 emits the high-order

address byte. In these applications it uses strong internal pullups when emitting

1s. During accesses to external Data Memory that use 8-bit addresses (MOVX @

Ri), Port2 emits the contents of the P2 Special Function Register. Port 2 also

receives the high-order address bits during program veriﬁcation.

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Table 1: PIN DESCRIPTION

Port 3

Port 3 is an 8-bit bi-directional I/O port with internal pullups. Port 3 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 3 pins that are externally being pulled low will

source current (I on the data sheet) because of the pullups. Port 3 also serves the

IL,

functions of various special features of the MCS-51 Family, as listed below:

Name

RXD

TXD

INT0

INT1

Alternate Function

Serial input line

Serial output line

External interrupt 0

P3.0

P3.1

P3.2

P3.3

P3.4

P3.5

P3.6

P3.7

External interrupt 1

Timer 0 external input

Timer 1 external input

External Data Memory Write strobe

External Data Memory Read strobe

Port 4

Port 5

Port 4 is an 8-bit bi-directional I/O port with internal pullups. Port 4 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 4 pins that are externally being pulled low will

source current (I on the data sheet) because of the internal pullups. In addition,

IL,

Port 4 also receives the low-order address bytes during program veriﬁcation.

Port 5 is an 8-bit bi-directional I/O port with internal pullups. Port 5 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 5 pins that are externally being pulled low will

source current (I , on the data sheet) because of the internal pullups.

Port 5 is also the multiplexed low-order address and data bus during accesses to

external program memory if EBEN is puled high. In this application it uses strong

pullups when emitting 1s.

Port 6

Port 6 is an 8-bit bi-directional I/O port with internal pullups. Port 6 pins that have

1s written to them are pulled high by the internal pullups, and in that state can be

used as inputs. As inputs, Port 6 pins that are externally pulled low will source

current (I , on the data sheet) because of the internal pullups. Port 6 emits the

high-order address byte during fetches from external Program Memory if EBEN

is pulled high. In this application it uses strong pullups when emitting 1s.

XTAL1 Input to the inverting oscillator ampliﬁer and input to the internal clock generating

circuits.

XTAL2 Output from the oscillator ampliﬁer.

RST

Reset input. A logic low on this pin for three machine cycles while the oscillator is

running resets the device. An internal pullup resistor permits a power on reset to

be generated using only an external capacitor to V . Although the GSC recog-

nizes the reset after three machine cycles, data may continue to be transmitted for

up to 4 machine cycles after Reset is ﬁrst applied.

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Table 1: PIN DESCRIPTION

ALE

Address Latch Enable output signal for latching the low byte of the address during

accesses to external memory.

In normal operation ALE is emitted at a constant rate of 1/6 the oscillator fre-

quency, and may be used for external timing or clocking purposes. Note, however,

that one ALE pulse is skipped during each access to external Data Memory. While

in Reset, ALE remains at a constant high level.

PSEN

Program Store Enable is the Read strobe to External Program Memory. When the

8XC152 is executing from external program memory, PSEN is active (low). When

the device is executing code from External Program Memory, PSEN is activated

twice each machine cycle, except that two PSEN activations are skipped during

each access to External Data Memory. While in Reset, PSEN remains at a con-

stant high level.

External Access enable. EA must be externally pulled low in order to enable the

8XC152 to fetch code from External Program Memory locations 0000H to

0FFFH.

EA must be connected to V for internal program execution.

EBEN

E-Bus Enable input that designates whether program memory fetches take place

via Ports 0 and 2 or ports 5 and 6. Table 2.1 shows how the ports are used in con-

junction with EBEN.

EPSEN E-bus program Store Enable is the Read strobe to external program memory when

EBEN is high. Table 2.1 shows when EPSEN is used relative to PSEN depending

on the status of EBEN and EA.

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2.2 Special function Registers

The following table lists the SFR’s present in 80152. Note that not all the addresses are occupied

by SFR’s. The unoccupied addresses are not implemented and should not be used by the cus-

tomer. Read access from these unoccupied locations will return unpredictable data, while write

accesses will have no effect on the chip

Table 2: SFR map for the cpu

IPN1

BCRL1

BCRL0

DARL1

DARL0

SARL1

SARL0

BCRH1

BCRH0

RFIFO

PBRS

MYSLOT

AMSK1

RSTAT

ACC

TSTAT

PSW

IEN1

DARH1 TCDCNT AMSK0

DARH0 BKOFF

ADR3

ADR2

ADR1

SARH1

SARH0

SLOTTM

IFS

SCON

SBUF

DCON0

TL0

DCON1 BAUD

ADR0

TH1

TCON

TMOD

TL1

TH0

DPL

DPH

GMOD

TFIFO

PCON

Note: SFR’s in marked column are bit addressable.

2.3 RESET

The RST pin is the input to a Schmitt Trigger whose output is used to generate the internal system

reset. In order to obtain a reset, the RST pin must be held low for at least four machine cycles,

while the oscillator is running. The CPU internal reset timings are shown in the Figure.

The external reset input RST is sampled on S5P2 in every machine cycle. If the sampled value is

high, then the processor responds with an internal reset signal at S3P1, two machine cycles after

the RST being sampled low. This means that there is an internal delay of 19 to 31 clock periods

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between the RST pin being pulled low and the internal reset being generated. During this time the

CPU continues its normal operations.

The internal reset signal clears the SFRs except the port SFRs which have FFh written into them

and the Stack Pointer which has 07h written to it. The SBUF is however in an indeterminate state.

The Program Counter is reset to 0000h. The internal RAM is not affected by the reset and their

contents remain unchanged. On power up, their contents is indeterminate.

S1 S2

S5 S6

S3 S4

RST

internal reset

ALE

PSEN

INST

ADDR

INST

ADDR

INST

ADDR

INST

ADDR

INST

ADDR

Reset Timing

Table 3: Reset Values of the SFRs

SFR Name

Reset Value

SFR Name

BRCL0-1

Reset Value

0000H

INDETERMINATE

ACC

00H

BCRH0-1

TL0

INDETERMINATE

00H

PSW

00H

TH0

00H

07H

TL1

00H

DPTR

0000H

TH1

00H

P0-6

FFH

ADR0-3

AMSK0-1

BAUD

BKOFF

SLOTTM

00H

RFIFO

RSTAT

SARH0-1

INDETERMINATE

00H

INDETERMINATE

00H

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Table 3: Reset Values of the SFRs

SFR Name

Reset Value

XXX00000B

SFR Name

SCON

Reset Value

00H

0XX00000B

00H

SBUF

INDETERMINATE

0XXX0000B

INDETERMINATE

00H

TMOD

TCON

DCON0-1

GMOD

IEN1

PCON

00H

DARL0-1

DARh0-1

IFS

00H

X0000000B

XX000000B

INDETERMINATE

MYSLOT

PRBS

00H

IPN1

00H

TCDCNT

TFIFO

TCON

TSTAT

00H

XX000100B

2.4 PORT STRUCTURES AND OPERATION

The ports are all bidirectional. Each port consists of two sections, the port SFR and the I/O pad.

The Ports 0 and 2 are involved in accesses to external memory. In this case, Port 0 outputs the

lower byte of the external memory address while port 2 outputs the higher byte of the external

address. The Port 0 bus is also used as a data bus for the data byte that is read or is to be written.

Therefore Port 0 is actually a time-multiplexed address/data bus. A point to note is that Port 2 out-

puts the upper 8 bits of the address only if the address is 16 bits wide, else it continues to emit the

Port 2 SFR contents.

In order that the alternate functions on the port pin are activated correctly, the corresponding bit

latch must be held at value 1. If this is not done then the corresponding port pin is stuck at 0, and

external or internal inputs will have no effect on the pin value.

I/O CONFIGURATIONS

Each individual port has different I/O pads to accommodate the different functions of each indi-

vidual port. The ﬁgure below shows a simpliﬁed diagram of the I/O pads for each port.

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ADDRESS

ADDR/DATA

IDNAHI

IDNAMX

VCC

Internal

Pullup

P2.X

P0.X

MUX

PORT2OP

PORT2IP

PORT0OP

PORT0IP

3. Port 2 I/O Pad

1. Port 0 I/O Pad

Alternate Output

Function

VCC

Weak Internal

Pullup

Weak Internal

Pullup

P1.X

P3.X

PORT1OP

PORT3OP

Alternate Output

Function

PORT1IP

PORT3IP

2. Port 1 I/O Pad

4. Port 3,4,5 &6 I/O Pad

Port bit I/O Pads

As shown in Figure above, Ports 0 and 2 can emit either their respective SFR contents or the

ADDR/DATA and ADDRESS bus, depending upon the control lines IDNAMX and IDNAHI.

During external memory accesses, the P2 SFR remains unchanged, but the P0 SFR is preset to

FFh.

Ports 1, 2, 4, 5 and 6 have internal pullups, while Port 0 has an open drain output.

Every single I/O line can be individually conﬁgured as an input or output. However Ports 0 and 2

cannot be used as I/O ports since they are used as the ADDRESS/ DATA bus. To use any port pin

as an input, the corresponding bit latch must contain a 1. This turns off the active pulldown FET.

Then, for Ports 1, 2 and 3, the pin is pulled high by the internal pullup. The Internal pullup is a

weak pullup and so the pin can be pulled low by a strong external source.

Port 0, however has no internal pullups. The active pullup FET is used only when the port pin is

emitting a 1 during external memory accesses, else the pullup is off. Hence, when the port is used

as an I/O pin; IDNAMX is 1; the pullup FET is always off. Writing a 1 to the bit latch will turn off

the active pulldown FET and as a result the port pin will ﬂoat.

As port 1, 2 and 3 have internal pullups, they will go high when conﬁgured as inputs and will

source current. Hence they are also known as “quasi-bidirectional” ports. Port 0 on the other hand

“ﬂoats” when conﬁgured as an input and hence is called a “true bidirectional” port.

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Writing to a Port

During the execution of an instruction that changes the value of a port SFR, the new value arrives

at the port latch during S6P2. However, the port latch contents do not appear on the port pins till

the next P1 phase. Therefore the new port data will appear on the port pins at S1P1 of the next

machine cycle.

Read-Modify-Write Feature

Each port is split into its SFR and its corresponding I/O pad. Therefore there are two options

available for a port read access. Either the SFR latch contents can be read or the input from the I/

O pads can be read. The instructions that read the latch, modify the value and write it back to the

latch are called “read-modify-write” instruction. In such instructions the latch and not the pin is

read. The instruction of this category are listed as follows

ANL

logical AND

ORL

logical OR

XRL

logical XOR

JBC

CPL

jump if bit = 1 and then clear bit

complement bit

INC

increment

DEC

decrement

DJNZ

decrement and jump if not zero

move carry bit to bit Y of Port X

clear bit Y of port X

set bit Y of X

MOV PX.Y, C

CLR PX.Y

SETB PX.Y

2.5 Ports 4,5 and 6

Ports 4,5 and 6 operation is identical to Ports 1-3 on the 80C51. Ports 5 and 6 exist only on the

“JB” and “JD” version of the C152 and can either function as standard I/O ports or can be conﬁg-

ured so that program memory fetches are performed with these two ports. To conﬁgure ports 5

and 6 as standard I/O ports, EBEN is tied to a logic low. When in this conﬁguration, ports 5 and 6

operation is identical to that of port 4 except they are not bit addressable. To conﬁgure ports 5 and

6 to fetch program memory, EBEN is tied to a logic high. When using ports 5 and 6 to fetch the

program memory, the signal EPSEN is used to enable the external memory device instead of

PSEN. Regardless of which ports are used to fetch program memory, all data memory fetches

occur over ports 0 and 2. The 80C152JB and 80C152JD are available as ROMless devices only.

ALE is still used to latch the address in all conﬁgurations. Table 2.1 summarizes the control sig-

nals and how the ports may be used.

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Table 4:

EBEN

Program

Fetch via

PSEN

EPSEN

Comments

P0, P2

N/A

Active

N/A

Inactive Addresses 0 - 0FFFFH

N/A

Invalid Combination

P5, P6

Inactive

Addresses 0 - 0FFFFH

P5, P6

P0, P2

Inactive

Active

Inactive

Addresses 0 - 1FFFH

Addresses 0 - 2000H

2.6 ACCESSING EXTERNAL MEMORY

External Memory is accessed if either of the following two conditions is met

1) The signal EA is low

2) Whenever the program counter (PC) contains an address greater than 0FFFh.

Accesses to external memory are of two type: External Program Memory accesses and External

Data Memory accesses. External Program Memory is accessed using the PSEN signal as the read

strobe. External Data Memory is accessed using the RD or WR pins to strobe the memory.

Fetches from the external Program Memory always use a 16 bit address, while External Data

Memory accesses can have addresses of 8 bit or 16 bit. Whenever a 16 bit address is used, Port 2

is used to emit the higher byte of the address. The point to note is that during external accesses,

port 2 uses strong pullups while emitting 1s. During the time Port 2 does not emit the higher

address byte, it continuously outputs the Port 2 SFR contents, which are not modiﬁed by the hard-

ware (unless of course the user writes to the Port 2 SFR). If an 8 bit address is being used, then the

Port 2 SFR contents will be outputed on the port pins throughout the external memory cycle.

The lower byte of the address is always multiplexed with the data byte on Port 0. The ADDR/

DATA bus can drive both the active pullup and pulldown FETs. Thus for external memory

accesses, the port 0 pins are not open-drain outputs and do not need any external pullups. The fall-

ing edge of the ALE signal can be used to store the address on Ports 0 and 2 in an external address

latch. During a write cycle, the data to be written to the external Data Memory is present on Port 0

pins and remains there until after WR is deactivated. In case of a read access, the data on Port 0

pins is read just before the deactivation of the RD signal.

During accesses to external memory, the CPU presets the Port 0 SFR to FFh in order to ﬂoat the

Port 0 pins. Therefore any data that was present in the SFR latch will be lost. If the user, writes

any data other than FFh to Port 0 during an external memory cycle, then the incoming code byte

will be corrupted. Therefore, DO NOT WRITE TO Port 0 if external memory is used.

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During External Memory Accesses, both Ports 0 and 2 are used for Address/ Data transfer and

therefore cannot be used for general I/O purposes. During external program fetches, Port 2 uses

strong pullups to emit 1s.

2.7 TIMER/COUNTERS

This has two 16-bit Timer/Counters, TM0 andTM1. Each of these Timer/Counters has two 8 bit

registers which form the 16 bit counting register. For Timer/Counter TM0 they are TH0, the upper

8 bits register and TL0, the lower 8 bit register. Similarly Timer/Counter TM1 has two 8 bit regis-

ters, TH1 and TL1 and Timer/Counter

When conﬁgured as a “Timer”, the register is incremented once every machine cycle. Since a

machine cycle consists of 12 clock periods, the timer clock can be thought of as 1/12 of the master

clock. In the “Counter” mode, the register is incremented on the falling edge of the external input

pin, T0 in case of TM0, T1 for TM1. The T0, T1 inputs are sampled in every machine cycle at

S5P2. If the sampled value is high in one machine cycle and low in the next, then a valid high to

low transition on the pin is recognized and the count register is incremented. Since it takes two

machine cycles to recognize a negative transition on the pin, the maximum rate at which counting

will take place is 1/24 of the master clock frequency. In either the “Timer” or “Counter” mode, the

count register will be updated in S3P1. Therefore, in the “Timer” mode, the recognized negative

transition on pin T0 and T1 can cause the count register value to be updated only in the machine

cycle following the one in which the negative edge was detected.

GATE C/T

M0 GATE C/T

Timer 1

Timer 0

GATE

This bit controls the gating operations. When this bit is set, the Timer/Counter “x”

is enabled only while “INTx” pin is high and “TRx” bit is set high. If this bit is

cleared, then Timer/Counter “x” is enabled only if “TRx” is set.

C/T

This bit selects the Timer or Counter mode of operation. If this bit is set then the

Counter mode is selected, else the Timer mode is selected.

M1, M0

M1 and M0 selects the operating mode for the Timer/Counter

Operating Mode

“THx” acts as a 8 bit Timer/Counter, with “TLx” as the 5 bit prescaler.

“THx” and “TLx” are cascaded to form a single 16 bit Timer/Counter.

This is the Auto Reload mode.

Timer 0: TL0 is an 8 bit Timer/Counter controlled by the Timer 0

control bits. TH0 is a 8 bit timer controlled by the Timer 1 control bits.

Timer 1: Timer/Counter is stopped.

TMOD: Timer/Counter Mode Control Register

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TF1 TR1 TF0 TR0 IE1

IT1

IE0

IT0

TF1

Timer 1 overﬂow ﬂag. Set by hardware on Timer/Counter overﬂow. Cleared by

hardware when processor vectors to timer 1 interrupt routine.

TR1

TF0

Timer 1 Run control bit. Set/Cleared by software to turn Timer/Counter on/off.

Timer 0 overﬂow ﬂag. Set by hardware on Timer/Counter overﬂow. Cleared by

hardware when processor vectors to timer 0 interrupt routine.

TR0

IE1

Timer 0 Run control bit. Set/Cleared by software to turn Timer/Counter on/off.

Interrupt 1 Edge ﬂag. Set by hardware when external interrupt edge is detected.

Cleared when interrupt is processed.

IT1

IE0

IT0

Interrupt 1 Type control bit. Set/ Cleared by software to specify falling edge/low

level triggered external interrupt.

Interrupt 0 Edge ﬂag. Set by hardware when external interrupt edge id detected.

Cleared when interrupt is processed.

Interrupt 0 Type control bit. Set/Cleared by software to specify falling edge /low

level triggered external interrupt.

TCON: Timer/Counter Control Register.

The “Timer” or “Counter” function is selected by the “C/T” bit in the TMOD Special Function

tion for Timer/Counter 0 and bit 6 of TMOD selects the function for Timer/Counter 1. In addition

each Timer/Counter can be set to operate in any one of four possible modes. The mode selection

is done by bits M0 and M1 in the TMOD SFR. Modes 0, 1 and 2 are identical for both the Timer/

Counters, but Mode 3 is different. The four modes of operation are described below.

MODE 0

In Mode 0, the timer/counters act as a 8 bit counter with a 5 bit, divide by 32 prescaler. In this

mode we have a 13 bit timer/counter. The 13 bit counter consists of 8 bits of THx and 5 lower bits

of TLx. The upper 3 bits of TLx are ignored.

The negative edge of the clock increments the count in the TLx register. When the ﬁfth bit in TLx

moves from 1 to 0, then the count in the THx register is incremented. When the count in THx

moves from FFh to 00h, then the overﬂow ﬂag TFx in TCON SFR is set.

The counted input is enabled only if TRx is set and either GATE = 0 or INTx = 1.

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OSC

S3P1

C/T

TLX THx

5 bits 8 bits

INTERRUPT

TFx

Tx pin

TRx

GATE

INTx

Timer/Counter in Mode 0

MODE 1

Mode 1 is similar to Mode 0 except that the counting register form a 16 bit counter, rather than a

13 bit counter. This means that all the bits of THx and TLx are used.

OSC

S3P1

C/T

TLX

8 bits

INTERRUPT

TFx

Tx pin

TRx

Reload

GATE

INTx

THx

8 bits

Timer/Counter in Mode 2

MODE 2

In Mode 2, the timer/counter is in the Auto Reload Mode. In this mode, TLx acts as a 8 bit count

TFx bit in TCON is set and TLx is reloaded with the contents of THx and the counting process

continues from here. The reload operation leaves the contents of the THx register unchanged

MODE 3

Mode 3 has different operating methods for the two timer/counters. For timer/counter TM1, mode

3 simply freezes the counter. This is the same as setting TR1 to 0.

Timer/Counter TM0 however conﬁgures TL0 and TH0 as two separate 8 bit count registers in this

mode. The logic for this mode is shown in the ﬁgure. TL0 uses the Timer/Counter TM0 control

bits C/T, GATE, TR0, INT0 and TF0. TH0 is forced as a machine cycle counter and takes over the

use of TR1 and TF1 from Timer/Counter TM1.

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Mode 3 is used in cases where an extra 8 bit timer is needed. With Timer 0 in Mode 3, Timer 1 can

be turned on and off by switching it out of and into its own Mode 3. It can also be used as a baud

rate generator for the serial port.

C/T

OSC

S3P1

T0 pin

TR0

TL0

5 bits

INTERRUPT

TF0

GATE

INT0

S3P1

TR1

TH0

8 bits

INTERRUPT

TF1

Timer/Counter 0 in Mode 3

2.8 Interrupts

The cpu has a provision for 11 different interrupt sources. These are the two external interrupts,

the three timer interrupts, the local serial port interrupt, dma interrupt and global serial port inter-

rupts.

ET1 EX1 ET0 EX0

Global Disable bit. If this bit is cleared it disables all the interrupts at once.

If EA is 1 then interrupts can be individually enabled/disabled by setting or

clearing its own enable bit.

reserved.

Local Serial port interrupt enable.

Timer 1 interrupt enable.

ET1

EX1

ET0

EX0

External interrupt 1 enable.

Timer 0 interrupt enable.

External interrupt 0 enable

IE: Interrupt Enable Register.

The External Interrupts INT0 and INT1 can be either edge triggered or level triggered, depending

on bits IT0 and IT1. The bits IE0 and IE1 in the TCON register are the ﬂags which are checked to

generate the interrupt. When an interrupt is generated by these external interrupt inputs, they will

be cleared on entering the Interrupt Service routine, only if the interrupt type is edge triggered. In

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case of level triggered interrupt, the IE0 and IE1 ﬂags are not cleared and will have to be cleared

by the software. This is because in the level activated mode, it is the external requesting source

that controls the interrupt ﬂag bit rather than the on-chip hardware.

The Timer 0 and 1 Interrupts are generated by the TF0 and TF1 ﬂags. These ﬂags are set by the

overﬂow in the Timer 0 and Timer 1. The TF0 and TF1 ﬂags are automatically cleared by the

hardware when the timer interrupt is serviced.

The Timer 0 and 1 ﬂags are set in S5P2 of the machine cycle in which the overﬂow occurs. The

values are polled in the next cycle.

The Local Serial block can generated interrupts on reception or transmission. There is however

only one interrupt source from the Local Serial block, which is obtained by oring the RI and TI

bits in the SCON SFR. These bits are not automatically cleared by the hardware and the user will

have to clear these bits using software.

All the bits that generate interrupts can be set or reset by hardware and thereby software initiated

interrupts can be generated. Each of the individual interrupts can be enabled or disabled by setting

or clearing a bit in the IE SFR. IE also has a global enable/disable bit EA, which can be cleared to

disable all the interrupts at once.

Priority Level Structure

There are two priority levels for the interrupts. Each interrupt source can be individually set to

either one of the two levels. Naturally, a higher priority interrupt cannot be interrupted by a lower

priority interrupt. However there exists a predeﬁned hierarchy amongst the interrupts themselves.

This hierarchy comes into play when the interrupt controller has to resolve simultaneous requests

having the same priority level. This hierarchy is deﬁned as shown below, the interrupts are num-

bered starting from the highest priority to the lowest.

PT1 PX1 PT0 PX0

reserved.

Local Serial port interrupt priority bit.

Timer 1 interrupt priority bit.

External interrupt 1 priority bit.

Timer 0 interrupt priority bit.

External interrupt 0 priority bit

PT1

PX1

PT0

PX0

IE: Interrupt Enable Register.

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EGSTE EDMA1 EGSTV EDMA0 EGSRE EGSRV

IEN1 (Additional interrupt enable register) (0C8H)

Interrupt enable register for DMA and GSC interrupts. A 1 in any bit position enables that interrupt.

IEN1.0 (EGSRV) - Enables the GSC valid receive interrupt.

IEN1.1 (EGSRE) - Enables the GSC receive error interrupt.

IEN1.2 (EDMA0) - Enables the DMA done interrupt for channel 0.

IEN1.3 (EGSTV) - Enables the GSC valid transmit interrupt.

IEN1.4 (EDMA1) - Enables the DMA done interrupt for Channel1.

IEN1.5 (EGSTE) - Enables the GSC transmit error interrupt.

PGSTE PDMA1 PGSTV PDMA0 PGSRE PGSRV

IPN1 (Additional interrupt priority register) (0F8H)

Allows the user software two levels of prioritization to be assigned to each of the interrupts in

IEN1. A 1 assigns the corresponding interrupt in IEN1 a higher interrupt than an interrupt with a

corresponding 0.

IPN1.0 (PGSRV) - Assigns the priority of GSC receive valid interrupt.

IPN1.1 (PGSRE) - Assigns the priority of GSC error receive interrupt.

IPN1.2 (PDMA0) - Assigns the priority of DMA done interrupt for Channel 0.

IPN1.3 (PGSTV) - Assigns the priority of GSC transmit valid interrupt.

IPN1.4 (PDMA1) - Assigns the priority of DMA done interrupt for Channel 1.

IPN1.5 (PGSTE) - Assigns the priority of GSC transmit error interrupt.

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The interrupt ﬂags are sampled in S5P2 of every machine cycle. In the next machine cycle, the

sampled interrupts are polled and their priority is resolved. If certain conditions are met then the

hardware will execute an internally generated LCALL instruction which will vector the process to

the appropriate interrupt vector address. The conditions for generating the LCALL are

1. An interrupt of equal or higher priority is not currently being serviced.

2. The current polling cycle is the last machine cycle of the instruction currently being executed.

3. The current instruction does not involve a write to IP or IE registers and is not a RETI.

If any of these conditions are not met, then the LCALL will not be generated. The polling cycle is

repeated every machine cycle, with the interrupts sampled at S5P2 in the previous machine cycle.

If an interrupt ﬂag is active in one cycle but not responded to, and is not active when the above

conditions are met, the denied interrupt will not be serviced. This means that active interrupts are

not remembered, every polling cycle is new.

The processor responds to a valid interrupts by executing an LCALL instruction to the appropriate

service routine. This may or may not clear the ﬂag which caused the interrupt. In case of Timer

interrupts, the TF0 or TF1 ﬂags, are cleared by hardware whenever the processor vectors to the

appropriate timer service routine. In case of External interrupt, INT0 and INT1, the ﬂags are

cleared only if they are edge triggered. In case of Local Serial interrupts, the ﬂags are not cleared

by hardware. The hardware LCALL behaves exactly like the software LCALL instruction. This

instruction saves the Program Counter contents onto the Stack, but does not save the Program Sta-

tus Word PSW. The PC is reloaded with the vector address of that interrupt which caused the

LCALL. These vector address for the different sources are as follows

Table 5:

Interrupt

Symbolic

name

Interrupt

Symbolic

Name

Priority

sequence

Priority

address

Priority

name

Vector

Address

IP.0

IPN1.0

IP.1

PX0

PGSRV

PT0

IE.0

IEN1.0

IE.1

EX0

EGSRV

ET0

03H

2BH

0BH

33H

3BH

13H

43H

IPN1.1

IPN1.2

IP.2

PGSRE

PDMA0

PX1

IEN1.1

IEN1.2

IE.2

EGSRE

EDMA0

EX1

IPN1.3

PGSTV

IEN1.3

EGSTV

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Table 5:

Interrupt

Symbolic

name

Interrupt

Symbolic

Name

Priority

sequence

Priority

address

Priority

name

Vector

Address

IPN1.4

IP.3

PDMA1

PT1

IEN1.4

1E.3

EDMA1

ET1

53H

1BH

4BH

23H

IPN1.5

IP.4

PGSTE

IEN1.5

IE.4

EGSTE

Execution continues from the vectored address till an RETI instruction is executed. On execution

of the RETI instruction the processor pops the Stack and loads the PC with the contents at the top

of the stack. The user must take care that the status of the stack is restored to what is was after the

hardware LCALL, if the execution is to return to the interrupted program. The processor does not

notice anything if the stack contents are modiﬁed and will proceed with execution from the

address put back into PC. Note that a RET instruction would do exactly the same process as a

RETI instruction, but it would not inform the Interrupt Controller, that the interrupt service rou-

tine is completed, an would leave the controller still thinking that the service routine is under

progress.

External Interrupts

There are two external interrupt sources in this processor, INT0 and INT1. These interrupts can be

programmed to be edge triggered or level activated, by setting bits IT0 and IT1 in TCON SFR. In

the edge triggered mode, the INTx inputs are sampled at S5P2 in every machine cycle. If the sam-

ple is high in one cycle and low in the next, then a high to low transition is detected and the inter-

rupts request ﬂag IEx in TCON is set. The ﬂag bit the requests the interrupt.Since the external

interrupts are sampled every machine cycle, they have to be held high or low for at least on com-

plete machine cycle. The IEx ﬂag is automatically cleared when the service routine is called.

If the level activated mode is selected, then the requesting source has to hold the pin low till the

interrupt is serviced. The IEx ﬂag will not be cleared by the hardware on entering the service rou-

tine. If the interrupt continues to be held low even after the service routine is completed, then the

processor may acknowledge another interrupt request from the same source.

Response Time

The response time for each interrupt source depends on several factors like nature of the interrupt

and the instruction under progress. In the case of external interrupt INT0 and INT1, they are sam-

pled at S5P2 of every machine cycle and then their corresponding interrupt ﬂags IE0 and IE1 will

be set or reset. Similarly, the Local Serial port ﬂags RI and TI are set in S5P2. The Timer 0 and 1

overﬂow ﬂags are set during S3 of the machine cycle in which overﬂow has occurred. These ﬂag

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values are polled only in the next machine cycle. If a request is active and all three conditions are

met, then the hardware generated LCALL is executed. This call itself takes two machine cycles to

be completed. Thus there is a minimum time of three machine cycles between the interrupt ﬂag

being set and the interrupt service routine being executed.

A longer response time should be anticipated if any of the three conditions are not met. If a higher

or equal priority is being serviced, then the interrupt latency time obviously depends on the nature

of the service routine currently being executed. If the polling cycle is not the last machine cycle of

the instruction being executed, then an additional delay is introduced. This is maximum in the

case of MUL and DIV instructions which are four machine cycles long. In case of a RETI or a

write to IP or IE registers the delay can be at most 5 machine cycles. This is because at most one

cycle is needed to complete the current instruction and at most 4 machine cycles to execute the

next instruction, which may be a MUL or DIV instruction.

Thus in a single-interrupt system the interrupt response time will always be more than 3 machine

cycles and not more than 8 machine cycles.

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2.9 Power Down and Idle

The processor has two Power Reduction modes, Idle and Power Down. Backup power is supplied

through the VCC pin in these operations. The processor can be put into the Idle or the Power

down mode by setting bits 0 or bit 1 respectively in the PCON SFR.

Any instruction sets the PD bit in PCON SFR, causes that instruction to be the last instruction

executed by the processor before going into the Power Down mode. In the Power Down mode, the

clock to the CPU and the peripheral blocks like Interrupt Controller, Serial Port, dma and Timer/

Counters is stopped. This causes the complete processor to stop its current activities. The status of

all the registers in the CPU, the ALU, the Program Counter, the Stack Pointer, the Program status

Word and the Accumulator are held at their current states. The port pins hold the value they had at

the time Idle was activated. ALE and PSEN are both held at logic low levels.

There are two ways to exit from the Power Down mode. One is a hardware reset. reset and the

other an external interrupt. The hardware reset redeﬁnes all the SFRs but the on-chip RAM is

unaffected.

With an external interrupt, INT0 or INT1 must be enabled and conﬁgures as level triggered inter-

rupts before entering the power down mode. Holding the pin low ends the power down mode con-

dition and bringing the pin high completes the exit. After the interrupt service routine is executed

the program will return to the next instruction following the one that put the device into Power

Down Mode.

Table 6: Status of the External Pins during Idle and Power Down

Program

Memory

Mode

ALE PSEN Port 0

Port 1

Port 2

Data

Port 3,4.

Data

Idle

Internal

External

Data

Float

Data

Float

Data

Address Data

Power Down Internal

Power Down External

Data

Any instruction which sets the IDL bit in PCON SFR, causes that instruction to be the last instruc-

tion executed by the processor before going into the idle mode. In the Idle mode, the clock to the

CPU is shut off while the peripheral blocks like Interrupt Controller, Serial Ports, dma and Timer/

Counters continue to receive the clock. This causes the CPU to stop its current activities. The sta-

tus of all the registers in the CPU, the ALU, the Program Counter, the Stack Pointer, the Program

status Word and the Accumulator are held at their current states. The port pins hold the value they

had at the time Idle was activated. ALE and PSEN are both held at logic high levels.

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SMOD

IDL

SMOD

Double Baud Rate bit. When cleared, the baud rate is halved, by dividing the

serial clock by 2.

Described later.

Power Down bit. Setting this bit activates this mode.

Idle Mode bit. Setting this bit activates this mode.

IDL

PCON: Power Control Register.

There are two ways to terminate the Idle mode. One way is to reset the processor and the other is

by activation of any enabled interrupt. On receiving an interrupt, the processor will vector to the

interrupt service routine and will start execution from here. After completion of the service rou-

tine, following the execution of RETI, the execution will continue from the instruction following

the one which put the processor into the Idle mode.

The signal at the RST pin is used to clear the IDL bit. The RST pin has to be held low for at least

24 clock periods to generate an internal reset. The clearing of IDL bit is done synchronously, so

that the CPU will restart from the same state phase condition that it went into Idle. This ensures

smooth transition from Idle mode to normal operation. Now the reset logic takes 19 clock periods

to activate the internal reset from the time where RST was sampled low. For this time the proces-

sor will continue operations carrying on from the next instruction which put the processor in the

Idle mode. On-chip hardware prevents any writes to RAM during this period. but accesses to ports

is not inhibited. To prevent unexpected outputs at the port pins, the instruction after the one which

causes the Idle mode should not be an assess to the ports or External RAM.

The GSC continues to operate in Idle as long as the interrupts are enabled. The interrupts need to

be enabled, so that the CPU can service the FIFO’s. In order to properly terminate a reception or

transmission the C152 must not be in idle when the EOF is transmitted or received. After servic-

ing the GSC, user software will need to again invoke the Idle command as the CPU does not auto-

matically re-enter the Idle mode after servicing the interrupts.

The GSC does not operate while in Power Down so the steps required prior to entering Power

Down become more complicated. The sequence when entering Power Down and the status of the

I/O is of major importance in preventing damage to the C152 or other components in the system.

since the only way to exit Power Down is with a Reset, several problems that merit careful consid-

eration are cases where the Power Down occurs during the middle of a transmission, and the pos-

sibility that other stations are not or cannot enter this same mode. The state of the GSC I/O pins

becomes critical and the GSC status will need to be saved before power down is entered. There

will also need to be some method of identifying to the CPU that the following Reset is probably

not a cold start and that other stations on the link may have already been initialized.

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The DMA circuitry stops operation in both Idle and power Down modes. Since operation is

stopped in both modes, the process should be similar in each case. Speciﬁc steps that need to be

taken include: notiﬁcation to other devices that DMA operation is about to cease for a particular

station or network, proper withdrawal from DMA operation, and saving the status of the DMA

channels. Again, the status of the I/O pins during Power Down needs careful consideration to

avoid damage to the C152 or other components.

Port 4 returns to its input state, which is high level using weak pullup devices.

2.10 Local Serial Channel

Local Serial port is a full duplex port. This means that it can simultaneously transmit and receive

data. In addition, the receive register is double buffered. This allows reception of the second data

byte before the ﬁrst byte is read from the receive register. The transmit register and the receive

buffer are both addressed as SBUF Special Function Register. However any write to SBUF will be

to the transmit register, while a read from SBUF will be from the receive buffer register.

The serial port can operate in four different modes.

MODE 0

This mode provides the synchronous communication with external devices. In mode 0, serial data

is transmitted and received on the RXD line. TXD is used to transmit the shift clock. This mode is

therefore a half duplex mode of serial communication. 8 bits are transmitted or received per

frame. The LSB is transmitted/received ﬁrst. The baud rate is ﬁxed at 1/12 of the oscillator fre-

quency.

Mode 0 is used to transmit data synchronously, in a half duplex format. The functional block dia-

gram is shown below. Data enters and leaves the Serial port on the RxD line. The Txd line is used

to output the shift clock. The shift clock is used to shift data into and out of the TBO and the

device at the other end of the line.

Any instruction that causes a write to SBUF will start the transmission. The shift clock will get

activated and data will be shifted out on RxD pin til all 8 bits are transmitted. The clock on TxD

then remains low for 6 clock periods and then goes high again. This ensures that at the receiving

end the data on RxD line can be clocked in either on the rising edge of the shift clock on TxD or

latched when the TxD clock is low. The TI ﬂag is set high in S1 following the end of transmission

of the last bit.

The Serial port will receive data when REN is 1 and RI is zero. The shift clock (TxD) will be acti

vated and the serial port will latch data sampled at S5P2 at the rising edge of shift clock. The

external device should therefore present data on the falling edge on the shift clock. This process

continues till all the 8 bits have been received. The RI ﬂag is set in S1 following the last rising

edge of the shift clock on TxD. This will stop reception, till the RI is cleared by software.

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Transmit Shift Register

CLOCK

RxD

P3.0 Alternate

Output function

SOUT

Internal

Data Bus

PARIN

Write to

SBUF

LOAD

CLOCK

TX START

TX CLOCK

TX SHIFT

SM2

SERIAL

CONTROLLER

Serial Interrupt

SHIFT

TxD

P3.1 Alternate

Output function

RX CLOCK

CLOCK

LOAD

SBUF

RX SHIFT

START

REN

CLOCK

PAROUT

Internal

Data Bus

SBUF

SIN

RxD

P3.0 Alternate

Input function

Read

SBUF

Receive Shift Register

Local Serial Port Mode 0

MODE 1

In Mode 1, the full duplex mode is used. Serial communication frames are made up of 10 bits

transmitted on TXD and received on RXD. The 10 bits consist of a start bit (0), 8 data bits (LSB

ﬁrst), and a stop bit (1). On receive, the stop bit goes into RB8 in the SFR SCON. The baud rate in

this mode is variable.In this mode 10 bits are transmitted (on TxD) or received (on RxD). The

frame consists of a start bit (0), 8 data bits (LSB ﬁrst), and a stop bit (1). On reception, the stop bit

is placed into RB8. Tthe baud rate is determined by the Timer 1 or timer 2 overﬂow rate, and so it

can be controlled by the user.

The ﬁgure below gives the simpliﬁed functional block for Mode 1.

Transmission begins with a write to SBUF. The serial data is brought out on to TxD pin following

the ﬁrst roll-over of divide by 16 counter. The next bit is placed on TxD pin following the next

rollover of the divide by 16 counter. Thus the transmission is synchronized to the divide by 16

counter and not directly to the write to SBUF signal. After all 8 bits of data are transmitted, the

stop bit is transmitted. The TI ﬂag is set in the S1 state after the stop bit has been put out on TxD

pin. This will be at the 10th rollover of the divide by 16 counter after a write to SBUF.

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Timer 1

Overﬂow

Transmit Shift Register

STOP

TxD

SOUT

Internal

Data Bus

PARIN

START

SMOD

Write to

SBUF

LOAD

CLOCK

TX START

TX CLOCK

TX SHIFT

SERIAL

CONTROLLER

Serial Interrupt

LOAD

SBUF

RX CLOCK

1-TO-0

DETECTOR

START

RX SHIFT

CLOCK

PAROUT

Internal

SBUF

RB8

Data Bus

BIT

DETECTOR

SIN

RxD

Read

SBUF

Receive Shift Register

Serial Port Mode 1

Reception is enabled only if REN is high. The serial port actually starts the receiving of serial

data, with the detection of a falling edge on the RxD pin. The 1-to-0 detector continuously moni-

tors the RxD line sampling it at the rate of 16 times the selected baud rate. When a falling edge is

detected, the divide by 16 counter is immediately reset. This helps to align the bit boundaries with

the rollovers of the divide by 16 counter.

The 16 states of the counter effectively divide the bit time into 16 slices. The bit detection is done

on a best of three basis. The bit detector samples the RxD pin, at the 8th, 9th and 10th counter

states. By using a majority 2 of 3 voting system, the bit value is selected. This is done to improve

the noise rejection feature of the serial port. If the ﬁrst bit detected after the falling edge of RxD

pin, is not 0, then it indicates an invalid start bit, and the reception is immediately aborted. the

serial port again looks for a falling edge in the RxD line. If a valid start bit is detected, then the

rest of the bits are also detected and shifted into the SBUF.

After shifting in 8 data bits, there is one more shift to do, after which the SBUF and RB8 are

loaded and RI is set. However certain conditions must be met before, The loading and setting of

RI can be done.

1. RI must be 0 and

2. Either SM2 = 0, or the received stop bit = 1.

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If these conditions are met, then the stop bit goes to RB8, the 8 data bits go into SBUF and RI is

set. Else the received frame may be lost. After the middle of the stop bit, the receiver goes back to

looking for a 1-to-0 transition on the RxD pin.

MODE 2

In this mode, 11 bits are transmitted on TXD and received on RXD. The 11 bits consist of a start

bit (0), 8 data bits (LSB ﬁrst), a programmable 9th data bit (TB8 in SCON) and a stop bit (1). The

9th bit can be assigned 1 or 0 by writing to bit 3 of SCON. On transmission, this bit will be

inserted into the data stream. On reception the received 9th bit goes into RB8 in SCON. The stop

bit is ignored. The baud rate is programmable to 1/32 or 1/64 of the oscillator frequency.

Transmission begins with a write to SBUF. The serial data is brought out on to TxD pin following

the ﬁrst roll-over of divide by 16 counter. The next bit is placed on TxD pin following the next

rollover of the divide by 16 counter. Thus the transmission is synchronized to the divide by 16

counter and not directly to the write to SBUF signal. After all 8 bits of data are transmitted, the

stop bit is transmitted. The TI ﬂag is set in the S1 state after the stop bit has been put out on TxD

pin. This will be at the 11th rollover of the divide by 16 counter after a write to SBUF.

Reception is enabled only if REN is high. The local serial port actually starts the receiving of local

serial data, with the detection of a falling edge on the RxD pin. The 1-to-0 detector continuously

monitors the RxD line sampling it at the rate of 16 times the selected baud rate. When a falling

edge is detected, the divide by 16 counter is immediately reset. This helps to align the bit bound-

aries with the rollovers of the divide by 16 counter.

The 16 states of the counter effectively divide the bit time into 16 slices. The bit detection is done

on a best of three basis. The bit detector samples the RxD pin, at the 8th, 9th and 10th counter

states. By using a majority 2 of 3 voting system, the bit value is selected. This is done to improve

the noise rejection feature of the local serial port. If the ﬁrst bit detected after the falling edge of

RxD pin, is not 0, then it indicates an invalid start bit, and the reception is immediately aborted.

the local serial port again looks for a falling edge in the RxD line. If a valid start bit is detected,

then the rest of the bits are also detected and shifted into the SBUF.

After shifting in 8 data bits, there is one more shift to do, after which the SBUF and RB8 are

loaded and RI is set. However certain conditions must be met before, The loading and setting of

RI can be done.

1. RI must be 0 and

2. Either SM2 = 0, or the received stop bit = 1.

If these conditions are met, then the stop bit goes to RB8, the 8 data bits go into SBUF and RI is

set. Else the received frame may be lost. After the middle of the stop bit, the receiver goes back to

looking for a 1-to-0 transition on the RxD pin.

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Clock / 2

Transmit Shift Register

STOP

PARIN

START

TxD

SOUT

TB8

Internal

Data Bus

SMOD

Write to

SBUF

LOAD

CLOCK

TX START

TX CLOCK

TX SHIFT

SERIAL

CONTROLLER

Serial Interrupt

LOAD

SBUF

RX CLOCK

1-TO-0

DETECTOR

START

RX SHIFT

CLOCK

PAROUT

Internal

Data Bus

SBUF

RB8

BIT

DETECTOR

SIN

RxD

Read

SBUF

Receive Shift Register

Local Serial Port Mode 2

MODE 3

This mode is similar to Mode 2 in all respects, except that the baud rate is programmable.

In all four modes, transmission is started by any instruction that uses SBUF as a destination regis-

ter. Reception is initiated in Mode 0 by the condition RI = 0 and REN = 1. Reception is initiated in

the other modes by the incoming start bit if REN = 1.

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Timer 1

Overﬂow

Transmit Shift Register

STOP

PARIN

START

TxD

SOUT

TB8

Internal

Data Bus

SMOD

Write to

SBUF

LOAD

CLOCK

TX START

TX CLOCK

TX SHIFT

SERIAL

CONTROLLER

Serial Interrupt

LOAD

SBUF

RX CLOCK

1-TO-0

DETECTOR

START

RX SHIFT

CLOCK

PAROUT

Internal

Data Bus

SBUF

RB8

BIT

DETECTOR

SIN

RxD

Read

SBUF

Receive Shift Register

Local Serial Port Mode 3

Baud Rates

In Mode 0 the baud rate is ﬁxed at 1/12 of the oscillator frequency.

In Mode 2 the baud rate depends on the value of bit SMOD in PCON SFR. If SMOD is 0 then the

baud rate is 1/64 of the oscillator frequency. If the bit is set to 1, then the baud rate is 1/32 of the

oscillator frequency.

Mode 2 Baud Rate = 2

( SMOD - 6 )

X Oscillator Frequency

The baud rates in Mode 1 and 3 are determined by the overﬂow rates of Timer 1.

Using Timer 1 to generate baud rates.

When Timer 1 is used as a baud rate generator, the baud rates in Modes 1 and 3 are determined by

the Timer 1 overﬂow rate and the value of SMOD.

( SMOD - 5 )

Modes 1 and 3 baud rate = 2

X Timer 1 overﬂow rate

The Timer 1 interrupt should be disabled in this mode. The timer itself can be conﬁgured as either

“timer” or “counter”, in any of its 3 operating modes. Commonly, Timer 1 is conﬁgured as a timer

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in auto-reload mode. In such a case the baud rate is given by

(SMOD - 5)

Mode 1,3 baud rate = 2

X Oscillator Frequency / (12 x [256 - TH1])

It is also possible to achieve very low baud rates by putting Timer 1 in Mode1 as a 16 bit timer and

enabling its interrupt. The Timer interrupt is used to reload the Timer 1 with a 16 bit value using

software methods. A list of commonly used baud rates and how they can be achieved is shown in

table 4

Table 7: Timer 1 generated commonly used Baud rates

Reload

Value

Baud Rate

fosc

SMOD

C/T

Mode

Mode 0 Max 1 MHZ

12 MHZ

Mode 2 Max 375K

Modes 1,3 62.5K

19.2 K

9.6 K

12 MHZ

FFh

FDh

FAh

F4h

E8h

1Dh

72h

11.059 MHZ

6 MHZ

4.8 K

2.4 K

1.2 K

137.5

110

12 MHZ

FEEBh

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2.11 SINGLE-STEP OPERATION

The processor does not have any pin which can directly force it to operate in the single-step mode.

However the user can use the interrupt structure to obtain single-step operation. The user will be

aware that the processor executes one more instruction after a RETI instruction and only then

does it respond to an interrupt request. Thus once an interrupt routine has begun, it cannot enter

another routine unless one instruction of the interrupted program has been executed. The external

interrupts can be used to good effect to achieve this. One of the interrupts, say INT0 is pro-

grammed to be level activated interrupt. Now when the processor detects a low on this pin, it will

vector to the interrupt service routine. The interrupt service routine will have a simple loop which

will wait till the INT0 pin is pulsed, high to low. The processor will now return to the interrupted

program and execute at least one instruction. If the INT0 pin is held low, then another interrupt

request is generated and the interrupt service routine is again executed.

JNB P3.2, $

; wait here till INT0 goes high

P3.2, $

; wait here till INT0 goes low

RETI

; return to the interrupted program and execute at least one more

; instruction

In this way a single instruction of the main program will be executed every time INT0 is pulsed.

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Table 8: Instruction Table

lsn

6-7

8-F

msn

nop

jbc

ajmp

acall

ajmp

acall

ajmp

acall

ajmp

acall

ajmp

acall

ljmp

rr a

inc a

inc d

dec d

inc @

dec @

inc rn

dec rn

lcall

rrc a

dec a

ret

rl a

add a, #

addc a, #

orl a, #

anl a, #

xrl a, #

mov a, #

add a, d

addc a, d

orl a, d

add a, @

addc a, @

orl a, @

add a, rn

addc a, rn

orl a, rn

jnb

reti

rlc a

orl d, a

anl d, a

xrl d, a

orl d, #

anl d, #

xrl d, #

jnc

anl a, d

anl a, @

xrl a, @

anl a, rn

xrl a, rn

xrl a, d

jnz

sjmp

orl c, bit

anl c, bit

mov bit, c

jmp @a+dptr

mov d, #

mov d, d

subb a, d

mov @, #

mov d, @

subb a, @

mov rn, #

mov d, rn

subb a, rn

movc a, @a+pc div ab

mov dptr, #

movc a,

subb a, #

@a+dptr

orl c, /bit

anl c, /bit

push

ajmp

acall

ajmp

acall

ajmp

acall

mov c, bit

cp bit

inc dptr

cpl c

mul ab

cjne a, #, rel

swap a

da a

dec dptr

mov @, d

cjne @, #, rel

xch a, @

mov rn, d

cjne rn, #, rel

xch a, rn

cjne a, d, rel

xch a, d

clr bit

clr c

pop

setb bit

setb c

djnz d, rel

mov a, d

mov d, a

xchd a, @

mov a, @

mov @, a

djnz rn, rel

mov a, rn

mov rn , a

movx a, @dptr

movx @dptr, a

movx a, @r0

movx @r0, a

movx a, @r1

movx @r1, a

clr a

cpl a

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3.0 GLOBAL SERIAL CHANNEL

3.1 Introduction

The Global Serial Channel (GSC) is a multi-protocol, high performance serial interface targeted

for data rates up to 2 MBPS with on-chip clock recovery, and 2.4 MBPS using the external clock

options. in applications using the serial channel, the GSC implements the Data Link Layer and

Physical Link Layer as described in the ISO reference model for open systems interconnection.

The GSC is designed to meet the requirements of a wide range of serial communications applica-

tions and is optimized to implement Carrier-Sense Multi-Access with Collision Detection

(CSMA/CD) and Synchronous Data Link Control (SDLC) protocols. The GSC architecture is

also designed to provide ﬂexibility in deﬁning non-standard protocols. This provides the ability to

retroﬁt new products into older serial technologies, as well as the development of proprietary

interconnect schemes for serial backplane environments.

The versatility of the GSC is demonstrated by the wide range of choices available to the user. The

various modes of operation are summarized in Table 3.1. In subsequent sections, each available

choice of operation will be explained in detail.

In using Table 3.1, the parameters listed vertically (on the left hand side) represent an option that

is selected (X). The parameters listed horizontally (along the top of the table) are all the parame-

ters that could theoretically be selected (Y). The Symbol at the junction of both X and Y deter-

mines the applicability of the option Y.

Note, that not all combinations are backwards compatible. for example, Manchester encoding

requires half duplex, but half duplex does not require Manchester encoding.

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Table 9:

Addr

recognition

Data

Flags

CRC

Duplex

Ack

N-Not available.

M-Mandatory.

O-Optional.

nr nr

P-Normally Preferred

X-N/A

Manchester (csma/cd)

NRZI (sdlc)

NRZ (ext clock)

Flags:01111110(sdlc)

11/IDLE

CRC: NONE

16-bit CCITT

32-bitAUTODIN II

Half Duples

Full Duplex

Acknowledge: None

Hardware

User deﬁned.

Address recog: none

8-bit

16 bit

Coll resol: Normal

Alternate

Deterministic

Preamble: None

8-bit

32-bit

64-bit

DC JAM

CRC JAM

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Table 9:

Addr

recognition

Data

Flags

CRC

Duplex

Ack

N-Not available.

M-Mandatory.

O-Optional.

nr nr

P-Normally Preferred

X-N/A

External clock

Internal clock

Control cpu

Control dma

Raw Receive

Raw Transmit

CSMA

SDLC

Table 10:

preamble

contr

backoff

Jam Clock

N-Not available.

M-Mandatory.

O-Optional.

8 3

P-Normally Preferred

X-N/A

it it

N O O

O O O

Manchester (csma/cd)

NRZI (sdlc)

NRZ (ext clock)

Flags:01111110(sdlc)

11/IDLE

O O

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Table 10:

contr

backoff

preamble

Jam Clock

N-Not available.

M-Mandatory.

O-Optional.

8 3

P-Normally Preferred

X-N/A

it it

CRC: NONE

16-bit CCITT

32-bitAUTODIN II

O O O

N O O

O O O

N O O

X N N

N X N

N N X

N N N

N O O

O O O

Half Duples

Full Duplex

Acknowledge: None

Hardware

User deﬁned.

Address recog: none

8-bit

16 bit

Coll resol: Normal

Alternate

Deterministic

Preamble: None

8-bit

32-bit

64-bit

DC JAM

CRC JAM

External clock

Internal clock

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Table 10:

contr

backoff

preamble

Jam Clock

N-Not available.

M-Mandatory.

O-Optional.

8 3

P-Normally Preferred

X-N/A

it it

O O O

Control cpu

Control dma

Raw Receive

Raw Transmit

CSMA

N O O

O O

SDLC

Note 1: Programmable in Raw transmit or receive mode.

Almost all the options available from Table can be implemented with the proper software to per-

form the functions that are necessary for the options selected. In Table 3.1, a judgment has been

made by the authors on which options are practical and which are not. What this means is that in

table , an “N” should be interpreted as meaning that the option is either not practical when imple-

mented with user software or that it cannot be done. An “O” is used when that function is one of

several that can be implement with the GSC without additional user software.

The GSC is targeted to operate at bit rates up to 2.4 Mbps using the external clock options and up

to 2 MBps using the internal baud rate generator, internal data formatting and on-chip clock

recovery. The baud rate generator allows most standard rates to be achieved. These standards

include the proposed IEEE802.3 LAN standard(1.0MBps) and the T1 standard (1.544Mbps). The

baud rate is derived from the crystal frequency. This makes crystal selection important when

determining the frequency and accuracy of the baud rate.

The user needs to be aware that after reset, the GSC is in CSMA/CD mode, IFS = 256 bit times,

and a bit time equals 8 oscillator periods. The GSC will remain in this mode until the interframe

space expires. If the user changes to SDLC mode or the parameters used in CSMA/CD, these

changes will not take effect until the interframe space expires. A requirement for the interframe

space timer to begin is that the receiver be in an idle state. This makes it possible for the GSC to

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be in some other mode than the user intends for a signiﬁcant amount of time after reset. To pre-

vent unwanted GSC errors from occurring, the user should not enable the GSC or the GSC inter-

rupts for 170 machine cycles ((256 X 8) /12) after LNI bit is set.

3.2 CSMA/CD Operation

3.2.1 CSMA/CD OVERVIEW

CSMA/CD Operates by sensing the transmission line for a carrier, which indicates link activity.

At the end of link activity, a station must wait a period of time, called the deference period, before

transmission may begin. The deference period is also known as the interframe space. The inter-

frame space is explained in Section 3.2.3

With this type of operation, there is always the possibility of a collision occurring after the defer-

ence period due to line delays. If a collision is detected after transmission is started, a jamming

mechanism is used to ensure that all stations monitoring the line are aware of the collision. A res-

olution algorithm is then executed to resolve the contention. There are three different modes of

collision resolution made available to the user on the C152. Re-transmission is attempted when a

resolution algorithm indicates that a station’s opportunity has arrived.

Normally, in CSMA/CD, re-transmission slot assignments are intended to be random. This

method gives all stations an equal opportunity to utilize the serial communication link but also

leaves the possibility of another collision due to two stations having the same slot assignment.

There is an option on the C152 which allows all the stations to have their slot assignments previ-

ously determined by user software. This pre-assignment of slots is called the deterministic resolu-

tion mode. This method allows resolution after the ﬁrst collision and ensures the access of the link

to each station during the resolution. Deterministic resolution can be advantageous when the link

is being heavily used and collisions are frequently occurring and in real time applications where

determinism is required. Deterministic resolution may also be desirable if it is known before hand

that a certain station’s communication needs to be prioritized over those of other stations if it is

involved in a collision.

3.2.2 CSMA/CD FRAME FORMAT

The frame format in CSMA/CD consists of preamble, Beginning of Frame ﬂag (BOF), address

ﬁeld, information ﬁeld, CRC, and End of Frame ﬂag (EOF) as shown in Figure

PREAMBLE

BOF

ADDRESS

INFO

CRC

EOF

Typical CSMA/CD Frame

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PREAMBLE - The preamble is a series of alternating 1s and 0s. The length of the preamble is

programmable to be 0, 8, 32, or 64 bits. The purpose of the preamble is to allow all the receivers

to synchronize to the same clock edges and identiﬁes to the other stations on-line that there is

activity indicating the link is being used. For these reasons zero preamble length is not compatible

with standard CSMA/CD, protocols. When using CSMA/CD, the BOF is considered part of the

preamble compared to SDLC, where the BOF is not part of the preamble. This means that if zero

preamble length were to be used in CSMA/CD, mode, no BOF would be generated. It is strongly

recommended that zero preamble length never be used in CSMA/CD mode. If the preamble con-

tains two consecutive 0s, The preamble is considered invalid. If the C152 detects an invalid pre-

amble, the frame is ignored.

BOF - In CSMA/CD the Beginning -Of-Frame is a part of the preamble and consists of two

sequential 1s. The purpose of the BOF is to identify the end of the preamble and indicate to the

receiver(s) that the address will immediately follow.

ADDRESS - The address ﬁeld is used to identify which messages are intended for which stations.

The user must assign addresses to each destination and source. How the addresses are assigned,

how they are maintained, and how each transmitter is made aware of which addresses are avail-

able is an issue that is left to the user. Some suggestions are discussed in Section 3.5.5. Generally,

each address is unique to each station but there are special cases where this is not true. In these

special cases a message is intended for more than one station. These multi-targeted messages are

called broadcast or multicast-group address usually is indicated by using a 1 as the ﬁrst address

bit. The user can choose to mask off all or selective bits of the address so that the GSC receives all

messages or multicast-group messages. The address length is programmable to be 8 or 16 bits. An

address consisting of all 1s will always be received by the GSC on the C152. The address bits are

always passed from the GSC to the CPU. With user software, the address can be extended beyond

16 bits, but the automatic address recognition will only work on a maximum of 16 bits. User soft-

ware will have to resolve any remaining address bits.

INFO - This is the information ﬁeld and contains the data that one device on the link wishes to

transmit to another device. It can be of any length the user wishes but needs to be in multiples of 8

bits. This is because multiples of 8 bits are used to transfer data into or out of the GSC FIFOs. The

information ﬁeld is delineated from the rest of the components of the frame by the preceding

address ﬁeld and the following CRC. The receiver determines the position of the end of the infor-

mation ﬁeld by passing the bytes through a temporary storage space. When the EOF is received

the bytes in temporary storage are the CRC, and the last bit received previous to the CRC consti-

tute the end of the information ﬁeld.

CRC - The Cyclic Redundancy Check (CRC) is an error checking algorithm commonly used in

serial communications. The C152 offers two types of CRC algorithms, a 16-bit and a 32-bit. The

16-bit algorithm is normally used in the SDLC mode and will be described in the SDLC section.

In CSMA/CD applications either algorithm can be used but IEEE 802.3 uses a 32-bit CRC. The

generation polynomial the C152 uses with the 32-bit CRC is:

G(X) = X + X +X +X +X +X +X +X + X + X +X +X +X + X +1

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The CRC generator, as shown in ﬁgure below, operates by taking each bit as it is received and

XOR’ing it with bit 31 of the current CRC. This result is then placed in temporary storage. The

result of XOR’ing bit 31 with the received bit is then XOR’d with bits 0,

1,3,4,6,7,9,10,11,15,21,22,25 as the CRC is shifted right, the temporary storage space holding the

result of XOR’ing bit 31 and the incoming bit is shifted into position 0. The whole process is then

repeated with the next incoming or outgoing bit.

Received

bit

The user has no access to the CRC generator or the bits which constitute the CRC while in

CSMA/CD. On transmission, the CRC is automatically appended to the data being sent, and on

reception, the CRC bits are not normally loaded into the receive FIFO. Instead, they are automati-

cally stripped. The only indication the user has for the status of the CRC is a pass/fail ﬂag. The

pass/fail ﬂag only operates during reception. A CRC is considered as passing when the CRC gen-

erator has 11000111 00000100 11011010 01111011 B as a remainder after all of the data, includ-

ing the CRC checksum, from the transmitting station has been cycled through the CRC generator.

The preamble, BOF and EOF are not included as part of the CRC algorithm. An interrupt is avail-

able that will interrupt the CPU if the CRC of the receiver is invalid. The user can enable the CRC

to be passed to the CPU by placing the receiver in the raw receive mode.

This method of calculating the CRC is compatible with IEEE 802.3

EOF - The End of Frame indicates when the transmission is completed. The end ﬂag in CSMA/

CD consists of an idle condition. An idle condition is assumed when there is no transitions and the

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link remains high for 2 or more bit times.

3.2.3 INTERFRAME SPACE

The interframe space is the amount of time that transmission is delayed after the link is sensed as

being idle and is used to separate transmitted frames. In alternate back off mode, the interframe

space may also be included in the determination of when retransmissions may actually begin. The

C152 allows programmable interframe spaces of even numbers of times from 2 to 256. The hard-

ware enforces the interframe space in SDLC mode as well as in CSMA/CD mode.

The period of the interframe space is determined by the contents of IFS. IFS is an SFR that is pro-

grammable from 0 to 254. The interframe space is measured in bit times. The value in IFS multi-

plied by the bit time equals the interframe space unless IFS equals 0. If IFS does equal 0, then the

interframe space will equal 256 bit times. One of the considerations when loading the IFS is that

only even numbers (LSB must be 0) can be used because only the 7 most signiﬁcant bits are

loaded into IFS. The LSB is controlled by the GSC and determines which half of the IFS is cur-

rently being used. In some modes, the interframe space timer is re-triggered if activity is detected

during the ﬁrst half of the period. The GSC determines which half of the interframe space is cur-

rently being used by examining the LSB. A one indicates the ﬁrst half and zero indicates the sec-

ond half of the IFS.

After reset IFS is 0, which delays the ﬁrst transmission for both SDLC and CSMA/CD by 256 bit

times (after reset, a bit time equals 8 oscillator clock periods).

In most applications, the period of the interframe space will be equal to or greater than the amount

of time needed to turn -around the received frame. The turn-around period is the amount of time

that is needed by user software to complete the handling of a received frame and be prepared to

receive the next frame. An interframe space smaller than the required turn-around period could be

used, but would allow some frames to be missed.

When a GSC transmitter has a new message to send, it will ﬁrst sense the link. If activity is

detected, transmission will be deferred to allow the frame in progress to complete. When link

activity ceases, the station continues deferring for one interframe space period.

As mentioned earlier, the interframe space is used during the collision resolution period as well as

during normal transmission. The backoff method selected affects how the deference period is han-

dled during normal transmission. If normal backoff mode is selected, the interframe space timer is

reset if activity occurs during approximately the ﬁrst half of the interframe space. If alternate

backoff or deterministic backoff is selected, the timer is not reset. In all cases when the interframe

space timer expires, transmission may begin, regardless if there is activity on the link or not.

Although the C152 resets the interframe space timer if activity is detected during the ﬁrst one-half

of the interframe space, this is not necessarily true of all CSMA/CD systems. (IEEE 802.3 recom-

mends that the interframe space be reset if activity is detected during the ﬁrst two-thirds or less of

the interframe space.)

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3.2.4 CSMA/CD DATA ENCODING

Manchester encoding/decoding is automatically selected when the user software selects CSMA/

CD transmission mode (See Figure below). In Manchester encoding the value of the bit is deter-

mined by the transition in the middle of the bit time, a positive transition is decoded as a 1 and a

negative transition is decoded as a 0. The Address and Info bytes are transmitted LSB ﬁrst. The

CRC is transmitted MSB ﬁrst.

MANCHESTER ENCODING

BIT

TIME

If the external 1X clock feature is chosen the transmission ode is always NRZ (see Section

3.5.11). Using CSMA/CD with the external clock option is not supported because the data needs

reformatting from NRZ to Manchester for the receiver to be able to detect code violations and col-

lisions.

3.2.5 COLLISION DETECTION

The GSC hardware detects collisions by detecting Manchester waveform violations at its GRXD

pin. Three kinds of waveform violations are detected: a missing 0-to-1 transition where one was

expected, a 1-to-0 transition where none was expected, and a waveform that stays low (or high)

for too short a time.

Jitter Tolerance

A valid Manchester waveform must have a transition at the midpoint of any bit cell, and may have

a transition at the edge of any bit cell. Therefore, transitions will nominally be separated by either

1/2 bit-time or 1 bit-time.

The GSC samples the GRXD pin at the rate of 8 x the bit rate. The sequence of samples for the

received bit sequence 001 would nominally be:

samples: 11110000 : 11110000 : 00001111

bit value:

:<-bit cell->: <-bit cell->: <-bit cell->:

The sampling system allows a jitter tolerance of + 1 sample for transitions that are 1/2 bit-time

apart, and +2 samples for transitions that are 1 bit-time apart.

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Narrow Pulses

A valid Manchester waveform must stay high or low for at least a half bit-time, nominally 4 sam-

ple-times. Jitter tolerance allows a waveform which stays high or low for 3 sample-times to also

be considered valid. A sample sequence which shows a second transition only 1 or 2 sample-times

after the previous transition is considered to be the result of a collision. Thus, sample sequences

such as 0000110000 and 111101111 are interpreted as collisions.

The GSC hardware recognizes the collision to have occurred within 3/8 to 1/2 bit-time following

the second transition.

Missing 0-to-1 Transition

A 0-to-1 transition is expected to occur at the center of any bit cell that begins with 0. If the previ-

ous 1-to-0 transition occurred at the bit cell edge, a jitter tolerance of +1 sample is allowed. Sam-

ple sequences such as 1111:00001111 and 1111:000001111 are valid, where “:” indicates a bit

cell edge. Sequences of the form 1111:000000XXX are interpreted as collisions.

For these kinds of sequences, the GSC recognizes the collision to have occurred within 1 to 11/8

bit-times after the previous 1-to-0 transition.

If the previous 1-to-0 transition occurred at the center of the previous bit cell, a jitter tolerance of

+2 samples is allowed. Thus, sample sequences such as 11110000:00001111 and

111100000:000001111 are valid. Sequences of the form 111100000:000000XXX are interpreted

as collisions.

For these kinds of sequences, the GSC recognizes the collision to have occurred within 1 5/8 to 1

3/4 bit-times after the previous1-to-0 transition.

Unexpected 1-to-0 Transition

If the line is at a logic 1 during the ﬁrst half of a bit cell, then it is expected to make a 1-to-0 tran-

sition at the midpoint of the bit cell. If the transition is missed, it is assumed that this bit cell is the

ﬁrst half of an EOF ﬂag (line idle for two bit-times). One bit-time later (which marks the midpoint

of the next bit cell), if there is still no 1-to-0 transition, a valid EOF is assumed and the line idle bit

(LNI in TSTAT) gets set.

However, if the assumed EOF ﬂag is interrupted by a 1-to-0 transition in the bit-time following

the ﬁrst missing transition, a collision is assumed. In that case the GSC hardware recognizes the

collision to have occurred within 1/2 to 5/8 bit-time after the unexpected transition.

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3.2.6 RESOLUTION OF COLLISIONS

How the GSC responds to a detected collision depends on what it was doing at the time the colli-

sion was detected. What it might be doing is either transmitting or receiving a frame, or it might

be inactive.

GSC Inactive

If the GSC is already in the process of receiving a frame at the time the collision is detected, its

response depends on whether the ﬁrst byte of the frame has been transferred into RFIFO yet or

not. If that hasn’t occurred, the GSC simply aborts the reception, but takes no other action unless

DCR has been selected. If DCR has been selected, the GSC participates in the resolution algo-

rithm.

If the reception has already progressed to the point where a byte has been transferred to RFIFO by

the time the collision is detected, the receiver is disabled (GREN = 0), and the Receive Error

Interrupt ﬂag RCABT is set. If DCR has been selected, the GSC participates in the resolution

algorithm.

Table 11: Response to a Detected Collision.

What the GSC was doing

Response

nothing

None, Unless DCR =1.

If DCR = 1, begin DCR countdown

Receiving a Frame, ﬁrst byte not in RFIFO yet.

Receiving a Frame, ﬁrst byte already in RFIFO.

Transmitting a Frame, ﬁrst byte still in TFIFO

None, unless DCR = 1.

If DCR = 1, begin DCR countdown.

Set RCABT, clear GREN.

If DCR = 1, begin DCR countdown.

Execute jam/backoff.

Restart if collision count< 8.

Transmitting a Frame, ﬁrst byte already taken from Execute jam/backoff.

TFIFO Set TCDT, clear TEN.

Note: References to DCR and the DCR Countdown have to do with the Deterministic Colli-

sion Resolution Algorithm.

Jam

The jam signal is generated by any 8XC152 that is involved in transmitting a frame at the time a

collision is detected at its GRXD pin. This is to ensure that if one transmitting station detects a

collision, all the other stations on the network will also detect a collision.

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If a transmitting 8XC152 detects a collision during the preamble/BOF part of the frame that it is

trying to transmit, it will complete the preamble/BOF and then begin the jam signal in the ﬁrst bit

time after BOF. If the collision is detected later in the frame., the jam signal will begin in the next

bit time after the collision was detected.

The jam signal lasts for the same number of bit times as the selected CRC length-either 16- or 32-

bit times.

The 8XC152 provides two types of jam signals that can be selected by user software. If the node

is DC-coupled to the network, the DC jam can be selected. In this case the GTXD pin is pulled to

a logic 0 for the duration of the jam. If the node is AC-coupled to the network, then AC jam must

be selected. In this case the GSC takes the CRC it has calculated thus far in the transmission,

inverts each bit, and transmits the inverted CRC.The selection of DC or AC jam is made by setting

or clearing the DCJ bit, which resides in the SFR named MYSLOT.

When the jam signal is completed, the 8XC152 goes into an idle state. Presumably, other stations

on the network are also generating their own jam signals, after which they too go into an idle state.

When the 8XC152 detects the idle state at its own GRXD pin, the backoff sequence begins.

Backoff

There are three software selectable collision resolution algorithms in the 8XC152. The selection is

made by writing values to 3 bits:

Table 12:

DCR M1 M0

Algorithm

Normal Random

Alternate Random

Deterministic

M1 and M0 reside in GMOD, and DCR is in MYSLOT.

In the Normal Random algorithm, the GSC backs off for a random number of slot times and then

decides whether to restart the transmission. The backoff time begins as soon as a line idle condi-

tion is detected.

The Alternate Random algorithm is the same as the Normal Random except the backoff time

doesn’t start until an IFS has transpired.

In the Deterministic algorithm, the GSC backs off to await its pre-determined turn.

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Random Backoff

In either of the random algorithms, the ﬁrst thing that happens after a collision is detected is that a

I gets shifted into the TCDCNT (Transmit Collision Detect Count) register, from the right.

Thus if the software cleared TCDCNT before telling the GSC to transmit, then TCDCNT keeps

track of how many times the transmission had to be aborted because of collisions:

TCDCNT = 00000000

00000001

ﬁrst attempt

ﬁrst collision

second collision

third collision

fourth collision

00000011

00000111

00001111

......................................

11111111

eighth collision

After TCDCNT gets a 1 shifted into it, the logical AND of TCDCNT and PRBS is loaded into a

countdown timer named BKOFF. PRBS is the name of an SFR which contains the output of a

pseudo-random binary sequence generator. Its function is to provide a random number for use in

the backoff algorithm.

Thus on the ﬁrst collision BKOFF gets loaded randomly with either 00000000 or 00000001. If

there is a second collision it gets loaded with the random selection of 00000000, 00000001,

00000010, or 00000011. On the third collision there will be a random selection among 8 possible

numbers. On the fourth, among 16, etc. Figure below shows the logical arrangement of PRBS,

TCDCNT, and BKOFF.

TCDCNT

PRBS

AND

LOAD

BKOFF

SLOT

CLOCK

BKOFF = 0

BKOFF

BKOFF= MYSLOT

COMP

MYSLOT

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BKOFF starts counting down from its preload value, counting slot times. At any time, the current

value in BKOFF can be read by the CPU, but CPU writes to BKOFF have no effect. While

BKOFF is counting down, if its current value is not 0, transmission is disabled. The output signal

“BKOFF = 0” is asserted when BKOFF reaches 0, and is used to re-enable transmission.

At that time transmission can proceed, subject of course to IFS enforcement, unless:

☛ shifting a 1 into TCDCNT from the right caused a 1 to shift out from the MSB of TCDCNT, or

☛ the collision was detected after TFIFO had been accessed by the transmit hardware.

In either of these cases, the transmitter is disabled (TEN = 0) and the Transmit Error ﬂag TCBT is

set. The automatic restart is canceled.

Where the Normal and Alternate Random backoff algorithms differ is that in Normal Random

backoff the BKOFF timer starts counting down as soon as a line idle condition is detected,

whereas in Alternate Random backoff the BKOFF timer doesn’t start counting down till the IFS

expires.

The Alternate Random mode was designed for networks in which the slot time is less than the

IFS. If the randomly assigned backoff time for a given transmitter happens to be 0, then it is free

to transmit as soon as the IFS ends. If the slot time is shorter than the IFS, Normal Random mode

would nearly guarantee that if there’s ﬁrst collision there will be a second collision. The situation

is avoided in Alternate Random mode, since the BKOFF countdown doesn’t start till the IFS is

over.

The unit of count to the BKOFF timer is the slot time. The slot time is measured in bit-times, and

is determined by a CPU write to the register SLOTTM. The slot time clock is a 1-byte down-

counter which starts its countdown from the value written to SLOTTM. It is decremented each bit

time when a backoff is in progress, and when it gets to 1 it generates one tick in the slot time

clock. The next state after 1 is the reload value which was written to SLOTTM. If 0 is the value

written to SLOTTM, the slot time clock will equal 256 bit times.

A CPU writes to SLOTTM accesses the reload register. A CPU read of SLOTTM accesses the

downcounter. In most protocols, the slot period must be equal to or greater than the longest round

trip propagation time plus the jam time.

Deterministic Backoff

In the Deterministic backoff mode, the GSC is assigned (in software) a slot number. The slot

assignment is written to the low 6 bits of the register MYSLOT. This same register also contains,

in the 2 high bit positions, the control bits DCJ and DCR.

Slot assignments therefore can run from 0 to 63. It will turn out that the higher the slot assign-

ment, the sooner the GSC will get to restart its transmission in the event of a collision.

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The highest slot assignment in the network is written by each station’s software into its TCDCNT

participate in the backoff algorithm.

In deterministic backoff mode a collision will not cause a 1 to be shifted into TCDCNT. TCDCNT

will still be ANDed with PRBS and the result loaded into BKOFF. In order to insure that all sta-

tions have the same value loaded into BKOFF, which determines the ﬁrst slot number to occur, the

PRBS should be loaded with OFFH; the PRBS will maintain this value until either the 8XC152 is

reset or the user writes some other value into PRBS. After BKOFF is loaded it begins counting

down slot times as soon as the IFS ends. Slot times are deﬁned by the user, the same way as

before, by loading SLOTTM with the number of bit times per slot.

When BKOFF equals the slot assignment (as deﬁned in MYSLOT), the signal “BKOFF = MYS-

LOT” in Figure shown above is asserted for one slot time, during which the GSC can restart its

transmission.

While BKOFF is counting down, if any activity is detected at the GRXD pin, the countdown is

frozen until the activity ends, a line idle condition is detected, and an IFS transpires. Then the

countdown resumes from where it left off.

If a collision is detected at the GRXD pin while BKOFF is counting down, the collision resolution

algorithm is restarted from the beginning.

In effect, the GSC “owns” its assigned slot number, but with one exception. Nobody owns slot

number 0. Therefore if the GSC is assigned slot number 0, then when BKOFF=0, this station and

any other station that has something to say at this time will have an equal chance to take the line.

3.2.7 HARDWARE BASED ACKNOWLEDGE

Hardware Based Acknowledge (HBA) is data link packet acknowledging scheme that the user

software can enable with CSMA/CD protocol. It is not an option with SDLC protocol however.

In general HBA can give improved system response time and increased effective transmission

rates over acknowledge schemes implemented in higher layers of the network architecture.

Another beneﬁt is the possibility of early release of the transmit buffer as soon as the acknowl-

edge is received.

The acknowledge consists of a preamble followed by an idle condition. A receiving station with

HABEN enabled will send an acknowledge only if the incoming address is unique to the receiving

station and if the frame is determined to be correct with no errors. For the acknowledge to be sent,

TEN must be set. For the transmitting station to recognize the acknowledge GREN must be set. A

zero as the LSB of the address indicates that the address is unique and not a group or broadcast

address. Errors can be caused by collisions, incorrect CRC, misalignment, or FIFO overﬂow. The

receiver sends the acknowledge as soon as the line is sensed to be idle. The user must program the

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interframe space and the preamble length such that the acknowledge is completed before IFS

expires. This is normally done by programming IFS larger than the preamble.

A transmitting station with HABEN enabled expects an acknowledge. It must receive one prior to

the end of the interframe space, or else an error is assumed and the NOACK bit is set. Setting of

the TDN bit is also delayed until the end of the interframe space. Collisions detected during the

interframe space will also cause NOACK to be set.

If the user software has enabled DMA servicing of the GSC, an interrupt is generated when TDN

is set. TDN will be set at the end of the interframe space if a hardware based acknowledge is

required and received. If the GSC is serviced by the CPU, the user must time out the interframe

space and then check TDN before disabling the transmitter or transmit error interrupts. NOACK

will generate a transmit error interrupt if the transmitter and interrupts are enabled during the

interframe space.

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3.3 SDLC Operation

SDLC is a communication protocol developed by IBM and widely used in industry. It is based on

a primary/ secondary architecture and requires that each secondary station have a unique address.

The secondary stations can only communicate to the primary station, and then, only when the pri-

mary station allows communication to take place. This eliminates the possibility of contention on

the serial line caused by the secondary station’s trying to transmit simultaneously.

In the C152, SDLC can be conﬁgured to work in either full or half duplex. When adhering to strict

SDLC protocol, full duplex is required. Full duplex is selected whenever a 16-bit CRC is selected.

At the end of a valid reset the 16-bit CRC is selected. To select half duplex with a 16-bit CRC, the

receiver must be turned off by user software before transmission. The receiver is turned off by

clearing the GREN bit (RSTAT.1). The receiver needs to be turned off because the address that is

transmitted is the address of the secondary station’s receiver. If not turned off, the receiver could

mistake the outgoing message as being intended for itself. When 32-bit CRCs are used, half

duplex is the only method available for transmission.

3.3.2 SDLC Frame Format

The format of an SDLC frame is shown in Figure. The frame consists of a Beginning of Frame

ﬂag, Address ﬁeld, Control Field, Information ﬁeld (optional), a CRC, and the End of Frame ﬂag.

BOF

ADDRESS

CONTROL

INFO

CRC

EOF

Typical SDLC Frame

BOF - The begin of frame ﬂag for SDLC is 01111110. It is only one of two possible combinations

that have six consecutive ones in SDLC. The other possibility is an abort character which consists

of eight or more consecutive ones. This is because SDLC utilizes a process called bit stufﬁng. Bit

stufﬁng is the insertion of a 0 as the next bit every time a sequence of ﬁve consecutive 1s is

detected. The receiver automatically removes a 0 after every consecutive group of ﬁve ones. This

removal of the 0 bit is referred to as bit stripping. Bit stufﬁng is discussed in Section 3.3.4. All the

procedures required for bit stufﬁng and bit stripping are automatically handled by the GSC.

In standard SDLC protocol the BOF signals the start of a frame and is limited to 8 bits in length.

Since there is no preamble in SDLC the BOF is considered an entire separate ﬁeld and marks the

beginning of the frame. The BOF also serves as the clock synchronization mechanism and the ref-

erence point for determining the position of the address and control ﬁelds.

ADDRESS - The address ﬁeld is used to identify which stations the message is intended for. Each

secondary station must have a unique address. The primary station must then be made aware of

which addresses are assigned to each station. The address length is speciﬁed as 8-bits in standard

SDLC protocols but it is expandable to 16-bits in the C152. User software can further expand the

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number of address bits, but the automatic address recognition feature works on a maximum of 16-

bits.

In SDLC the address are normally unique for each station. However, there are several classes of

messages that are intended for more than one station. These messages are called broadcast and

group addressed frames. An address consisting of all 1s will always be automatically received by

the GSC, this is deﬁned as the broadcast address in SDLC. A group address is an address that is

common to more than one station. The GSC provides address masking bits to provide the capabil-

ity of receiving group address.

If desired, the user software can mask off all the bits of the address. This type of masking puts the

GSC in a pomiscuous mode so that all addresses are received.

CONTROL - The control ﬁeld is used for initialization of the system, identifying the sequence of

a frame, to identify if the message is complete, to tell secondary stations if a response is expected,

and acknowledgment of previously sent frames. The user software is responsible for insertion of

the control ﬁeld as the GSC hardware has no provisions for the management of this ﬁeld. The

interpretation and formation of the control ﬁeld must also be handled by user software. The infor-

mation following the control ﬁeld is typically used for information transfer, error reporting, and

various other functions. These functions are accomplished by the format of the control ﬁeld.

There are three formats available. The types of formats are informational, Supervisory, or Unnum-

bered.

INFO - This is the information ﬁeld and contains the data that one device on the link wishes to

transmit to another device. It can be of any length the user wishes, but must be a multiple of 8 bits.

It is possible that some frames may contain no information ﬁeld. The information ﬁeld is identi-

ﬁed to the receiving stations by the preceding control ﬁeld and the following CRC. The GSC

determines where the last of the information ﬁeld is by passing the bits through the CRC genera-

tor. When the last bit or EOF is received the bits that remain constitute the CRC.

CRC -The Cyclic Redundancy Check (CRC) is an error checking sequence commonly used in

serial communications. The C152 offers two types of CRC algorithms, a 16-bit and a 32-bit. The

32-bit algorithm is normally used in CSMA/CD applications and is described in section 3.2.2. In

most SDLC applications a 16-bit CRC is used and the hardware conﬁguration that supports 16-bit

CRC is shown in Figure below The generating polynomial that the CRC generator uses with the

16-bit CRC is:

G(X) = X + X + X + 1

The way the CRC operates is that as a bit is received it is XOR’d with bit 15 of the current CRC

and placed in temporary storage. The result of XOR’ing bit 15 with the received bit is then XOR’d

with bit 4 and bit 11 as the CRC is shifted one position to the right. The bit in temporary storage is

shifted into position 0.

The required CRC length for SDLC is 16 bits. The CRC is automatically stripped from the frame

and not passed on to the CPU. The last 16 bits are then run through the CRC generator to insure

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that the correct remainder is left. The remainder that is checked for is 001110100001111B (1D0F

Hex). If there is a mismatch, an error is generated. The user software has the option of enabling

this interrupt so the CPU is notiﬁed.

16-Bit CRC

RECEIVED

BIT

EOF - The End Of Frame (EOF) indicates when the transmission is complete. The EOF is identi-

ﬁed by the end ﬂag. An end ﬂag consists of the bit pattern 01111110, The EOF can also serve as

the BOF for the next frame.

3.3.3 DATA ENCODING

The transmission of data in SDLC mode is done via NRZI encoding as shown in Figure below.

NRZI encoding transmits data by changing the state of the output whenever a 0 is being transmit-

ted. Whenever a 1 is transmitted the state of the output remains the same as the previous bit and

remains valid for the entire bit time. When SDLC mode is selected it automatically enables the

NRZI encoding on the transmit line and NRZI decoding on the receive line. The Address and Info

bytes are transmitted LSB ﬁrst. The Address and Info bytes are transmitted LSB ﬁrst. The CRC is

transmitted MSB ﬁrst.

NRZI

BIT

TIME

3.3.4 BIT STUFFING/STRIPPING

In SDLC mode one of the primary rules of the protocol is that in any normal data transmission,

there will never be an occurrence of more than 5 consecutive 1s. The GSC takes care of this

housekeeping chore by automatically inserting a 0 after every occurrence of 5 consecutive 1s and

the receiver automatically removes a zero after receiving 5 consecutive 1s. All the necessary steps

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required for implementing bit stufﬁng and striping are incorporated into the GSC hardware. This

makes the operation transparent to the user. About the only time this operation becomes apparent

to the user, is if the actual data on the transmission medium is being monitored by a device that is

not aware of the automatic insertion of 0s. The bit stufﬁng/stripping guarantees that there will be

at least one transition every 6 bit times while the line is active.

3.3.5 SENDING ABORT CHARACTER

An abort character is one of the exceptions to the rule that disallows more than 5 consecutive 1s.

The abort character consists of any occurrence of seven or more consecutive ones. The simplest

way for the C152 to send an abort character is to clear the TEN bit. This causes the output to be

disabled which, in turn, forces it to a constant high state. The delay necessary to insure that the

link is high for seven bit times is a task that needs to be handled by user software. Other methods

of sending an abort character are using the IFS register or using the Raw Transmit mode. Using

IFS still entails clearing the TEN bit, but TEN can be immediately re-enabled. The next message

will not begin until the IFS expires. The IFS begins timing out as soon as DEN goes high which

identiﬁes the end of transmission. This also requires that IFS contain a value equal to or greater

than 8. This method may have the undesirable effect that DEN goes high and disables the external

drivers. The other alternative is to switch to Raw Transmit mode. Then, writing OFFH to TFIFO

would generate a high output for 8 bit times. This method would leave DEN active during the

transmission of the abort character.1

When the receiver detects seven or more consecutive 1s and data has been loaded into the receive

FIFO, the RCABT ﬂag is set in RSTAT and the frame is ignored. If no data has been loaded into

the receive FIFO, there are no abort ﬂags set and that frame is just ignored. A retransmitted frame

may immediately follow an abort character, provided the proper ﬂags are used.

3.3.6 LINE IDLE

If 15 or more consecutive 1s are detected by the receiver the Line Idle (LNI) in TSTAT is set. The

seven 1s from the abort character may be included when sensing for a line idle condition. The

same methods used for sending the Abort character can be used for creating the Idle condition.

However, the values would need to be changed to reﬂect 15 bit times, instead of seven bit times.

3.3.7 ACKNOWLEDGEMENT

Acknowledgment in SDLC is an implied acknowledge and is contained in the control ﬁeld. Part

of the control frame is the sequence number of the next expected frame. This sequence number of

is called the Receive Count. In transmitting the Receive Count, the receiver is in fact acknowledg-

ing all the previous frames prior to the count that was transmitted. This allows for the transmis-

sion of up to seven frames before an acknowledge is required back to the transmitter. The

limitation of seven frames is necessary because the Receive Count in the control ﬁeld is limited to

three binary digits. This means that if an eighth transmission occurred this would cause the next

Receive Count to repeat the ﬁrst count that still is waiting for an acknowledge. This would defeat

the purpose of the acknowledgment. The processing and general maintenance of the sequence

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count must be done by the user software. The Hardware Based Acknowledge option that is pro-

vided in the C152 is not compatible with standard SDLC protocol.

3.3.8 PRIMARY/SECONDARY STATIONS

All SDLC networks are based upon a primary/secondary station relationship. There can be only

one primary station in a network and all the other stations are considered secondary. All commu-

nication is between the primary and secondary station. Secondary station to secondary station

direct communication is prohibited. If there is a need for secondary to secondary communication,

the user software will have to make allowances for the master to act as an intermediary. Secondary

stations are allowed use of the serial line only when the master permits them. This is done by the

master polling the secondary stations to see if they have a need to access the serial line. This

should prevent any collisions from occurring, provided each secondary station has its own unique

address. This arrangement also partially determines the types of networks supported. Normally

SDLC networks consists of point-to-point, multi-drop, or ring conﬁgurations and the C152 sup-

ports all of these. However, some SDLC processors support an automatic one bit delay at each

node that is not supported by the C152. In a “Loop Mode” conﬁguration, is necessary that the

transmission be delayed from the reception of the frames from the upstream station before passing

the message to the downstream station. This delay is necessary so that a station can decode its

own address before the message is passed on. The various networks are shown in Figure.

PRIMARY

SECONDARY

Point-to-point Network

PRIMARY

SECONDARY

SECONDARY SECONDARY

Multi-Drop Network

PRIMARY

Ring Network

SECONDARY

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3.3.9 HDLC/SDLC COMPARISON

HDLC (High level Data Link Control) is a standard adopted by the International Standards Orga-

nization (ISO). The HDLC standard is deﬁned in the ISO document # ISO6159 -HDLC unbal-

anced classes of procedure. IBM developed the SDLC protocol as a subset of HDLC. SDLC

conﬁrms to HDLC protocol requirements, but is more restrictive. SDLC contains a more precise

deﬁnition on the modes of operation.

Some of the major differences between SDLC and HDLC are:

SDLC

HDLC

Unbalanced (primary/secondary)

Modulo 8(no extensions allowed,

acknowledge is required)

up to 7 out-standing frames before

acknowledge is required)

8-bit addressing only

Balanced (peer to peer)

Modulo 128 (up to 127 outstanding frames before

Extended addressing

Variable size of data

Byte aligned data

The C152 does not support HDLC implementation requiring data alignment other than byte align-

ment. The user will ﬁnd that many of the protocol parameters are programmable in the C152

which allows easy implementation of proprietary or standard HDLC network. User software

needs to implement the control ﬁeld functions.

3.3.10 USING A PREAMBLE IN SDLC

When transmitting a preamble in SDLC mode, the user should be aware that the pattern of

10101010.....is output. NRZI encoding is used in SDLC when the internal baud rate generator is

the clock source and this means that a transition will occur every two bit times, when a 0 is trans-

mitted. This compares with some others SDLC devices, most of which transmit the pattern

00000000..... which will cause a transition every bit time. Our past experience has shown that the

C152 preamble does not cause a problem with most other devices. This is because the preamble is

used only to deﬁne the relative bit time boundaries within some variation allowed by the receiving

station, and the C152 does not have any problems with receiving a preamble consisting of all 0s.

One note of caution however. If idle ﬁll ﬂags are used in conjunction with a preamble, the address

00(00)H and 55(55) H should not be assigned to any C152 as the preamble following the idle ﬁll

ﬂags will be interpreted as an address.

3.4 User Deﬁned Protocols

The explanation on the implementation of user deﬁned protocols would go beyond the scope of

this manual, but examining Table 3.1 should give the reader a consolidated list of most of the pos-

sibilities. In this manual, any deviation from the documents that cover the implementation of

CSMA/CD or SDLC are considered user deﬁned protocols. Examples of this would be the use of

SDLC with the 32-bit CRC selected or CSMA/CD with hardware based acknowledge.

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3.5 USING THE GSC

3.5.1 LINE DISCIPLINE

Line discipline is how the management of the transfer of data over the physical medium is con-

trolled. Two types of line discipline will be discussed in this section: full duplex and half duplex.

Full duplex is the simultaneous transmission and reception of data.Full duplex uses anywhere

from two to four wires. At least one wire is needed for transmission and one write for reception.

Usually there will also be a ground reference on each signal if the distance from station to station

is relatively long. Full-duplex operation in the dC152 requires that both the receive and the trans-

mit portion of the GSC are functioning at the same time. Since both the transmitter and receiver

are operating, two CRC generators are also needed. The C152 handles this problem by having one

32-bit CRC generator and one 16-bit CRC generator. When supporting full-duplex operation, the

32-bit CRC generator is modiﬁed to work as a 16-bit CRC generator. When ever the 16-bit CRC

is selected, the GSC automatically enters the full duplex mode. Half duplex with a 16-bit CRC is

discussed in the following paragraph.

Half duplex is the alternate transmission and reception of data over a single common wire. Only

one or two wires are needed in half-duplex systems. One wire is needed for the signal and if the

distance to be covered is long there will also be a wire for the found reference. In half-duplex

mode, only the receiver or transmitter can operate at one time. When the receiver or transmitter

operates is determined by user software, but typically the receiver will always be enabled unless

the GSC is transmitting. When using the C152 in half-duplex and the receiver is connected to the

transmitter it is possible that a station will receive its’ own transmission. This can occur if a

broadcast address is sent, the address mask register(s) are ﬁlled with all 1s, or the address being

sent matches the sending stations address through the use of the address masking registers. the

receiver must be disabled by the user while transmitting if any of these conditions will occur,

unless the user wants a station to receive its own transmission. The receiver is disabled by clearing

GREN (and GAREN if used). Half-duplex operation in the C152 is supported with either 16-bit or

32-bit CRCs. Whenever a 32-bit CRC is selected, only half-duplex operation can be supported by

the GSC. It is possible to simulate full-duplex operation with a 32-bit CRC, but this would require

that the CRC be performed with software. Calculating the CRC with the CPU would greatly

reduce the data rates that could be used with the GSC. Whenever a 16-bit CRC is selected, full-

duplex operation is automatically chosen and the GSC must be reconﬁgured if half duplex opera-

tion is preferred.

3.5.2 PLANNING FOR NETWORK CHANGES AND EXPANSIONS

A complete explanation on ho to plan for network expansion will not be covered in this manual as

there are far too many possibilities that would need to be discussed. But there are several areas

that will have major impact when allowing for changes in the system. In cases where there will

never be any changes allowed, expansion plans become a mute issue. However, it is strongly sug-

gested that there always be some allowance for future modiﬁcations.

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Some of the general areas that will impact the overall scheme on how to incorporate future

changes to the system are:

1) Communication of the change to all the stations or the primary station.

2) Maximum distance for communication. This will affect the drivers used and the slot time.

3) More stations may be on the line at one time. This may impact the interframe space or the col-

lision resolution used.

4) If using CSMA/CD without deterministic resolution, any increase in network size will have a

negative impact on the average throughput of the network and lower the efﬁciency. The user will

have to give careful consideration when deciding how large a system can ultimately be and still

maintain adequate performance.

3.5.3 DMA SERVICING OF GSC CHANNELS

There are two sources that can be used to control the GSC. The ﬁrst is CPU control and the sec-

ond is DMA control.

CPU control is used when user software takes care of the tasks such as: loading the TFIFO, read-

ing the RDIFO, checking the status ﬂags, and general tracking of the transmission process. As the

number of tasks grow and higher data transfer rates are used, the overhead required by the CPU

becomes the dominant consumption of time. Eventually, a point is reached where the CPU is

spending 100% of its time responding to the needs of the GSC. An alternative is to have the DMA

channels control the GSC

A detailed explanation on the general use of the DMA channels is covered in Section 4. In this

section only those details required for the use of the DMA channels with the GSC will be covered.

The DMA channels can be conﬁgured by user software so that the GSC data transfers are serviced

by the DA controller. Since there are two DMA channels, one channel can be used to service the

receiver, and one channel can be used to service the transmitter. In using the DMA channels, the

CPU is relived of much of the time required to do the basic servicing of the GSC buffers. The

types of servicing that the DMA channels can provide are: loading of the transmit FIFO, remov-

ing data from the receive FIFO, notiﬁcation of the CPU when the transmission or reception has

ended, and response to certain error conditions. When using the DMA channels the source or des-

tination of the data intended for serial transmission can be internal data memory, external data

memory, or any of the SFRs.

The only tasks required after initialization of the DMA and GSC registers are enabling the proper

interrupts and informing the DMA controller when to start. After the DMA channels are started

all that is required of the CPU is to respond to error conditions or wait until the end of transmis-

sion.

Initialization of the DMA channels requires setting up the control, source, and destination address

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registers. On the DMA channel servicing the receiver, the control register needs to be loaded as

follows: DCONn.2 = 0, this sets the transfer mode so that response is to GSC interrupts and put

the DMA control in alternate cycle mode; DCONn.3 = 1, this enables the demand mode;

DCONn.4 = 0, this clears the automatic increment option for the source address; and DCONn.5 =

1, this deﬁnes the source as SFR, The DMA channel servicing the receiver also needs its source

address register to contain the address of RFIFO (SHRHN = XXH, SARLN = 0F4H). On the

DMA channel servicing the transmitter, the control register needs to be loaded as follows:

DCONn.2 = 0; DCONn.3 = 1; DCONn.6 = 0, this clears the automatic increment option for the

destination address; and DCONn.7 = 1, this sets the destination as SFR. The DMA channel serv-

ing the transmitter also requires that its destination address register contains the address of TFIFO

(DARHN = XXH, DARLN = 85H). Assuming that DCON0 would be serving the receiver and

DCON1 the transmitter, DCON0 would be loaded with XX1010X0B. The contents of SARH0

and DARH1 do not have any impact when using internal SFRs as the source or destination.

When using the DMA channels to service the GSC, the byte count registers will also need to be

initialized.

The Done ﬂag for the DMA channel servicing the receiver should be used if ﬁxed packet lengths

only are being transmitted or to insure that memory is not over-written by long received data

packets. Overwriting of data can occur when using a smaller buffer than the packet size. In these

cases the servicing of the DMA and/or GSC would be in response to the DMA Done ﬂag when

the byte count reaches zero.

In some cases the buffer size is not the limiting factor and the packet length will be unknown. in

these cases it would be desirable to eliminate the function of the Done ﬂag. To effectively disable

the Done ﬂag for the DMA channel servicing the receiver, the byte count should be set to some

number larger than any packet that will be received, up to 64K. If not using the Done ﬂag, then

GSC servicing would be driven by the receive Done (RDN) ﬂag and /or interrupt. RDN is set

when the EOF is detected. When using the RDN ﬂag, RFNE should also be checked to insure that

all the data has been emptied out of the receive FIFO.

The byte count register is used for all transmissions and this means that all packets going out will

have to be of the same length or the length of the packet to be sent will have to be known prior to

the start of transmission. When using the DMA channels to service the GSC transmitter, there is

no practical way to disable the Done ﬂag. This is because the transmit done ﬂag (TDN) is set

when the transmit FIFO is empty and the last message bit has been transmitted. But, when using

the DMA channel to service the transmitter, loads to the TFIFO continue to occur until the byte

count reaches 0. This makes it impossible to use TDN as a ﬂag to stop the DMA transfers to

TFIFO. It is possible to examine some other registers or conditions, such as the current byte

count, to determine when to stop the DMA transfers to TFIFO, but this is not recommended as a

way to service the DMA and GSC when transmitting because frequent reading of the DMA regis-

ters will cause the effective DMA transfer rate to slow down.

When using the DMA channels, initialization of the GSC would be exactly the same as normal

except that TSTAT.0 = 1 (DMA), this informs the GSC that the DMA channels are going to be

used to service the GSC. Although only TSTAT is written to, both the receiver and transmitter use

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this same DMA bit.

The interrupts EGSTE (IEN1.5), GSC transmit error; EGSTV (IEN1.3), GSC transmit valid;

EGSRE (IEN1.1), GSC receive error; and EGSRV (IEN1.0), GSC receive valid; need to be

enabled. The DMA interrupts are normally not used when servicing the GSC with the DMA chan-

nels. To ensure that the DMA interrupts are normally not used when servicing the GSC with the

DMA channels. To ensure that the DMA interrupts are not responded to is a function of the user

software and should be checked by the software to make sure they are not enabled. priority for

these interrupts can also be set at this time. Whether to use high or low priority needs to be

decided by the user. When responding to the GSC interrupts, if a buffer is being used to store the

GSC information, then the DMA registers used for the buffer will probably need updating.

After this initialization, all that needs to be done when the GSC is actually going to be used is:

load the byte count, set-up the source addresses for the DMA channel servicing the transmitter,

set-up the destination addresses for the DMA channel servicing the receiver, and start the DMA

transfer. The GSC enable bits should be set ﬁrst and then the GO bits for the DMA. This initiates

the data transfers.

This simpliﬁes the maintenance of the GSC and can make the implementation of an external

buffer for packetized information automatic.

An external buffer can be used as the source of data for transmission, or the destination of data

from the receiver. In this arrangement, the message size is limited to the RAM size or 64K, which-

ever is smaller. By using an external buffer, the data can be accessed by other devices which may

want access to the serial data. The amount of time required for the external data moves will also

decrease. Under CPU control, a “MOVX” command would take 24 oscillator periods to complete.

Under DMA control, external to internal, or internal to external, data moves take only 12 oscilla-

tor periods.

3.5.4 BAUD RATE

The GSC baud rate is determined by the contents of the SFR, BAUD, or the external clock. The

formula used to determine the baud rate when using the internal clock is:

(fosc)/ ((BAUD +1)*8)

For example if a 12 MHz oscillator is used the baud rate can vary from:

12,000,000/ ((0+1)*8) = 1.5 MBPS

to:

12,000,000/((255+1)*8) = 5.859 KBPS

There are certain requirements that the external clock will need to meet. These requirements are

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speciﬁed in the data sheet. For a description of the use of the GSC with external clock please read

Section 3.5.11.

3.5.5 INITIALIZATION

Initialization can be broken down into two major components, 1) initialization of the component

so that its serial port is capable of proper communication; and 2) initialization of the system or a

station so that intelligible communication can take place.

Most of the initialization of the component has already been discussed in the previous sections.

Those items not covered are the parameters required for the component to effectively communi-

cate with other components. These types of issues are common to both system and component ini-

tialization and will be covered in the following text.

Initialization of the system can be broken down into several steps. First, are the assumptions of

each network station.

The ﬁrst assumption is that the type of data encoding to be used is predetermined for the system

and that each station will adhere to the same basic rules deﬁning that encoding. The second

assumption is that the basic protocol and line discipline is predetermined and known. This means

that all stations are using CSMA/CD or SDLC or whatever, and that all stations are either full or

half duplex. The third assumption is that the baud rate is preset for the whole system. Though the

baud rate could probably be determined by the microprocessor just by monitoring the link, it will

make it much simpler if the baud rate is known in advance.

One of the ﬁrst things that will be required during system initialization is the assignment of

unique addresses for each station. in a two-station only environment this is not necessary and can

be ignored. However, keep in mind, that all systems should be constructed for easy future expan-

sions. Therefore, even in only a two station system, addresses should be assigned. There are three

basic ways in which addresses can be assigned. The ﬁrst, and most common is preassigned

addresses that are loaded into the station by the user. This could be done with a DIP-switch,

through a keyboard. The second method of assigning addresses is to randomly assign an address

and then check for its uniqueness throughout the system, and the third method is to make an

inquiry to the system for the assignment of a unique address. Once the method of address assign-

ment is determined, the method should become part of the speciﬁcations for the system to which

all additions will have to adhere. This, then, is the ﬁnal assumption.

The negotiation process may not be clear for some readers. The following two procedure are

given as a guideline for dynamic address assignment.

In the ﬁrst procedure, a station assumes a random address and then checks for its uniqueness

throughout the system. As a station is initialized into the system it sends out a message containing

its assumed address. The format of the message should be such that any station decoding the

address recognizes it as a request for initialization. if that address is already used, the receiving

station returns a message, with its own address starting that the address in question is already

taken. The initializing station then picks another address. When the initializing station sends its

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inquiry for the address check, a timer is also started. If the timer expires before the inquiry is

responded to, then that station assumes the address chosen is okay.

In the second procedure, an initializing station asks for an address assignment from the system.

This requires that some station on the link take care of the task of maintaining a record of which

addresses are used. This station will be called station-1. When the initializing station, called sta-

tion-2, gets on the link, it sends out a message with a broadcast address. The format of the mes-

sage should be such that all other stations on the link recognize it as a request for address

assignment. Part of the message from station-2 is a random number generated by the station

requesting the address. Station-2 then examines all received messages for this random number.

The random number could be the address of the received message or could be within the informa-

tion section of a broadcast frame. All the stations, except station-1, on the link should ignore the

initialization request. Station-1, upon receiving the initialization requests, assigns an address and

returns it to station-2. Station-1 will be required to format the message in such a manner so that all

stations on the link recognize it as a response to initialization. This means that all stations except

station-2 ignore the return message.

3.5.6 TEST MODES

There are two test modes associated with the GSC that are made available to the user. The test

modes are named Raw Receive and Raw Transmit. The test modes are selected by the proper set-

ting of the two mode bits in GMOD (M0 = GMOD.5, M1 = GMOD.6). If M1, M0 = 0,1 then Raw

Transmit is selected. If M1,M0 = 1,0 then Raw Receive is enabled. The 32-bit CRC cannot be

used in any of the test modes, or else CRC errors will occur.

In Raw Transmit, the transmit output is internally connected to the Receiver input. This is

intended to be used as a local loop-back test mode, so that all data written to the transmitter will

be returned by the receiver. Raw Transmit can also be used to transmit user data. If Raw Transmit

is used in this way the data is emitted with no preamble, ﬂag, address CRC, nd no bit insertion.

The data is still encoded with whatever format is selected, Manchester with CSMA/CD, NRZI

with SDLC or as NRZ if external clocks are used. The receiver still operates as normal and in this

mode most of the receive functions can be tested.

In Raw Receive, the transmitter should be externally connected to the receiver. To do this a port

pin should be sued to enable an external device to connect the two pins together. in Raw Receive

mode the receiver acts as normal except that all bytes following the BOF are loaded into the

receive FIFO, including the CRC. Also address recognition is not active but needs to be per-

formed in software. If SDLC is selected as the protocol, zero-bit deletion is still enabled. The

transmitter still operates as normal and in this mode most of the transmitter functions and an

external transceiver can be tested. This is also the only way that the CRC can be read by the CPU,

but the CRC error bit will not be set.

3.5.7 EXTERNAL DRIVER INTERFACE

A signal is provided from the C152 to enable transmitter drivers for the serial link. This is pro-

vided for systems that require more than what the GSC ports are capable of delivering. The volt-

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age and currents that the GSC is capable of providing are the same levels as those for normal port

operation. The signal used to enable the external drivers is DEN. No similar signal is needed for

the receiver.

DEN is active one bit time before transmission begins. In CSMA/CD DEN remains active for two

bit times after the CRC is transmitted. In SDLC DEN remains active until the last bit of the EOF

is transmitted.

3.5.8 JITTER (RECEIVE)

Data jitter is the difference between the actual transmitted waveform and the exact calculated

value(s). In NRZI, data jitter would be how much the actual wave-form exceeds or falls short of

one calculated bit time. A bit time equal 1/ baud rate. If using Manchester encoding, there can be

two transitions during one bit time as shown in Figure below. This causes a second parameter to

be considered when trying to ﬁgure out the complete data jitter amount. This other parameter is

the half-bit jitter.The half-bit jitter is comprised of the difference in time that the half-bit transition

actually occurs and the calculated value. Jitter is important because if the transition occurs too

soon it is considered noise, and if the transition occurs too late, then either the bit is missed or a

collision is assumed.

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LOCAL VALUE

MANCHESTER

ENCODING

“1” BIT TIME

(8 X BAUD)

RECEIVE

SAMPLING

RATE

RECEIVED

DATA

“1” BIT TIME

“0” BIT TIME

RECEIVED

DATA

3.5.9 Transmit Waveforms

The CSC is capable of three types of data encoding, Manchester, NRZI, and NRZ. Figure shows

example of all three types of data encoding.

BIT

TIME

NRZ

NRZI

MANCHESTER

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3.5.10 Receiver Clock Recovery

The receiver is always monitored at eight times the baud rate frequency, except when an external

clock is used. When using an external clock the receiver is loaded during the clock cycle.

In CSMA/CD mode the receiver synchronizes to the transmitted data during the preamble. If a

pulse is detected as being too short it is assumed to be noise or a collision. If a pulse is too long it

is assumed to be a collision or an idle condition.

In SDLC the synchronization takes place during the BOF ﬂag. In addition, pulses less than four

sample periods are ignored, and assumed to be noise. This sets a lower limit on the pulse size of

received zeros.

In CSMA/CD the preamble consists of alternating 1s and 0s. Consequently, the preamble looks

like the waveform in Figure A and B.

CSMA/CD Clock Recovery

IDEL

WAVE FORM

8X SAMPLING

RATE

ACTUAL

WAVEFORM

RECOVERED

BIT STREAM

CLOCK

SDLC Clock Recovery

IDLE

WAVEFORM

8X SAMPLING

RATE

ACTUAL

WAVE FORM

RECOVERED

BIT STREAM

CLOCK

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3.5.11 External Clocking

To select external clocking, the user is given three choices. External clocking can be used with the

transmitter, with the receiver, or with both. To select external clocking for the transmitter, XTCLK

(GMOD.7) has to be set to a 1. To select external clocking for the receiver, XRCLK (PCON.3) has

to be set to a 1. Set ting both bits to 1 forces external clocking for the receiver and transmitter. The

minimum frequency the GSC can be externally clocked at is 0 Hz (D.C.).

The external transmit clock is applied to pin 4 (TXC), P1.3. The external receive clock is applied

to pin 5 (RXC), P1.4. To enable the external clock function on the port pin, that pin has to be set to

a 1 in the appropriate SFR, P1.

Whenever the external clock option is used, the format of the transmitted and received data is

restricted to NRZ encoding and the protocol is restricted to SDLC. With external clock, the bit

stufﬁng/stripping is still active with SDLC protocol.

3.5.12 Determining Receiver Errors

It is possible that several receiver error bits will be set in response to a single cause. The multiple

errors that can occur are:

AE and CRCE may both be set when an alignment error occurs due to a bad CRC caused by the

misaligned frame.

RCABT, AE, and CRCE may be set when an abort occurs.

OVR, AE, and CRCE may be set when a overrun occurs.

In order to determine the correct cause o the error a speciﬁc order should be followed when exam-

ining the error bits. This order is:

1) OVR

2) RCBAT

3) AE

4) CRCE

3.5.13 Addressing

There are four 8-bit address registers (ADR0, ADR1, ADR2, ADR3) and two 8-bit address mask

registers (AMSK0, AMSK1) in the C152. These function with the GSC receiver only. The trans-

mitted address is treated like any other data. The address is transmitted under software control by

placing the address byte(s) at the proper location (usually ﬁrst) in the sequence of bytes to be out-

put in the outgoing packet.

The C152 can have up to four different 8-bit addresses or two different 16-bit addresses assigned

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to each station. When using 16-bit addressing, ADR0:ADR1 form one address and ADR2:ADR3

form the second address. If the receiver is enabled, it looks for a matching address after every

BOF ﬂag is detected. As the data is received, if the 8th (or 16th) bit does not match the address

recognition circuitry, the rest of the frame is ignored and the search continues for another ﬂag. if

the address does match the address recognition circuitry, the address and all subsequent data is

passed into the receive FIFO until the EOF ﬂag or an error occurs. The address is not stripped and

is also passed to RFIFO.

The address masking registers, AMSK0 and AMSK1, work in conjunction with ADR0 and ADR1

respectively to identify “don’t care” bits. A 1 in any position in the AMSKn register makes the

respective bit in the ADRn register irrelevant. These combinations can then be used for form

group addresses. If the masking registers are ﬁlled with all 1s, the C152 will receive all packets,

which is called the promiscuous mode. If 16-bit addressing is used, AMSK0:AMSK1 form one

16-bit address mask.

3.6 GSC Operation

3.6.1 Determining Line Discipline

In normal operation the GSC uses full or half duplex operation. When using a 32-bit CRC

(GMOD.3 = 1), operation can only be half duplex. If using a 16-bit CRC (GMOD.3 = 0), full

duplex is selected by default. When using a 16-bit CRC the receiver can be turned off while trans-

mitting (RSTAT.1 = 0), and the transmitter can be turned off during reception (TSTAT.1 = 0). This

simulates half-duplex operation when using a 16-bit CRC.

Normally, HDLC uses a 16-bit CRC, so half duplex is determined by turning off the receiver or

transmitter. This is so that the receiver will not detect its own address as transmission takes place.

This also needs to be done when using CSMA/CD with a 16-bit CRC for the same reason.

3.6.2 CPU/DMA CONTROL OF THE GSC

The data for transmission or reception can be handled by either the CPU (TSTAT.0 = 0) or DMA

controller (TSTAT.0 = 1). This allows the user two sets of ﬂags to control the FIFO. Associated

with these ﬂags are interrupts, which may be enabled by the user software. Either one or both sets

of ﬂags may be used at the same time.

In CPU control mode the ﬂags (RFNE, TFNF) are generated by the condition of the receive or

transmit FIFO’s. After loading a byte into the transmit FIFO, there is a one machine cycle latency

until the TFNF ﬂag is updated. Because of this latency, the status of TFNF should not be checked

immediately following the instruction to load the transmit FIFO. If using the interrupts to service

the transmit FIFO, the one machine cycle of latency must be considered if the TFNF ﬂag is

checked prior to leaving the subroutine.

When using the CPU for control, transmission normally is initiated by setting TEN bit (TSTAT.1)

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and then writing to TFIFO. TEN must be set before loading the transmit FIFO, as setting TEN

clears the transmit FIFO. TCDCNT should also be checked by user software and cleared if a col-

lision occurred on a prior transmission.

To enable the receiver, GREN (RSTAT.1) is set. After GREN is set, the GSC begins to look for a

valid BOF. After detecting a valid BOF the GSC attempts to match the received address byte(s)

against the address match registers. When a match occurs the frame is loaded into the GSC. Due

to the CRC strip hardware, there is a 40 or 24 bit time delay following the BOF until the ﬁrst data

byte is loaded into RFIFO if the 32 or 16 bit CRC is chosen. If the end of frame is detected before

data is loaded into the receive FIFO, the receiver ignores that frame.

If the receiver detects a collision during reception in CSMA/CD mode and if any bytes have been

loaded into the receive FIFO, the RCABT ﬂag is set. The GSC hardware then halts reception and

resets GREN. The user software needs to ﬁlter any collision fragment data which may have been

received. If the collision occurred prior to the data being loaded into RFIFO the CPU is not noti-

ﬁed and the receiver is left enabled. At the end of a reception the RDN bit is set and GREN is

cleared. In HABEN mode this causes an acknowledgment to be transmitted if the frame did not

have a broadcast or multi-cast address. The user software can enable the interrupt for RDN to

determine when a frame is completed.

In DMA mode the interrupts are generated by the internal “transmit/receive done” (TDN,RDN)

conditions. When the CPU responds to TDN or RDN, checks are performed to see if the transmit

underrun error has occurred. The underrun condition is only checked when using the DMA chan-

nels.

Upon power up the CPU mode is initialized. General DMA control is covered in Section 4.0

DMA control of the GSC is covered in Section 3.5.4. If DMA is to be used for serving the GSC, it

must be conﬁgured into the serial channel demand mode and the DMA bit in TSTAT has to be set.

3.6.3 COLLISIONS AND BACKOFF

The actions that are taken by the GSC if a collision occurs while transmitting depend on where the

collision occurs. If a collision occurs in CSMA/CD mode following the preamble and BOF ﬂag,

the TCDT ﬂag is set and the transmit hardware completes a jam. When this type of collision

occurs, there will be no automatic retry at transmission. After the jam, control is returned to the

CPU and user software must then initiate whatever actions are necessary for a proper recovery.

The possibility that data might have been loaded into or from the GSC deserves special consider-

ation. If these fragments of a message have been passed on to other devices, user software may

have to perform some extensive error handling or notiﬁcation. Before starting a new message, the

transmit and receive FIFOs will need to be cleared. If DMA servicing is being used the pointers

must also be reinitialized. It should be noted that a collision should never occur after the BOF ﬂag

in a well designed system, since the system slot time will likely be less than the preamble length.

The occurrence of such a situation is normally due to a station on the link that is not adhering to

proper CSMA/CD protocol or is not using the same timing s as the rest of the network.

A collision occurring during the preamble or BOF ﬂag is the normal type of collision that is

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expected. When this type of collision occurs the GSC automatically handles the retransmission

attempts for as many as eight tries. If on the eighth attempt a collision occurs, the transmitter is

disabled, although the jam and back off are performed. If enabled, the CPU is then interrupted.

The user software should then determine what action to take. The possibilities range from just

reporting the error and aborting transmission to reinitializing the serial channel registers and

attempt retransmission.

If less than eight attempts are desired TCDCNT can be loaded with some value which will reduce

the number of collisions possible before TCDCNT overﬂows. The value loaded should consist of

all 1s as the least signiﬁcant bits, e.g. 7, 0FH, 3FH. A solid block of 1s is suggested because TCD-

CNT is used as a mask when generating the random slot number assignment. The TCDCNT reg-

ister operates by shifting the contents one bit position to the left as each collision is detected. As

each shift occurs a 1 is loaded into the LSB. When TCDCNT overﬂows, GSC operation stops and

the CPU is notiﬁed by the setting of the TCDT bit which can ﬂag an interrupt.

The amount of time that the GSC has before it must be ready to retransmit after a collision is

determined by the mode which is selected. The mode is determined M0 (GMOD.5) and M1

(GMOD.6). If M0 and M1 equal 0,0 (normal backoff) then the minimum period before retrans-

mission will be either the interframe space or the backoff period, whichever is longer. If M0 and

M1 equal 1,1 (alternate backoff) then the minimum period before retransmission will be the inter-

frame space plus the backoff period. Both of these are shown in table below. Alternate backoff

must be enabled if using deterministic resolution. If the GSC is not ready to retransmit by the time

its assigned slot becomes available, the slot time is lost and the station must wait until the colli-

sion resolution time period has passed.

Table 13:

What the GSC was doing

Response

nothing

None, Unless DCR =1.

If DCR = 1, begin DCR countdown

Receiving a Frame, ﬁrst byte not in RFIFO yet.

Receiving a Frame, ﬁrst byte already in RFIFO.

Transmitting a Frame, ﬁrst byte still in TFIFO

None, unless DCR = 1.

If DCR = 1, begin DCR countdown.

Set RCABT, clear GREN.

If DCR = 1, begin DCR countdown.

Execute jam/backoff.

Restart if collision count< 8.

Transmitting a Frame, ﬁrst byte already taken from

TFIFO

Execute jam/backoff.

Set TCDT, clear TEN.

Instead of waiting for the collision resolution to pass, the transmission could be aborted. The deci-

sion to abort is usually dependent on the number of stations on the link and how many collisions

have already occurred. The number of collisions can be obtained by examining the register, TCD-

CNT. The abort is normally implemented by clearing TEN. The new transmission begins by set-

ting TEN and loading TFIFO. The minimum amount of time available to initiate a retransmission

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would be one interframe space period after the line is sensed as being idle.

As the number of stations approach 256 the probability of a successful transmission decreases

rapidly. If there are more than 256 stations involved in the collision there would be no resolution

since at least two of the stations will always have the same backoff interval selected.

All the stations monitor the link as long as that station is active, even if not attempting to transmit.

This is to ensure that each station always defers the minimum amount of time before attempting a

transmission and so that addresses are recognized. However, the collision detect circuitry operates

slightly differently.

In normal back-off mode, a transmitting station always monitors the link while transmitting. If a

collision is detected one or more of the transmitting stations apply the jam signal and all transmit-

ting stations enter the back-off algorithm. The receiving stations also constantly monitor for a col-

lision but do not take part in the resolution phase. This allows a station to try to transmit in the

middle of a resolution period. This in turn may or may not cause another collision. If the new sta-

tion trying to transmit on the link does so during an unused slot time then there will probably not

be a collision. If trying to transmit during a used slot time, then there will probably be a collision.

The actions the receiver does take when detecting a collision is to just stop receiving data if data

has not been loaded into RFIFO or to stop reception, clear receiver enable (REN) and set the

receiver abort ﬂag (RCABT - RSTAT.6).

If deterministic resolution is used, the transmitting stations go through pretty much the same pro-

cess as in normal back-off, except that the slots are predetermined. All the receivers go through

the back-off algorithm and may only transmit during their assigned slot.

3.6.4 SUCCESSFUL ENDING OF TRANSMISSIONS AND RECEPTIONS

In both CSMA/CD and SDLC modes, the TDN bit is set and TEN cleared at the end of a success-

ful transmission. The end of the transmission occurs when the TFIFO is empty and the last byte

has been transmitted. In CSMA/CD the user should clear the TCDCNT register after successful

transmission.

At the end of a successful reception, the RDN bit is set and GREN is cleared. The end of reception

occurs when the EOF ﬂag is detected by the GSC hardware.

3.7 GSC Register Descriptions

ADR0,1,2,3 (95H, 0A5H, 0B5H, 0C5H) - Address Match Registers 0,1,2,3 - Contains the address

match values which determines which data will be accepted as valid. In 8 bit addressing mode, a

match with any of the four registers will trigger acceptance. In 16 bit addressing mode a match

with ADR1:ADR0 or ADR3:ADR2 will be accepted. Addressing mode is determined in

GMOD(AL).

AMSK0,1 (0D5H, 0E5H) - Address Mask 0,1 - Identiﬁes which bits in ADR0,1 are “don’t care”

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bits. Writing a one to a bit in AMSK0,1 masks out that corresponding bit in ADDR0,1.

BAUD (94H) -GSC Baud Rate Generator - Contains the value of the programmable baud rate.

The data rate will equal (frequency of the oscillator)/((BAUD +1) x (8)). Writing to BAUD actu-

ally stores the value in a reload register. The reload register contents are copied into the BAUD

A read during GSC operation will give a value that may not be current because the timer could

decrement between the time it is read by the CPU and by the time the value is loaded into its des-

tination.

BKOFF(OC4H) - Backoff Timer -The backoff timer is an eight bit count-down timer with a clock

period equal to one slot time. The backoff time is used in the CSMA/CD collision resolution algo-

rithm. The user software may read the timer but the value may be invalid as the timer is clocked

asynchronously to the CPU. Writing to 0C4H will have no effect.

GMOD(84H)

PL1

PL0

XTCLK

GMOD.0 (PR) - Protocol - If set, SDLC protocols with NRZI encoding and SDLC ﬂags are used.

If cleared, CSMA/CD link access with Manchester encoding is used. The user software is respon-

sible for setting or clearing this ﬂag.

GMOD.1,2 (PL0,1) - Preamble length

PL1

PL0

LENGTH (BITS)

The length includes the two bit Begin Of Frame (BOF) ﬂag in CSMA/CD but does not include the

SDLC ﬂag. In SDLC mode, the BOF is an SDLC ﬂag, otherwise it is two consecutive ones. Zero

length is not compatible in CSMA/CD mode. The user software is responsible for setting or clear-

ing these bits.

GMOD.3 (CT)-CRC Type - If set, 32 bit AUTODINII -32 is used. If cleared, 16 bit CRC-CCITT

is used. The user software is responsible for setting or clearing this ﬂag.

GMOD.4 (AL) - Address Length - If set, 16 bit addressing is used. In 8 bit mode a match with any

of the 4 address registers will be accepted (ADR0, ADR1, ADR2, ADR3). “Don’t Care” bits may

be masked in ADR0 and ADR1 with AMSK0 and AMSK1. In 16 bit mode, addresses are

matched against “ADR1:ADR0” or “ADR3: ADR2”. Again, “Don’t Care” bits in ADR1:ADR0

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can be masked in AMSK1:AMSK0. A received address of all ones will always be recognized in

any mode. The user software is responsible for setting or clearing this ﬂag.

GMOD.5,6 (M0,M1) - Mode Select - Two test modes, an optional “alternate backoff” mode, or

normal back-off can be enabled with these two bits. The user software is responsible for setting or

clearing the mode bits.

Mode

Normal

Raw Transmit

Raw Receive

Alternate Backoff

In raw receive mode, the receiver operates as normal except that all the bytes following the BOF

are loaded into the receive FIFO, including the CRC. The transmitter operates as normal.

In raw transmit mode the transmit output is internally connected to the receiver input. The internal

connection is not at the actual port pin, but inside the port latch. All data transmitted is done with-

out a preamble, ﬂag or zero bit insertion, and without appending a CRC. The receiver operates as

normal. Zero bit deletion is performed.

In alternate backoff mode the standard backoff process is modiﬁed so the backoff is delayed until

the end of the IFS. This should help to prevent collisions constantly happening because the IFS

time is usually larger than the slot time.

GMOD.7 (XTCLK) - External Transmit Clock -If set an external 1X clock is used for the trans-

mitter. If cleared the internal baud rate generator provides the transmit clock. The input clock is

applied to P1.3 (T X C). The user software is responsible for setting or clearing this ﬂag. External

receive clock is enabled by setting PCON.3

IFS (0A4H) - Interframe Spacing - Determines the number of bit times separating transmitted

frames in CSMA/CD and SDLC.A bit time is equal to 1/baud rate. Only even interframe space

periods can be used. The number written into this register is divided by two and loaded in the

most signiﬁcant seven bits. Complete interframe space is obtained by counting this seven bit num-

ber down to zero twice. A user software read of this register will give a value where the seven

most signiﬁcant bit shows a one for the ﬁrst count-down and a zero for the second count. The

value read may not be valid as the timer is clocked in periods not necessarily associated with the

CPU read of IFS. Loading this register with zero results in 256 bit times.

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MYSLOT (0F5H) - Slot Address Register

SA2

DCR

SA3

SA1 SA0

SA5

DCJ

SA4

SAn = SLOT ADDRESS (BITS 5 - 0)

MYSLOT.0, 1,2,3,4,5 - Slot Address -The six address bits choose 1 of 64 slot addresses. Address

63 has the highest priority and address 1 has lowest. A value of zero will prevent a station from

transmitting during the collision resolution period by waiting until all the possible slot times have

elapsed. The user software normally initializes this address in the operating software.

MYSLOT.6 (DCR) - Deterministic Collision Resolution Algorithm - When set, the alternate colli-

sion resolution algorithm is selected. Retriggering of the IFS on reappearance of the carrier is also

disabled. When using this feature Alternate Backoff Mode must be selected and several other reg-

isters must be initialized. User software must initialize TCDCNT with the maximum number of

slots that are most appropriate for a particular application. The PRBS register must be set to all

ones. The disables the PRBS by freezing it’s contents at 0FFH. The backoff timer is used to count

down the number of slots based on the slot timer value setting the period of one slot. The user

software is responsible for setting or clearing this ﬂag.

MYSLOT.7 (DCJ) - D.C.Jam - When set selects D.C. type jam, when clear, selects A.C. type jam.

The user software is responsible for setting or clearing this ﬂag.

PCON (087H)

REQ

SMOD ARB

GAREN

XRCLK GFIEN

IDL

PCON contains bits for power control, LSC control, DMA control, and GSC control. The bit used

for the GSC are PCON.2, PCON.3, and PCON.4.

PCON.2 (GFIEN) - GSC Flag Idle Enable -Setting GFIEN to a 1 caused idle ﬂags to be generated

between transmitted frames in SDLC mode. SDLC idle ﬂags consists of 01111110 ﬂags creating

the sequence 01111110011111110..........011111110. A possible side effect of enabling GFIEN is

that the maximum possible latency from writing to TFIFO until the ﬁrst bit is transmitted

increased from approximately 2 bit-times to around 8 bit-times. GFIEN has not effect with

CSMA/CD.

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PCON.3(XRCLK) -GSC External Receive Clock Enable- Writing a 1 to XRCLK enables an

external clock to be applied to pin 5(Port 1.4). The external clock is used to determine when bits

are loaded into the receiver.

PCON.4 (GAREN) - GSC Auxiliary Receiver Enable Bit - This bit needs to be set to a 1 to enable

the reception of back-to-back SDLC frames. A back-to-back SDLC frame is when the EOF and

BOF is shared between two sequential frames intended for the same station on the link. If

GAREN contains a 0 then the receiver will be disabled upon reception of the EOF and by the time

user software re-enables the receiver the ﬁrst bit(s) may have already passed, in the case of back-

to-back frames. Setting GAREN to a 1, prevents the receiver from being disabled by the EOF but

GREN will be cleared and can be checked by user software to determine that an EOF has been

received. GAREN has no effect if the GSC is in CSMA/CD mode.

PRBS (0E4H) - Pseudo-Random Binary Sequence - This register contains a pseudo-random num-

ber to be used in the CSMA/CD backoff algorithm. The number is generated by using a feedback

shift register clocked by the CPU phase clocks. Writing all ones to the PRBS will freeze the value

at all ones. Writing any other value to it will restart the PRBS generator.The PRBS is initialized to

all zero’s during RESET. A read of location 0E4H will not necessary give the seed used in the

backoff algorithm because the PRBS may have been altered between the time when the seed was

generated and before a READ has been internally executed.

RFIFO (0F4H) - Receive FIFO - RFIFO is a 3 byte buffer that is loaded each time the GSC

receiver has a byte of data. Associated with RFIFO is a pointer that is automatically updated with

each read of the FIFO. A read of RFIFO fetches the oldest data in the FIFO.

RSTAT (0E8H) - Receive Status Register

CRCE

RDN

RFNE GREN HABEN

RCABT

RSTAT.0 (HABEN) - Hardware Based Acknowledge Enable - If set, enables the hardware based

acknowledge feature. The user software is responsible for setting or clearing this ﬂag.

RSTAT.1 (GREN) - Receiver Enable -When set, the receiver is enabled to accept incoming

frames. The user must clear RFIFO with software before enabling the receiver. RFIFO is cleared

by reading the contents of RFIFO until RFIFO=0. After each read of RFIFO, it takes one machine

cycle for the status of RFNE to be updated. Setting GREN is cleared by hardware at the end of a

reception or if any receive errors are detected. The user software is responsible for setting this ﬂag

and the GSC or user software can clear it. The status of GREN has no effect on whether the

receiver input circuitry always monitors the receive pin.

RSTAT.2 (RFNE) - Receive FIFO Not Empty - If set, indicates that the receive FIFO contains

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data. The receive FIFO is a three byte buffer into which the receive data is loaded. A CPU read of

the FIFO retrieves the oldest data and automatically updates the FIFO pointers. Setting GREN to

a one will clear the receive FIFO. The status of this ﬂag is controlled by the GSC. It is cleared if

user empties receive FIFO.

RSTAT.3 (RDN) - Receive Done - If set, indicates the successful completion of a receiver opera-

tion. Will not be set if a CRC, alignment, abort, or FIFO overrun error occurred. The status of this

ﬂag is controlled by the GSC.

RSTAT.4 (CRCE) - CRC Error - If set, indicates that a properly aligned frame was received with a

mismatched CRC. The status of this ﬂag is controlled by the GSC.

RSTAT.5 (AE) - Alignment Error - In CSMA/CD mode, AE is set if the receiver shift register (an

internal serial-to-parallel converter) is not full and the CRC is bad when an EOF is detected. In

CSMA/CD the EOF is a line idle condition (see LNI) for two bit times. If the CRC is correct

while in CSMA/CD mode, AE is not set and any mis-alignment is assumed to be caused by drib-

ble bits as the line went idle. In SDLC mode, AE is set if a non-byte-aligned ﬂag is received.

CRCE may also be set. The setting of this ﬂag is controlled by the GSC.

RSTAT.5 (AE) - Alignment Error -In CSMA/CD mode, AE is set if the receiver shift register (an

internal serial-to-parallel converter) is not full and the CRC is bad when an EOF is detected. In

CSMA/CD the EOF is a line idle condition (see LNI) for two bit times. If the CRC is correct

while in CSMA/CD mode, AE is not set and any mis-alignment is assumed to be caused by drib-

ble bits as the line went idle. In SDLC mode, AE is set if a non-byte-aligned ﬂag is received.

CRCE may also be set. The setting of this ﬂag is controlled by the GSC.

RSTAT.6 (RCABT) - Receiver Collision/Abort Detect - If set, indicates that a collision was

detected after data had been loaded into the receive FIFO in CSMA/CD mode. In SDLC mode,

RCABT indicates that 7 consecutive ones were detected prior to the end ﬂag but after data has

been loaded into the receive FIFO. AE may also be set. The setting of this ﬂag is controlled by the

GSC.

RSTAT.7 (OVR) - Overrun -If set, indicates that the receive FIFO was full and new shift register

data was written into it. AE and/or CRCE may also be set. The setting of this ﬂag is controlled by

the GSC and it is cleared by user software.

SLOTTM (0BH) - Slot Time -Determines the length of the slot time used in CSMA/CD. A slot

time equals (SLOTTM) X (1/ baud rate). A read of SLOTTM will give the value of the slot time

timer but the value may be invalid as the timer is clocked asynchronously to the CPU. Loading

SLOTTM with 0 results in 256 bit times.

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TCDCNT (0D4H) - Transmit Collision Detect Count Contains the number of collisions that have

occurred if probabilistic CSMA/CD is used. The user software must clear this register before

transmitting a new frame so that the GSC backoff hardware can accurately distinguish a new

frame from a retransmit attempt.

In deterministic backoff mode, TCDCNT is used to hold the maximum number of slots.

TFIFO (85H) - GSC Transmit FIFO - TFIFO is a 3 byte buffer with an associated pointer that is

automatically updated for each write by user software. Writing a byte to TFIFO loads the data into

the next available location in the transmit FIFO. Setting TEN clears the transmit FIFO so the

transmit FIFO should not be written to prior to setting TEN. If TEN is already set transmission

begins as soon as data is written to TFIFO.

LNI

TCDT

TDN

TEN

NOACK

TFNF

DMA

TSTAT (0D8) - Transmit Status Register

TSTAT.0 (DMA) - DMA Select - if set, indicates that DMA channels are used to service the GSC

FIFO’s and GSC interrupts occur on TDN and RDN, and also enables UR to become set. If

cleared, indicates that the GSC is operating in its normal mode and interrupts occur on TFNF and

RFNE. For more information on DMA servicing please refer to the DMA section on DMA serial

demand mode (4.2.2.3). The user software is responsible for setting or clearing this ﬂag.

TSTAT.1 (TEN) - Transmit Enable - When set causes TDN, UR, TCDT, and NOACK ﬂag to be

reset and the TFIFO cleared. The transmitter will clear TEN after a successful transmission, a col-

lision during the data, CRC, or end ﬂag. The user software is responsible for setting but the GSC

or user software is responsible for setting but the GSC or user software may clear this ﬂag. If

cleared during a transmission the GSC transmit pin goes to a steady state high level. This is the

method used to send an abort character in SDLC. Also DEN is forced to a high level. The end of

transmission occurs whenever the TFIFO is emptied.

TSTAT.2 (TFNF) - Transmit FIFO not full - When set, indicates that new data may be written into

the transmit FIFO. The transmit FIFO is a three byte buffer that loads the transmit shift register

with data. The status of this ﬂag is controlled by the GSC.

TSTAT.3 (TDN) - Transmit Done -When set, indicates the successful completion of a frame trans-

mission. If HABEN is set, TDN will not be set until the end of the IFS following the transmitted

message, so that the acknowledge can be checked. If an acknowledge is expected and not

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received, TDN is not set. An acknowledge is not expected following a broadcast or multi-cast

packet. The status of this ﬂag is controlled by the GSC.

TSTAT.4(TCDT) - Transmit Collision Detect - If set, indicates that the transmitter halted due to a

collision. It is set if a collision occurs during the data or CRC or if there are more than eight colli-

sions. The status of this ﬂag is controlled by the GSC.

TSTAT.5 (UR) - Underrun - If set, indicates that in DMA mode the last bit was shifted out of the

transmit register and that the DMA byte count did not equal zero. When an underrun occurs, the

transmitter halts without sending the CRC or the end ﬂag. The status of this ﬂag is controlled by

the GSC.

TSTAT.6 (NOACK) - No Acknowledge - If set, indicates that no acknowledge was received for

the previous frame. Will be set only if HABEN is set and no acknowledge is received prior to the

end of the IFS. NOACK is not set following a broadcast or a multicast packet. The status of this

ﬂag is controlled by the GSC.

TSTAT.7(LNI) - Line Idle - If set, indicates the receive line is idle. In SDLC protocol it is set if 15

consecutive ones are received. In CSMA/CD protocol, line idle is set if GR x D remains high for

approximately 1.6 bit times. LNI is cleared after a transition on GR X D. The status of this ﬂag is

controlled by the GSC.

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4.0 DMA Operation

The C152 contains DMA (Direct Memory Accessing) logic to perform high speed data transfers

between any two of

Internal Data RAM

Internal SFRs

External Data RAM

In external RAM is involved, the Port 2 and Port 0 pins are used as the address/data bus, and RD

and WR signals are generated as required.

Hardware is also implemented to generate a Hold Request signal and await a Hold Acknowledge

response before commencing a DMA that involves external RAM.

Alternatively, The Hold/Hold Acknowledge hardware can be programed to accept a Hold Request

signal from an external device and generate a Hold Acknowledge signal in response, to indicate to

the requesting device that the C152 will not commence a DMA to or from external RAM while

the Hold Request is active.

4.1 DMA with the 80C152

The C152 contains two identical general purpose 8-it DMA channels with 16-bit address ability:

DMA0 and DAM1. DMA transfers can be executed by either channel independent of the other,

but only by one channel at a time. During the time that a DMA transfer is being executed, pro-

gram execution is suspended. A DMA transfer takes one machine cycle (12 oscillator periods) per

byte transferred, except when the destination and source are both in External Data RAM. In that

case the transfer takes two machine cycles per byte. The term DMA Cycle will be used to mean

the transfer of a single data byte, whether it takes 1 or 2 machine cycles.

Associated with each channel are seven SFRs, shown below SARLn and SARHn holds the low

and high bytes of the source address. Taken together they form a 16-bit Source Address Register.

DARLn and DARHn hold the low and high bytes of the destination address, and together form the

Destination Address Register. BCRLn and BCRHn hold the low and high bytes of the number of

bytes to be transferred, and together form the Byte Count Register. DCONn contains control and

ﬂag bits.

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DMA CHANNEL 1

DMA CHANNEL 0

DARLO

DARHO

DARH1

DARL1

DESTINATION ADDRESS

SARHO

SARLO

SARH1

SARL1

SOURCE ADDRESS

BCRH1

BCRHO

BCRL1

BYTE COUNT

BCRLO

BYTE COUNT

DCON1

DCONO

DMA1 CONTROL

DMAO CONTROL

PCON

Two new bits in PCON control

Hold/Hold Acknowledge logic

DMA Registers

Two bits in DCONn are used to specify the physical destination of the data transfer. These bits are

DAS (Destination Address Space) and IDA (Increment Destination Address). If DAS = 0, the des-

tination is in data memory external to the C152. If DAS = 1, the destination is internal to the

C152. If DAS = 1 and IDA = 0, the internal destination is a Special Function Register (SFR). IF

DAS = 1 and IDA =1, the internal destination is in the 256-byte data RAM.

In any case, if IDA = 1, the destination address is automatically incremented after each byte trans-

fer. If IDA = 0, it is not.

Two other bits in DCONs specify the physical source of the data to be transferred. These are SAS

(Source Address Space) and ISA (Increment Source Address). If SAS = 0, the source is in data

memory external to the C152. If SAS = 1, the source is internal. If SAS = 1 and ISA = 0, the inter-

nal source is an SFR. If SAS = 1 and ISA = 1, the internal source is in the 256-byte data RAM.

In any case, if ISA = 1, the source address is automatically incremented after each byte transfer. If

ISA = 0, it is not.

The functions of these four control bits are summarized below:

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DAS

IDA

Destination

Auto-Increment

External RAM

SFR

yes

Internal RAM

yes

SAS

ISA

Source

Auto-Increment

External RAM

SFR

yes

Internal RAM

yes

There are four modes in which the DMA channel can operate. These are selected by the bits DM

and TM (Demand Mode and Transfer Mode) in DCONn:

Operating Mode

Alternate Cycles Mode

Burst Mode

Serial Port Demand Mode

External Demand Mode

The operating modes are described below.

4.1.1 ALTERNATE CYCLE MODE

In Alternate Cycles Mode the DMA is initiated by setting the GO bit in DCONn. Following the

instruction that set the GO bit, one more instruction is executed, and then the ﬁrst data byte is

transferred from the source address to the destination address. Then another instruction is exe-

cuted, and then another byte of data is transferred, and so on in this manner.

Each time a data byte is transferred, BCRn (Byte Count Register for DMA Channel n) is decre-

mented. When it reaches 0000H, on-chip hardware clears the GO bit and sets the DONE bit, and

the DMA ceases. The DONE bit ﬂags an interrupt.

4.1.2 BURST MODE

Burst Mode differs from Alternate Cycles mode only in that once the data transfer has begun, pro-

gram execution is entirely suspended until BCRn reaches 0000H, indicating that all data bytes

that were to be transferred have been transferred. The interrupt control hardware remains active

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during the DMA, so interrupt ﬂags may get set, but since program execution is suspended, the

interrupts will not be serviced while the DMA is in progress.

4.1.3 SERIAL PORT DEMAND MODE

In this mode the DMA can be used to service the Local Serial Channel (LSC) or the Global Serial

Channel (GSC).

In Serial Port Demand Mode the DMA is initiated by any of the following conditions, if the GO

bit is set:

Source Address = SBUF

.AND. RI = 1

Destination Address = SBUF

Source Address = RFIFO

Destination Address = TFIFO

.AND. TI = 1

.AND. RFNE = 1

.AND. TFNF = 1

Each time one of the above conditions is met, one DMA Cycle is executed; that is, one data byte is

transferred from the source address to the destination address. On-chip hardware then clears the

ﬂag (RI, TI, RFNE, or TFNF) that initiated the DMA, and decrements BCRn. Note that since the

ﬂag that initiated the DMA is cleared, it will not generate an interrupt unless DMA servicing may

be held off when alternate cycle is being used or by the status of the HOLD/HLDA logic. In these

situations the interrupt for the LSC may occur before the DMA can clear the RI or TI ﬂag. This is

because the LSC is serviced according to the status of RI and TI, whether or not the DMA chan-

nels are being used for the transferring of data. The GSC does not use RFNE or TFNF ﬂags when

using the DMA channels so these do not need to be disabled. When using the DMA channels to

service the LSC it is recommended that the interrupts (RI and TI) be disabled. If the decremented

BCRn is 0000H, on-chip hardware then clears the GO bit and sets the DONE bit. The DONE bit

ﬂags an interrupt.

4.1.4 EXTERNAL DEMAND MODE

In External Demand Mode the DMA is initiated by one of the External Interrupt pins, provided

the GO bit is set. INT0 initiates a Channel 0 DMA, and INT1 initiates a Channel 1 DMA.

If the external interrupt is conﬁgured to be transition activated, then each 1-to-0 transition at the

interrupt pin sets the corresponding external interrupt ﬂag, and generates one DMA Cycle. Then,

BCRn is decremented. No more DMA Cycles take place until another 1-to-0 transition is seen at

the external interrupt pin. IF THE DECREMENTED bcrN = 0000H, on-chip hardware clears the

GO bit and sets the DONE bit. If the external interrupt is enabled, it will be serviced.

If the external interrupt is conﬁgured to be level-activated, then DMA Cycles commence when the

interrupt pin is pulled low, and continue for as long as the pin is held low and BCRn is not 0000H.

If BCRn reaches 0 while the interrupt pin is still low, the GO bit is cleared, the DONE bit is set,

and the DMA ceases. If the external interrupt is enabled, it will be serviced.

If the interrupt pin is pulled up before BCRn reaches 0000H, then the DMA ceases, but the GO bit

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is still 1 and the DONE bit is still 0. An external interrupt is not generated in this case, since in

level-activated mode, pulling the pin to a logical 1 clears the interrupt ﬂag. If the interrupt pin is

then pulled low again, DMA transfers will continue from where they were previously stopped.

The timing for the DMA Cycle in the transition - activated mode, or for the ﬁrst DMA Cycle in

the level-activated mode is as follows: If the 1-to-0 transition is detected before the ﬁnal machine

cycle of the instruction in progress, then the DMA commences as soon as the instruction in

progress is completed. Otherwise, one more instruction will be executed before the DMA starts.

No instruction is executed during any DMA Cycle.

4.2 Timing Diagrams

Timing diagrams for single-byte DMA transfers are shown in following ﬁgures for four kinds of

DMA Cycles: internal memory to internal memory, internal memory to external memory, external

memory to internal memory, and external memory to external memory. In each case we assume

the C152 is executing out of external program memory. If the C152 is executing out of internal

program memory, the PSEN is inactive, and the Port 0 and Port 2 pins emit P0 and P2 SFR data. If

External Data Memory is involved, the Port 0 and Port 2 pins are used as the address/data bus, and

RD and /or WR signals are generated as needed, in the same manner as in the execution of a

MOVX @ DPTR instruction.

12 OSC.PERIODS

ALE

PSEN

P1 INST

FLOAT

P2 SFR

PCL FLOAT INST

P2 PCH

PCH

DMA CYCLE

RESUME PROGRAM

EXECUTION

DMA Transfer from Internal Memory to Internal Memory

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DMA Transfer from Internal Memory to External Memory

12 OSC. PERIODS

ALE

PSEN

PCL INST

PCH

DMA DATA OUT

DARHn

P 0 INST

P 2

PCH

RESUME PROGRAM

EXECUTION

DMA CYCLE

12 OSC. PERIODS

ALE

PSEN

FLOAT

DATA IN

PCL

FLOAT

SARHn

INST

PO INST

SARLn

PCH

RESUME PROGRAM

EXECUTION

DMA CYCLE

DMA Transfer from External Memory to Internal Memory

DMA Transfer from External Memory to External Memory

12 OSC.PERIODS

12 OSC. PERIODS

ALE

PSEN

DATA IN

FLOAT

DATA OUT

P 0 INST

DARLn

FLOAT

SARln

PCL INST

PCH

P 2

PCH

SARHn

DARHn

DMA CYCLE

RESUME PROGRAM

EXECUTION

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4.3 Hold/Hold Acknowledge

Two operating modes of Hold/Hold Acknowledge logic are available, and either or neither may be

invoked by software. In one mode, the C152 generates a Hold Request signal and awaits a Hold

Acknowledge response before commencing a DMA that involves external RAM. This is called

the Requester Mode.

In the other mode, the C152 accepts a Hold Request signal from an external device and generates

a Hold Acknowledge signal in response, to indicate to the requesting device that the C152 will not

commence a DMA to or from external RAM while the Hold Request is active. This is called the

Arbiter mode.

4.3.1 REQUEST MODE

The Requester Mode is selected by setting the control bit REQ, which resides in PCON. In that

mode, when the C152 wants to do a DMA to External Data Memory, it ﬁrst generates a Hold

Request signal, HLD, and waits for a Hold Acknowledge signal, HLDA, before commencing the

DMA operation. Note that program execution continues while HLDA is awaited. The DMA is not

begun until a logical 0 is detected at the HLDA pin. Then, once the DMA has begun, it goes to

completion regardless of the logic level at HLDA.

The protocol is activated only for DMA (not for program fetches or MOVX operations), and only

for DMAs to or from External Data Memory. If the data destination and source are both internal

to the C152, the HLD/HLDA protocol is not used.

The HLD output is an alternate function of port pin P1.5, and the HLDA input is an alternate

function of port pin P1.6

4.3.2 ARBITER MODE

For DMAs that are to be driven by some device other than the C152, a different version of the

Hold/Hold Acknowledge protocol is available. In this version, the device which is to drive the

DMA sends a Hold Request signal, HLD, to the C152. I f the C152 is currently performing a

DMA to or from External Data Memory, it will complete this DMA before responding to the Hold

Request. When the C152 responds to the Hold Request, it does so by activating a Hold Acknowl-

edge signal, HLDA. This indicates that the C152 will not commence a new DMA to or from

External Data Memory while HLD remains active.

Note that in the Arbiter Mode the C152 does not suspend program execution at all, even if it is

executing from external program memory. It does not surrender use of its own bus.

The Hold Request input, HLD, is at P1.5. The Hold Acknowledge output, HLDA, is at P1.6. This

version of the Hold/Hold Acknowledge feature is selected by setting the control bit ARB in

PCON.

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The functions of the ARB and REQ bits in PCON, then, are

ARB

REQ

Hold/Hold Acknowledge Logic

Disabled

C152 generates HLD, detects HLDA

C152 detects HLD, generates HLDA

Invalid

4.3.3 USING THE HOLD/HLDA ACKNOWLEDGE

The HOLD/HOLDA logic only affects DMA operation with external RAM and don’t affect other

operations with external RAM, such as MOVX instruction.

Figure shows a system in which two 83C152s are sharing a global RAM. In this system, both

CPUs are executing from internal ROM. Neither CPU uses the bus except to access the shared

RAM, and such accesses are done only through DMA operations, not by MOVX instructions.

Two 83C152s Sharing External RAM

8 X 10 kohms

83C152

ARB

ALE

HLDA

HLD

ALE SWITCH

HLD

HLDA

ALE

83C152

REQ

DATA

LOW HIGH

SHARED RAM

ADDR

One CPU is programmed to be the Arbiter and the other, to be the Requester. The ALE Switch

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selects which CPU’s ALE signal will be directed to the address latch. The Arbiter’s ALE is

selected if HLDA is high, and the Requester’s ALE is selected if HLDA is low.

The ALE Switch logic can be implemented as shown in below.

ALE

(ARB)

ALE (ARB)

IF HLDA = 1

HLDA

ALE (AEQ)

IF HLDA = 0

ALE

(REQ)

4.3.4 INTERNAL LOGIC OF THE ARBITER

The internal logic of the arbiter is shown in ﬁgure below. In operation an input low at HLD sets

Q2 if the arbiter’s internal signal DMXRQ is low. DMXRQ is the arbiter’s “DMA to XRAM

Request”. SettingQ2 activates HLDA through Q3. Q2 being set also disables any DMAs to

XRAM that the arbiter might decide to do during the requester’s DMA.

Internal Logic of the Arbiter

Inhibit Arbiter’s

DMA to XRAM

DMXRQ

HLD Input

HLDA

Clock 1

Clock 2

Waveform below shows the minimum response time, 4 to 7 CPU oscillator periods, between a

transition at the HLD input and the response at HLDA.

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HLD Input

CPU Osc.

Periods

Clock 1

Clock 2

HLDA Output

2 Osc.

4 Osc.

Periods

When the arbiter wants to DMA the XRAM, it ﬁrst activates DMXRQ. This signal prevents Q2

from being set if it is not already set.An output low from Q2 enables the arbiter to carry out its

DMA to XRAM, and maintains an output high at HLDA. When the arbiter completes its DMA,

the signal DMXRQ goes to O, which enables Q2 to accept signals from the HLD input again.

4.3.5 Internal Logic of the Requester

The internal logic of the requester is shown below. Initially, the requester’s internal signal

DMXRQ (DMA to XRAM Request) is at 0, so Q2 is set and the HLD output is high. As long as

Q2 to be cleared (but doesn’t clear it), and, if HLDA is high, also activates the HLD output.

Inhibit Requester’s DMA to XRAM

DMXRQ

HLDA

HLD

Clock 2

HLD

(Q3)

Clock 1

Q1A

Clock 2

A 1-to-0 transition from HLDA can now clear Q2, which will enable the requester to commence

its DMA to XRAM. Q2 being low also maintains an output low at HLD. When the DMA is com-

pleted, DMXRQ goes to 0, which sets Q2 and de-activates HLD.

Only DMXRQ going to 0 can set Q2. That means once Q2 gets cleared, enabling the requester’s

DMA to proceed, the arbiter has no way to stop the requester’s DMA in progress. At this point,

de-activating HLDA will have no effect on the requester’s use of the bus. Only the requester itself

can stop the DMA in progress, and when it does, it de-activates both DMXRQ and HLD.

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If the DMA is in alternate cycles mode, then each time DMA cycle is completed DMXRQ goes to

0, thus de-activating HLD. Once HLD has been de-activate, it can’t be re-asserted till after HLDA

has been to go high (through ﬂip-ﬂop Q1A). Thus every time the DMA is suspended to allow an

instruction cycle to proceed, the requester gives up the bus and must renew the request and receive

another acknowledge before another DMA cycle to XRAM can proceed. Obviously in this case,

the “alternate cycles” mode may consist of single DMA cycles separated by any number of

instruction cycles, depending on how long it takes the requester to regain the bus.

A channel 1 DMA in progress will always be overridden by a DMA request of any kind from

channel 0. If a channel 1 DMA to XRAM is in progress and is over-ridden by a channel 0 DMA

which does not require the bus, DMXRQ will go to 0 during the channel 0 DMA, thus de-activat-

ing HLD. Again, the requester must re-new its request for the bus, and must receive a new 1-to-0

transition in HLDA before channel 1 can continue its DMA to XRAM.

4.4 DMA Arbitration

The DMA Arbitration described in this section is not arbitration between two devices wanting to

access a shared RAM, but on-chip arbitration between the two DMA channels on the 8XC152.

The 8XC152 provides two DMA channels, either of which may be called into operation at any

time in response to real time conditions in the application circuit. Since a DMA cycle always uses

the 8XC152’s internal bus, and there’s only one internal bus, only one DMA channel can be ser-

viced during a single DMA cycle. Executing program instructions also requires the internal bus,

so program execution will also be suspended in order for a DMA to take place.

ARBITRA-

TION

LOGIC

INSTRUCTION

CYCLE

DMAO

CYCLE

DMA 1

CYCLE

WRITE

YES

DMA

REG?

Figure above shows the three tasks to which the internal bus of the 8XC152 can be dedicated. In

this ﬁgure, Instruction Cycle means the complete execution of a single instruction, whether it

takes 1,2 or 4 machine cycles. DMA Cycle means the transfer of a single data byte from source to

destination, whether it takes 1 or 2 machine cycles. Each time a DMA Cycle or an Instruction

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Cycle is executed, on-chip arbitration logic determines which type of cycle is to be executed next.

Note that when an instruction is executed, if the instruction wrote to a DMA register (excluding

PCON), then another instruction is executed without further arbitration. Therefore, a single write

or a series of writes to DMA registers will prevent a DMA from taking place, and will continue to

prevent a DMA from taking place until at least one instruction is executed which does not write to

any DMA register.

The logic that determines whether the next cycle will be a DMA0 cycle, a DMA1 cycle, or an

Instruction Cycle is shown as a pseudo-HLL function. The statements are executed sequentially

unless an “if” condition is satisﬁed, in which case the corresponding “return” is executed and the

remainder of the function is not. The return value of 0,1, or 2 is passed to the arbitration logic

block to determine which exit path from the block is used.

arbitration_logic :

if (G00 = 1.AND. mode_logic (0) = 1) return 0;

else if (G01 = 1.AND. mode_logic (1) = 1) return 1;

else return 2;

end arbitration_logic;

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mode_logic (n)

if (DCONn indicates burst_mode) return 1:

else if (DCONn indicates external_demand_mode)

{

if (demand_ﬂag = 1) return 1:

else return 0;

}

else if (DCONn indicates SP_demand_mode)

{

if {SARn = SBUF .AND. RI = 1) return 1;

else if (DARn = SBUF .AND. TI = 1) return 1;

else if (SARn = RFIFO .AND. RFNE = 1) return 1;

else if (DARn = TFIFO .AND. TFNE = 1 .AND.

previous_cycle = instruction _cycle) return 1;

else return 0;

}

else if (DCONn indicates alt_cycles_mode)

{

if (DCONm indicates .NOT. alt _cycles_mode

.OR. G0m = 0)

{

if (previous _cycle = instruction-cycle) return 1;

else return 0;

}

else if (previous_cycle = instruction_cycle

.AND. previous_dma_cyle = .NOT. DMAn)

return 1;

}

return 0;

end mode_logic (n);

If the channel is conﬁgured to External Demand mode, then the ﬁrst if-condition is not satisﬁed

but the second one is. In that case the block of statements following that if-condition and delim-

ited by {...} is executed: if the demand ﬂag (IEO for channel 0 and IE1 for channel 1) is set, the

“return 1” expression is executed and the remainder of the function is not. If the demand ﬂag is

not set, the “return 0” expression is executed and the remainder of the function is not.

If the channel is conﬁgured to Serial Port Demand mode, the source and destination addresses,

SARn and DARn, have to be checked to see which Serial Port buffer is being addressed, and

whether its demand ﬂag is set.

SARn refers to the 16-bit source address for “this channel”. Note that the condition “SARn =

SBUF” cannot be true unless the SAS and IAS bits in DOCNn are conﬁgured to select SFR space.

If SARn is numerically equal to the address of SBUF(99H), and SAS and ISA are conﬁgured to

select internal RAM rather than SFR space, then SARn refers to location 99H in the “upper 128”

of internal RAM, not to SBUF.

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If the test for SARn = SBUF is true, and if the ﬂag RI is set, mode_logic (n) returns as 1 and the

remainder of the function is not executed. Otherwise, execution proceeds to then exit if-condition,

testing DARn against SBUF and T1 against 1.

The same considerations regarding SAS and ISA in the SARn test are now applied to DAS and

IDA in the DARn test. If SFR space isn’t selected, no Serial Port buffer is being addressed.

Note that if DMA channel n is conﬁgured to Alternate Cycles mode, the logic must examine the

other DCON register, DCONm, to determine if the other channel is also conﬁgured to Alternate

Cycles mode and whether its GO bit is set. In the previous ﬁgure, the symbol DCONn refers to the

DCON register for “this channel,” and DCONm refers to “the other channel.”

A careful examination of the logic will reveal some idiosyncrasies that the user should be aware

of. First, the logic allows sequential DMA cycles to be generated to service RFIFO, but not to ser-

vice TFIFO. This idiosyncrasy is due to internal timing conﬂicts, and results in each individual

DMA cycle to TFIFO having to be immediately preceded by an Instruction cycle. The logic disal-

lows that there be two DMAs to TFIFO in a row.

If the user is unaware of this idiosyncrasy, it can cause problems in situations where one DMA

channel is servicing TFIFO and the other is conﬁgured to a completely different mode of opera-

tion.

For example, consider the situation where channel 0 is conﬁgured to service TFIFO and channel 1

is conﬁgured to Alternate Cycles mode. Then DMAs to TFIFO will always override the alternate

cycles of channel 1. If TFIFO needs more than 1 byte it will receive them in precedence over

channel 1, but each DMA to TFIFO must be preceded by an Instruction cycle. The sequence of

cycles might be:

DMA1 cycle

Instruction cycle

DMA 1 cycle, during which TFNF gets set

Instruction cycle

DMA0 cycle

Instruction cycle

DMA0 cycle, as a result of which TFNF gets cleared

Instruction cycle

DMA1 cycle

Instruction cycle

DMA1 cycle

Instruction cycle

......

The requirement that a DMA to TFIFO be preceded by an Instruction cycle can result in the nor-

mal precedence of channel 0 over channel 1 being thwarted. Consider for example the situation

where channel 0 is conﬁgured to service TFIFO, and is in the process of doing so, and channel 1

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decides it wants to do Burst mode DMA. The sequence of events might be:

Instruction cycle (sets GO bit in DCON1)

Instruction cycle (during which TFNF gets set)

DMA0 cycle

DMA1 cycle

......

DMA1 cycle (completes channel 1 burst)

Instruction cycle

DMA0 cycle

Instruction cycle

...........

This sequence begins with two Instruction cycles. The ﬁrst one accesses a DMA register

(DCON1), and therefore is followed by another Instruction cycle, which presumably does not

access a DMA register. After the second Instruction cycle both channels are ready to generate

DMA cycles, and channel 0 of course takes precedence. After the DM0 cycle, channel 0 must

wait for an Instruction cycle before it can access TFIFO again. Channel 1, being in Burst mode,

doesn’t have that restriction, and is therefore granted a DMA1 cycle. After the ﬁrst DMA1 cycle,

channel 0 is still waiting for an Instruction cycle and channel 1 still does not have that restriction.

There follows another DMA1 cycle.

The result is that in this particular case channel 0 has to wait until channel 1 completes its Burst

mode DMA, and then has to wait for an Instruction cycle to be generated, before it can continue

its own DMA to TFIFO. The delay in servicing TFFIO can cause an Underﬂow condition in the

GSC transmission.

The delay will not occur if channel 1 is conﬁgured to Alternate Cycles mode, since channel 0

would then see the Instruction cycles it needs to complete its logic requirements for asserting its

request.

4.4.1 DMA Arbitration with Hold/Hold Ack

The Hold/Hold Acknowledge feature is invoked by setting either the ARB or REQ bit in PCON.

Their effect is to add the requirements of the Hold/Hold Ack protocol to mode_logic ( ). This

amounts to replacing every expression “return 1” in with the expression “return hld_hlda_logic (

) “, where hld_hlda_logic ( ) is a function which returns 1 if the Hold/Hold Ack protocol is satis-

ﬁed, and returns 0 otherwise. A suitable deﬁnition for hld_hlda_logic () is shown in Figure 4.14.

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hold_holda ( ):

if (ARB = 0 .AND. REQ = 0) return 1;

if SARn = XRAM .OR. DARn =XRAM)

{

if (ARB =1 .AND. HLDA = 1) return 1;

if (REQ = 1 .AND. HLDA = 0) return 1;

else return 0 ;

}

return 1;

end hold_holda( ) ;

4.5 Summary of DMA Control Bits

DAS

DCONs

IDA

ISA

DONE

SAS

DAS speciﬁes the Destination Address Space. If DAS = 0, the destination is in External Data

Memory. If DAS = 1 and IDA = 0, the destination is a Special Function Register (SFR). If DAS =

1 and IDA = 1, the destination is in Internal Data RAM.

IDA (Increment Destination Address) If IDA= 1, the destination address is automatically incre-

mented after each byte transfer. If IDA = 0, It is not.

SAS speciﬁes the Source Address Space. If SAS = 0, the source is in External Data Memory. If

SAS = 1 and ISA = 0, the source is an SFR. If SAS = 1 and ISA = 0, the source is an SFR. If SAS

= 1 and ISA = 1, the source is Internal Data RAM.

ISA (Increment Source Address) If ISA = 1, the source address is automatically incremented after

each byte transfer. If ISA = 0, it is not.

DM (Demand Mode) If DM = 1, the DMA Channel operates in Demand Mode. In Demand Mode

the DMA is initiated either by an external signal or by a Serial Port Flag, depending on the value

of the TM bit. If DM = 0, the DMA is requested by setting the GO bit in software.

TM (Transfer Mode) If DM = 1 then TM selects whether a DMA is initiated by an external signal

(TM = 1) or by a Serial Port ﬂag (TM =0). If DM = 0 then TM selects whether the data transfers

are to be in bursts (TM = 1) or in alternate cycles (TM = 0).

DONE indicates the completion of a DMA operation and ﬂags an interrupt. It is set to 1 by on-

chip hardware when BCRn = 0, and is cleared to 0 by on-chip hardware when the interrupt is vec-

tored to. It can also be set or cleared by software.

GO is the enable bit for the DMA Channel itself. The DMA Channel is inactive if GO= 0.

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SMOD ARB

REQ

GAREN XRCLK GFIEN PDN IDL

PCON

ARB enables the DMA logic to detect HLD and generate HLDA. After it has activated HLDA,

the C152 will not begin a new DMA to or from External Data Memory as long as HLD is seen to

be active. This logic is disabled when ARB = 0, and enabled when ARB = 1.

REQ enables the DMA logic to generate HLD and detect HLDA before performing a DMA to or

from External Data Memory. After it has activated HLD, the C152 will not begin the DMA until

HLDA is seen to be active. This logic is disabled when REQ = 0, and enabled when REQ = 1.

5.0 INTERRUPT STRUCTURE

The 8XC152 retains all ﬁve interrupts of the 80C51BH. Six new interrupts are added in the

8XC152, to support its GSC and DMA features. They are all listed below, and the ﬂag that gener-

ate them are shown in Figure below.

RFNE

RDN

DMA = 0

GSCRV

DMA = 1

TCDT

GSCTE

NOACK

CRCE

RCABT

OVR

GSCRE

DONE

DMA0

DMA1

(DCON0.1)

TFNF

TDN

DMA = 0

DONE

(DCON1.1)

GSCTV

DMA = 1

GSCRV - GSC Receive Valid

GSCRE - GSC Receive Error

GSCTV - GSC Transmit Valid

GSCTE - GSC Transmit Error

DMA0 - DMA Channel0 Done

DMA1 - DMA Channel1 Done

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As shown in Figure above, the Receive Valid interrupt can be signalled either by the RFNE ﬂag

(Receive FIFO Not Empty), or by the RDN ﬂag (Receive Done). Which one of these ﬂags causes

the interrupt depends on the setting of the DMA bit in the SFR named TSTAT.

DMA = 0 means the DMA hardware is not conﬁgured to service the GSC, so the CPU will service

it in software in response to the Receive FIFO not being empty. In that case, RFNE generates the

Receive Valid interrupt.

DMA = 1 means the DMA hardware is conﬁgured to service the GSC, in which case the CPU

need not be interrupted till the receive is complete. In that case RDN generates the Receive Valid

interrupt.

Similarly the Transmit Valid interrupt can be signalled either by the TFNF ﬂag (Transmit FIFO

Not Full), or by the TDN ﬂag (Transmit Done), depending on whether the DMA bit is 0 or 1.

Note that setting the DMA bit does not itself conﬁgure the DMA channels to service the GSC.

That job must be done by software writes to the DMA registers. the DMA bit only selects whether

the GSCRV and GSCTV interrupts are ﬂagged by a FIFO needing service or by an “operation

done” signal.

The Receive and Transmit Error interrupt ﬂags are generated by the logical OR of a number of

error conditions, which are described in Section 3.6.5.

Each interrupt is assigned a ﬁxed location in Program Memory, and the interrupt causes the CPU

to jump to that location. All the interrupt ﬂags are sampled at S5P2 of every machine cycle, and

then the samples are sequentially polled during the next machine cycle. If more than one interrupt

of same priority is active, the one that is highest in the polling sequence is serviced ﬁrst. The inter-

rupts and their ﬁxed locations in Program Memory are listed below in the order of their polling

sequence

Table 14:

Interrupt

IE0

Location

0003H

002BH

000BH

0033H

003BH

0013H

0043H

Name

External Interrupt 0

GSC Receive Valid

Timer 0 Overﬂow

GSCRV

TF0

GSCRE

DMA0

IE1

GSC Receive Error

DMA channel 0 Done

External interrupt 1

GSC Transmit Valid

GSCTV

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Table 14:

Interrupt

DMA1

TF1

Location

0053H

Name

DMA Channel 1 Done

Timer 1 Overﬂow

001BH

004BH

0023H

GSCTE

TI+RI

GSC Transmit Error

UART Transmit/Receive

Note that the locations of the basic 8051 interrupts are the same as in the rest of the MCS-51 Fam-

ily. And relative to each other they retain the same positions in the polling sequence.

The locations of the new interrupts all follow the locations the basic 8051 interrupts in the Pro-

gram Memory, but they are interleaved with them in the polling sequence.

To support the new interrupts a second Interrupt Enable register and A second Interrupt Priority

in the 8XC152 are as follows:

ET1

EX1

ET0

EX0

IE:

Address of IE in SFR space = 0A8H (bit-addressable)

EGSTE EDMA1 EGSTV EDMA0 EGSRE EGSRV

IEN1:

Address of IE1 in SFR space = 0C8H (bit-addressable)

The bits in the IE are unchanged from the standard 8051 IE register. The bits in IEN1 are as fol-

lows:

EGSTE = 1 Enable GSC Transmit Error Interrupt

= 0 Disable

EDMA1 = 1 Enable DMA Channel 1 Done Interrupt

= 0 Disable

EGSTV = 1 Enable GSC Transmit Valid Interrupt

= 0 Disable

EDMA0 = 1 Enable DMA Channel 0 Done Interrupt

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= 0 Disable

EGSRE = 1 Enable GSC Receive Error Interrupt

= 0 Disable

EGSRV = 1 Enable GSC Receive Valid Interrupt

= 0 Disable

The two Interrupt Priority registers in the 8XC152 are as follows:

PT1

PX1

PT0

PX0

IP:

Address of IP in SFR space = 0B8H (bit-addressable)

IPN1:

PGSTE PDMA1 PGSTV PDMA0 PGSRE PGSRV

Address of IPN1 in SFR space = 0F8H (bit -addressable)

The bits in IP are unchanged from the standard 8051 IP register. The bits in IPN1 are as follows:

PGSTE = 1 GSC Transmit Error Interrupt Priority to high

= 0 Priority to Low

PDMA1 = 1 DMA Channel 1 Done Interrupt Priority to high

= 0 Priority to Low

PGSTV = 1 GSC Transmit Valid Interrupt Priority to high

= 0 Priority to Low

PDMA0 = 1 DMA Channel 0 Done Interrupt Priority to high

= 0 Priority to Low

PGSRE = 1 GSC Receive Error Interrupt Priority to high

= 0 Priority to Low

PGSRV = 1 GSC Receive Valid Interrupt Priority to high

= 0 Priority to Low

Note that these registers all have unimplemented bits ("-"). If these bits are read, they will return

unpredictable values. If they are written to, the value written goes nowhere.

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It is recommended that user software should never write 1s to unimplemented bits in MCS-51

devices. Further versions of the device may have new bits installed in these locations. If so, their

reset value will be 0. Old software that writes 1s to newly implemented bits may unexpectedly

invoke new features.

The MCS-51 interrupt structure provides hardware support for only two priority levels, High and

Low. With as many interrupt sources as the 8XC152 has, it may be helpful to know how to aug-

ment the priority structure in the software. Any number of priority levels can be implemented in

software by saving and redeﬁning the interrupt enable registers within the interrupt service rou-

tines.

5.1 GSC Transmitter Error Conditions

The GSC Transmitter section reports three kinds of error conditions:

TCDT - Transmitter Collision Detector

UR - Underrun in Transmit FIFO

NOACK - No Acknowledge

These bits reside in the TSTAT register. User software can read them, but only the GSC hardware

can write to them. The GSC hardware will set them in response to the various error conditions that

they represent. When the user software sets the TEN bit, the GSC hardware will at that time clear

these ﬂags. This is the only way these ﬂags can be cleared.

The logical OR of these three bits ﬂags the GSC Transmit Error interrupt (GSCTE) and clears the

TEN bit, as shown in Figure below. Thus any detected error condition aborts the transmission. No

CRC bits are transmitted. In the SDLC mode an EOF is generated. In CSMA/CD mode an EOF is

generated by default, since the GTXD pin is pulled to a logic 1 and held there.

TCDT

NOACK

GSCTE

Clear

TEN

Set

TDN

TRANSMIT

EOF

(Logic for Clearing TEN, Setting TDN)

Transmit Error Flags

The TCDT bit can get set only if the GSC is conﬁgured to CSMA/CD mode. In that case the GSC

hardware sets the TCDT when a collision is detected during a transmission, and the collision was

detected after TFIFO has been accessed. Also, the GSC hardware sets TCDT when a detected col-

lision causes the TCDCNT register to overﬂow.

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The UR bit can be set only if the DMA bit in the TSTAT is set. The DMA bit being set informs the

GSC hardware that TFIFO is being serviced by DMA. In that case if the GSC goes to fetch

another byte from TFIFO and ﬁnds it empty, and the byte count register of the DMA channel ser-

vicing TFIFO is not zero, it sets the UR bit.

If the DMA hardware is not being used to service TFIFO, the UR bit cannot get set. If the DMA

bit is 0, then when the GSC ﬁnds TFIFO empty, it assumes that the transmission of data is com-

plete and the transmission of CRC bits can begin.

The NOACK bit is functional only in CSMA/CD mode, and only when the HABEN bit in RSTAT

is set. The HABEN bit turns on the Hardware Based Acknowledge feature, as described in Sec-

tion 3.2.6. If this feature is not invoked, the NOACK bit will stay at 0.

If the NOACK bit gets set, it means the GSC has completed a transmission, and was expecting to

receive a hardware based acknowledge from the receiver of the message, but did not receive the

acknowledge, or at least did not receive it cleanly. There are three ways the NOACK can get set:

1. The acknowledge signal (an unattached preamble) was not received before the IFS was com-

plete.

2. A collision was detected during the IFS

3. The line was active during the last bit-time of the IFS

The ﬁrst condition is an obvious reason for setting the NOACK bit, since that’s what the hardware

based acknowledge is for. The other two ways the NOACK bit can get set are to guard against the

possibility that the transmitting station might mistake an unrelated transmission or transmission

fragment for an acknowledge signal.

5.2 GSC Receiver Error Conditions

The GSC Receiver section reports four kinds of error conditions:

CRCE - CRC error

AE - Alignment Error

RCABT - Receive Abort

OVR - Overrun in Receive FIFO

These bits reside in the RSTAT register. User software can read them, but only GSC hardware can

write to them. The GSC hardware will set them in response to the various error conditions that

they represent. When user software sets the GREN bit, the GSC hardware will at that time clears

these ﬂags. This is the only way these ﬂags can be cleared.

The logical OR of these four bits ﬂags the GSC Receive Error interrupt (GSCRE) and clears the

GREN bit, as shown in Figure below. Note in this ﬁgure that any error condition will prevent

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RDN from being set.

CRCE

GSCRE

RCABT

OVR

Clear

GREN

Set

EOF

RDN

RECEIVED

(Logic for Clearing GREN, Setting RDN)

Receive Error Flag

A CRC Error means the CRC generator did not come to its correct value after calculating the

CRC of the message plus received CRC. An alignment error means the number of bits received

between the BOF and EOF was not a multiple of 8.

In SDLC mode, the CRC bit gets set at the end of any frame in which there is a CRC error, and the

AE bit gets set at the end of any frame in which there is an Alignment Error.

In CSMA/CD mode, if there is no CRC Error, neither CRCE nor AE will get set. If there is a CRC

error and no Alignment Error, the CRCE bit will get set, but not the AE bit. If there is both a CRC

Error and an Alignment Error, then the AE bit will get set, but not the CRCE bit. Thus in CSMA/

CD mode, the CRCE and AE bits are mutually exclusive.

The Receive Abort ﬂag, RCABT, gets set if an incoming frame was interrupted after received data

had already passed to the Receive FIFO. In the SDLC mode, this can happen if a line idle condi-

tion is detected before an EOF ﬂag is. In CSMA/CD mode, this can happen if there is a collision.

In either case, the CPU will have to re-initialize whatever pointers and counters it might have been

using.

The Overrun Error ﬂag, OVR, gets set if the GSC Receiver is ready to push a newly received byte

onto the Receive FIFO, but the FIFO is full.

Up to 7 “dribble bits” can be received after the EOF without causing an error condition.

6.0 GLOSSARY

ADR0,1,2,3 (95H, 0A5H, 0B5H, 0C5H) - Address Match Registers 0, 1, 2, 3 - The contents of

these SFR’s are compared against the address bits from the serial data on the GSC. If the address

matches the SFR, then the C152 accepts that frame. If in 8 bit addressing mode a match with any

of the four registers will trigger acceptance. In the 16 bit addressing mode, a match with

ADR1:ADR0 or ADR3:ADR2 will be accepted. Address length is determined by GMOD (AL).

AE - Alignment Error, see RSTAT.

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AL - Address Length, see GMOD.

AMSK0,1 (0D5H, 0E5H) - Address Match Mask 0,1 - Identiﬁes which bits in ADR0,1 are “don’t

care” bits. Setting a bit to 1 in AMSK0,1 identiﬁes the corresponding bit in ADDR0,1 as not to be

examined when comparing address.

BAUD - (94H) Contains the programmable value for the baud rate generator for the GSC. The

baud rate will equal (fosc) / ((BAUD+1) x 8).

BCRL0,1 (0E2H, 0F2H) - Byte Count Register Low 0,1 - Contains the lower byte of the byte

count. Used during the DMA transfer to identify to the DMA channels when the transfer is com-

plete.

BCRH0,1 (0E3H, 0F3H) - Byte Count Register High 0,1 - Contains the upper byte of the byte

count.

BKOFF (0C4H) - Backoff Timer - The Backoff timer is an eight bit count-down timer with a

clock period equal to one slot time. The Backoff time is used in CSMA/CD collision resolution

algorithm.

BOF - Beginning of Frame Flag - A term commonly used when dealing packetized data. Signiﬁes

the beginning of a frame.

CRC - Cyclic Redundancy Check - An error checking routine that mathematically manipulates a

value depending on the incoming data. The purpose is to identify when a frame has been received

in error.

CRCE - CRC Error, See RSTAT.

CSMA/CD - Stands for Carrier Sense, Multiple Access, with Collision Detection.

CT- CRC Type, see GMOD.

DARL0/1 (0C2H, 0D2H) - Destination Address Register Low 0/1 - Contains the lower byte of the

destinations’ address when performing DMA transfers.

DARH0/1 (0C3H, 0D3H) - Destination Address Register Low 0/1 - Contains the upper byte of the

destinations’ address when performing DMA transfers.

DAS - Destination Address Space, see DCON.

DCJ - D.C.Jam, see MYSLOT.

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DCON0/1 (092H,093H)

DAS

IDA

SAS

ISA

DONE

The DCON register control the operation of the DMA channels by determining the source of data

to be transferred, the destination of the data to be transfer, and the various modes of operation.

DCON.0 (GO) - Enables DMA Transfer - When set it enables a DMA channel. If block mode is

set then DMA transfer starts as soon as possible under CPU control. If demand mode is set then

DMA transfer starts when a demand is asserted and recognized.

DCON.1 (DONE) - DMA Transfer is Complete - When set the DMA transfer is complete. It is set

when BCR equals 0 and is automatically reset when the DMA vectors to its interrupt routine. If

DMA interrupt is disabled and user software executes a jump on the DONE bit, then the user soft-

ware must also reset the done bit. If DONE bit is not set, then the DMA transfer is not complete.

DCON.2 (TM) - Transfer Mode - When set, DMA burst transfers are used if the DMA channel is

conﬁgured in the block mode or external interrupts are used to initiate a transfer if in Demand

Mode. When TM is cleared, Alternate cycle transfers are used if DMA is in the Block Mode, or

Local Serial Channel/GSC interrupts are used to initiate a transfer if in Demand Mode.

DCON.3 (DM) - DMA Channel Mode - When set, Demand Mode is used and when cleared Block

Mode is used.

DCON.4 (ISA) - Increment Source Address - When set, the source address registers are automat-

ically incremented during each transfer. When cleared the source registers are not incremented.

DCON.5 (SAS) - Source Address Space - When set the source of data for the DMA transfers is

internal data memory if autoincrement is also set. If auto increment is not set but SAS is, then the

source for the data will be one of the Special Function Register. When SAS is cleared, the source

for the data is external data memory.

DCON.6 (IDA) - Increment destination Address Space - When set, destination address registers

are incremented once after each byte is transferred. When cleared, the destination address regis-

ters are not automatically incremented.

DCON.7 (DAS) - Destination Address Space - When set, destination of data to be transferred is

internal data memory if autoincrement mode is also set. If auto increment is not set the destination

will be one of the Special Function Registers. When DAS is cleared then the destination is exter-

nal data memory

DCR - Deterministic Resolution, see MYSLOT.

DEN - An alternate function of one of the port 1 pins (P1.2). Its purpose is to enable external driv-

ers when the GSC is transmitting data. This function is always active when using the GSC and if

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P1.2 is programmed to a 1.

DM - DMA Mode, see DCON0

DMA - Direct Memory Access mode, see TSTAT.

DONE - DMA done bit, see DCON0.

DPH - Data Pointer High, an SFR that contains the high order byte of a general purpose pointer

called the data pointer(DPTR).

DPL - Data Pointer Low, an SFR that contains the low order byte of the data pointer.

EDMA0 - Enable DMA Channel 0 interrupt, see IEN1.

EDMA1 - Enable DMA Channel 1 interrupt, see IEN1.

EGSRE - Enable GSC Receive Error interrupt, see IEN1.

EGSRV - Enable GSC Receive Valid interrupt, see IEN1.

EGSTE - Enable GSC Transmit Error interrupt, see IEN1.

EGSTV - Enable GSC Transmit Valid interrupt, see IEN1.

EOF - A general term used in serial communications. Eof stands for End Of Frame and signiﬁes

when the last bits of data are transmitted when using packetized data.

ES - Enable LSC service interrupt, see IE.

ET0 - Enable Timer 0 interrupt, see IE.

ET1 - Enable Timer 1 interrupt, see IE.

EX0 - Enable External interrupt 0, see IE.

GMOD (84H)

PL1

XTCLK M1

PL0

The bits in this SFR, perform most of the conﬁguration on the type of data transfers to be used

with the GSC. Determines the mode, address length, preamble length, protocol select, and enables

the external clocking of the transmit data.

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GMOD.0 (PR) - Protocol - If set SDLC protocols with NRZI encoding, zero bit insertion, and

SDLC ﬂags are used. If cleared, CSMA/CD link access with Manchester encoding is used.

GMOD.1,2 (PL0,1) - Preamble length

PL1 PL0 LENGTH (BITS)

0 0

0 1

1 0

1 1

The length includes the two bit Begin of Frame (BOF) ﬂag in CSMA/CD but does not include

SDLC ﬂag. In SDLC mode, the BOF is an SDLC ﬂag, otherwise it is two consecutive ones. Zero

length is not compatible in CSMA/CD mode.

GMOD.3 (CT) - CRC Type - If set, 32 bit AUTODIN-II-32 is used. If cleared, 16 bit CRC-CCITT

is used.

GMOD.4 (AL) - Address Length - If set, 16 bit addressing is used. In 8 bit mode, a match with

any of the 4 address registers will allow the frame to be accepted (ADR0, ADR1, ADR2, ADR3).

“Don’t Care” bits may be masked in ADR0 and ADR1 with AMSK0 and AMSK1. In 16 bit

mode, address are matched against “ADR1:ADR0” or “ADR3:ADR2”. Again don’t care bits in

ADR1:ADR0 can be masked in AMSK1:AMSK0. A received address of all ones will be always

be recognized in any mode.

GMOD.5,6 (M0,M1) - Mode select - Two test modes, an optional “alternate backoff” mode, or

normal backoff can be enabled with these two bits.

M1 M0 Mode

0 0 Normal

0 1 Raw Transmit

1 0 Raw Receive

1 1 Alternate Backoff

GMOD.7 (XTCLK) - External Transmit Clock - If set an external 1X clock is used the transmitter.

If cleared the internal baud rate generator provides the transmit clock. The input clock is applied

to the P1.3 (TxC). The user software is responsible for setting or clearing this ﬂag. External

receive clock is enabled by setting PCON.3

GO - DMA Go bit, see DCON0.

GRxD - GSC Receive Data input, an alternate function of one of the port 1 pins (PI.0). This pin is

used as the receive input for the GSC. P1.0 must be programmed to a 1 for this function to oper-

ate.

GSC - Global Serial Channel - A high level, multi-protocol, serial communication controller

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added to the 8051BH core to accomplish high-speed transfers of packetized serial data.

GTxD - GSC Transmit Data output, an alternate function of one of the port 1 pins (P1.1). This pin

is used as the transmit output for the GSC. P1.1 must be programmed to a 1 for this function to

operate.

HBAEN - Hard Ware Based Acknowledge Enable, see RSTAT.

HLDA - Hold Acknowledge, an alternate function of one of the port 1 pins (P1.6). This pin is

used to perform the “HOLD ACKNOWLEDGE” function for DMA transfers. HLDA can be an

input or an output, depending on the conﬁguration of the DMA channels. P1.6 must be pro-

grammed to a logic 1 for this function to operate.

HOLD - Hold, an alternate function of one of the port 1 pins (P1.5). This pin is used to perform

the “HOLD” function for the DMA transfers. HOLD can be an input or an output, depending on

the conﬁguration of the DMA channels. P1.5 must be programmed to a logic 1 for this function to

operate.

IDA - Increment Destination Address, see DCON0.

IE (0A8H)

ET1 EX1 ET0 EX0

Interrupt Enable SFR, used to individually enable the Timer and Local Serial Channel interrupts.

Also contains the global enable bit which must be set to a 1 to enable any interrupt to be automat-

ically recognized by the CPU.

IE.0 (EX0) - Enables the external interrupt INT0 on P3.2.

IE.1 (ET0) - Enables the Timer0 interrupt.

IE.2 (EX1) - Enables the external interrupt INT1 on P3.3.

IE.3 (ET1) - Enables the Timer 1 interrupt.

IE.4 (ES) - Enable the Local Serial Channel interrupt.

IE.7 (EA) - The global interrupt enable bit. This bit must be set to a 1 for any other interrupt to be

enabled.

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IEN1 (0C8H)

EGSTE EDMA1 EGSTV EDMA0 EGSRE EGSRV

Interrupt enable register for DMA and GSC interrupts. A 1 in any bit position enables that interrupt.

IEN1.0 (EGSRV) - Enables the GSC valid receive interrupt.

IEN1.1 (EGSRE) - Enables the GSC receive error interrupt.

IEN1.2 (EDMA0) - Enables the DMA done interrupt for channel 0.

IEN1.3 (EGSTV) - Enables the GSC valid transmit interrupt.

IEN1.4 (EDMA1) - Enables the DMA done interrupt for Channel1.

IEN1.5 (EGSTE) - Enables the GSC transmit error interrupt.

IFS - (0A4H) Interframe Space, determines the number of bit times separating transmitted frames

in CSMA/CD and SDLC.

IP (0B8H)

PT1

PX1

PT0

EX0

Allows the user software two levels of prioritization to be assigned to each of the interrupts in IE.

A 1 assigns the corresponding interrupt in IE a higher interrupt than an interrupt with a correspond-

ing 0.

IP.0 (PX0) - Assigns priority of external interrupt, INT0.

IP.1 (PT0) - Assigns the priority of Timer 0 interrupt, T0.

IP.2 (PX1) - Assigns priority of external interrupt, INT1.

IP.3 (PT1) - Assigns the priority of Timer1 interrupt, T1.

IP.4 (PS) - Assigns the priority of the LSC interrupt, SBUF.

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IPN1 (0F8H)

PGSTE PDMA1 PGSTV PDMA0 PGSRE PGSRV

Allows the user software two levels of prioritization to be assigned to each of the interrupts in

IEN1. A 1 assigns the corresponding interrupt in IEN1 a higher interrupt than an interrupt with a

corresponding 0.

IPN1.0 (PGSRV) - Assigns the priority of GSC receive valid interrupt.

IPN1.1 (PGSRE) - Assigns the priority of GSC error receive interrupt.

IPN1.2 (PDMA0) - Assigns the priority of DMA done interrupt for Channel 0.

IPN1.3 (PGSTV) - Assigns the priority of GSC transmit valid interrupt.

IPN1.4 (PDMA1) - Assigns the priority of DMA done interrupt for Channel 1.

IPN1.5 (PGSTE) - Assigns the priority of GSC transmit error interrupt.

ISA - Increment Source Address, see DCON0.

LNI - Line Idle, see TSTAT.

LSC - Local Serial Channel - The asynchronous serial port found on all MCS-51 devices. Uses

start/stop bits and can transfer only 1 bit at a time.

M0 - One of two GSC mode bits, see TMOD.

M1 - One of the two GSC mode bits, see TMOD.

MYSLOT - (0F5H)

DCJ DCR SA5 SA4

SA3 SA2

SA1 SA0

Determines which type of Jam is used, which backoff algorithm is used and DCR slot address for

the GSC.

MYSLOT.0,1,2,3,4,5 (SA0,1,2,3,4,5) - These bits determine which slot address is assigned to the

C152 when deterministic backoff during CSMA/CD operations on the GSC. Maximum slots avail-

able is 63. An address of 00H prevents that station from participating in the backoff process.

MYSLOT.6 (DCR) - Determines which collision resolution algorithm is used. If set to 1, then the

deterministic backoff is used. If cleared, then a random slot assignment is used.

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MYSLOT.7 (DCJ) - Determines the type of Jam used during CSMA/CD operation when a collision

occurs. If set to a 1 then a low D.C level is used as the jam signal. If cleared, then CRC is used as

the jam signal. The jam is applied for a length of time equal to the CRC length.

NOACK - No Acknowledgment error bit, see TSTAT.

NRZI - Non-Return to Zero Inverted, a type of data encoding where a 0 is represented by a change

in the level of the serial link. A 1 is represented by no change.

OVR - Overrun Error bit, see RSTAT.

PR - Protocol select bit, see GMOD.

PCON (87H)

SMOD ARB REQ GAREN XRCLK GFIEN

PD IDL

PCON.0 (IDL) - Idle bit, used to place the C152 into the idle power saving mode.

PCON.1 (PD) - Power Down bit, used to place the C152 into the power down power saving mode.

PCON.2 (GFIEN) - GSC Flag Idle Enable bit, when set, enables idle ﬂags (01111110) to be gen-

erated between transmitted frames in SDLC mode.

PCON.3 (XRCLK) - External Receive Clock bit, used to enable an external clock to be used for

only the receiver portion of the GSC.

PCON.4 (GAREN) - GSC Auxiliary Receive Enable bit, used to enable the GSC to receive back-

to-back SDLC frames. This bit has no effect in CSMA/CD mode.

PCON.5 (REQ) - Requester mode bit, set to a 1 when C152 is to be operated as the requester station

during DMA transfers.

PCON.6 (ARB) - Arbiter mode bit, set to a 1 when C152 is to be operated as the arbiter during

DMA transfers.

PCON.7 (SMOD) - LSC mode bit, used to double the baud rate on the LSC.

PDMA0 - Priority bit for DMA Channel 0 interrupt, see IPN1.

PDMA1 - Priority bit for DMA Channel 1 interrupt, see IPN1.

PGSRE - Priority bit for GSC Receive Error interrupt, see IPN1.

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PGSRV - Priority bit for GSC Receive Valid interrupt see IPN1.

PGSTE - Priority bit for GSC Transmit Valid interrupt, see IPN1.

PL0 - One of the two bits that determines the Preamble Length, see GMOD.

PL1 - One of the two bits that determines the Preamble Length, see GMOD.

PRBS - (0E4H) - Pseudo-Random Binary Sequence generates the pseudo-random number to be

used in CSMA/CD backoff algorithms.

PS - Priority bit for the LSC service interrupt, see IP.

PT0 - Priority bit for Timer 0 interrupt, see IP.

PT1 - Priority bit for Timer 1 interrupt, see IP.

PX0 - Priority bit for External Interrupt 0, see IP.

PX1 - Priority bit for External interrupt 1, see IP.

RCABT - GSC Receive Abort error bit, see RSTAT.

RDN - GSC Receive Done bit, see RSTAT.

GREN - GSC Receive Enable bit, see RSTAT.

RFNE - GSC Receive FIFO Not Empty bit, see RSTAT.

RI - LSC Receive Interrupt bit, see SCON.

RFIFO - (F4H) - RFIFO is a 3-byte FIFO that contains the receive data from GSC.

RSTAT (0E8H) - Receive Status Register

OVR RCABT AE CRCE RDN RFNE GREN HABEN

RSTAT.0 (HABEN) - Hardware Based Acknowledge Enable - If set, enables the hardware based

acknowledge feature.

RSTAT.1 (GREN) - Receiver Enable - When set, the receiver is enabled to accept incoming frames.

The user must clear RFIFO with software before enabling the receiver. RFIFO is cleared by reading

the contents of RFIFO until RFNE = 0. After each read of RFIFO, it takes one machine cycle ﬁr

the status of RFNE to be updated. Setting GREN also clears RDN, CRCE, AE and RCABT. GREN

is cleared by hardware at the end of a reception or if any receive errors are detected. The status of

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GREN has no effect on whether the receiver detects a collision in CSMA/CD mode as the receiver

input circuitry always monitors the receive pin.

RSTAT.2 (RFNE) - Receive FIFO Not Empty - If set, indicates that the receive FIFO contains data.

The receive FIFO is a three byte buffer into which the receive data is loaded. A CPU read of the

FIFO retrieves the oldest data and automatically updates the FIFO pointers. Setting GREN to a one

will clear the receive FIFO. The status of this ﬂag is controlled by the GSC. This bit is cleared if

the user software empties receive FIFO.

RSTAT.3 (RDN) - Receive Done - If set, indicates the successful completion of a receiver opera-

tion. Will not ne set is a CRC, alignment, abort or FIFO overrun error occurred.

RSTAT.4 (CRCE) - CRC Error - If set, indicates that a properly aligned frame was received with a

mismatched CRC.

RSTAT.5 (AE) - Alignment Error - In CSMA/CD mode, AE is set if the receiver shift register (an

internal serial-to-parallel converter) is not full and CRC is bad when an EOF is detected. In CSMA/

CD the EOF is a line idle condition (see LNI) for two bit times. If the CRC is correct while in

CSMA/CD mode, AE bit is not set and any mis-alignment us assumed to be caused by dribble bits

as the line went idle. in SDLC mode. AE is set if a non-byte-aligned ﬂag is received. CRC may also

be set. The setting of this ﬂag is controlled by the GSC.

RSTAT.6 (RCABT) - Receiver Collision/Abort Detect - If set, indicates a collision was detected

after data had been loaded into the receive FIFO in CSMA/CD mode. In SDLC mode, RCABT

indicates that 7 consecutive 1’s were detected prior to the end of ﬂag but after data has been loaded

into the receive FIFO. AE may also be set if RCABT is set.

RSTAT.7 (OVR) - Overrun - If set, indicates that the receive FIFO was full and new shift register

data was written into it. It is cleared by the user software. AE and/or CRCE may also be set if OVR

is set.

SARH0 (0A3H) - Source Address Register High 0, contains the high byte of the source address for

the DMA Channel 0.

SARH1 (0B3H) - Source Address Register High 1, contains the high byte of the source address for

the DMA Channel 1.

SARL0 (0A2H) - Source Address Register Low 0, contains the low byte of the source address for

the DMA Channel 0.

SARL1 (0B2H) - Source Address Register Low 1, contains the low byte of the source address for

the DMA Channel 1.

SAS - Source Address Space bit, see DCON0.

SBUF (099H) - Serial Buffer, both the receive and transmit SFR location for the LSC.

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SCON (098H)

SM0 SM1

SM2 REN TB8 RB8 TI

SCON.0 (RI) - Receive Interrupt Flag.

SCON.1 - Transmit Interrupt Flag.

SCON.2 (RB8) - Receive Bit 8, contains the ninth bit that was received in Modes 2 and 3 and stop

bit in Mode 1 if SM20. Not used in Mode 0.

SCON.3 (TB8) - Transmit Bit 8, the ninth bit to be transmitted in Modes 2 and 3.

SCON.4 (REN) - Receive Enable, enables reception for the LSC.

SCON.5 (SM2) - Enables the multiprocessor communication feature in Modes 2 and 3 for the LSC.

SCON.6 (SM1) - LSC mode speciﬁer.

SCON.7 (SM2) - LSC mode speciﬁer.

SDLC - Stands for synchronous Data Link Communication and is a protocol developed by IBM.

SLOTTM - (0B4H) Determines the length of the slot time in CSMA/CD.

SP (081H) - Stack Pointer, an eight bit pointer register used during a PUSH, POP, CALL, RET or

RETI.

TCDCNT (0D4H) Contains the number of collisions in the current frame if using probabilistic

CSMA/CD and contains the maximum number of slots in the deterministic mode.

TCDT - Transmit Collision Detect, see TSTAT.

TCON (088H)

TF1 TR1

TF0 TR0

IE1 IT1 IE0 IT0

TCON.0 (IT0) - Interrupt 0 mode control bit.

TCON.1 (IE0) - External interrupt 0 edge ﬂag.

TCON.2 (IT1) - Interrupt 1 mode control bit.

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TCON.3 (IE1) - External Interrupt 1 edge ﬂag.

TCON.4 (TR0) - Timer 0 run control bit.

TCON.5 (TF0) - Timer 0 overﬂow ﬂag.

TCON.6 (TR1) - Transmit Done ﬂag, see TSTAT.

TCON.7 (TF1) - Timer 1 overﬂow ﬂag.

TDN - Transmit Done Flag, see TSTAT

TEN - Transmit Enable bit, see TSTAT.

TFNF - Transmit FIFO Not Full Flag, see TSTAT.

TFIFO - (85H) TFIFO is a 3 byte FIFO that contains the transmission data for the GSC.

TH0 (08CH) - Timer 0 High Byte, contains the high byte for timer/counter 0.

TH1 (08DH) - Transmit Interrupt, see SCON.

TL0 (08AH) - Timer 0 Low byte, contains the low byte for timer/counter 0.

TL1 (08BH) - Timer 1 Low byte, contains the low byte for timer/counter 1.

TM - Transfer Mode, see DCON0.

TMOD (089H)

GATE C/T

M1 M0

GATE C/T M1

TMOD.0 (M0) - Mode selector bit for Timer 0.

TMOD.1 (M1) - Mode selector bit for Timer 0.

TMOD2 (C/T) - Timer/Counter selector bit for Timer 0.

TMOD3 (GATE) - Gating Mode bit for Timer 0.

TMOD.4 (M0) - Mode selector bit for Timer 1.

TMOD.5 (M1) - Mode selector bit for Timer 1.

TMOD6 (C/T) - Timer/Counter selector bit for Timer 1.

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TMOD7 (GATE) - Gating Mode bit for Timer 1.

TSTAT (0D8H) - Transmit Status Register

LNI NOACK UR TCDT TDN TFNF TEN DMA

TSTAT.0 (DMA) - DMA Select - If set, indicates that DMA channels are used to service the GSC

FIFO’s and GSC interrupts occur on TDN and RDN, and also enables UR to become set. If cleared,

indicates that the GSC is operating in it normal mode and interrupt occur on TFNE and RFNE. For

more information on DMA servicing please refer to DMA section on DMA serial demand mode

(4.2.2.3).

TSTAT.1 (TEN) - Transmit Enable - When set, causes TDN, UR, TCDT and NOACK ﬂags to be

reset and TFIFO cleared. The transmitter will clear TEN after a successful transmission, a collision

during the data, CRC, or end ﬂag. If cleared during transmission the GSC transmit pin goes to a

steady state high level. This is the method used to send an abort character in SDLC. Also DEN is

forced to high level. The end of transmission occurs whenever the TFIFO is emptied.

TSTAT.2 (TFNF) - Transmit FIFO Not Full - When set, indicates that the new data may be written

into the transmit FIFO. The transmit FIFO is a three byte buffer that loads the transmit shift register

with data.

TSTAT.3 (TDN) - Transmit Done - When set, indicates the successful completion of a frame trans-

mission. If HBAEN is set, TDN will not be set until the end of the IFS following the transmitted

message, so that the acknowledgment can be checked. If an acknowledgment is nor expected fol-

lowing a broadcast or multi-cast packet.

TSTAT.4 (TCDT) - Transmit Collision Detect- If set, indicates that the transmitter halted due to

collision. It is set if a collision occurs during the data or CRC or is there are more than eighth col-

lisions.

TSTAT.5 (UR) - Underruns - If set, indicates that in the DMA mode last bit was shifted out of the

transmit register and the DMA byte count did not equal zero. When an underrun occurs, the trans-

mitter halts without sending the CRC or the end ﬂag.

TSTAT.6 (NOACK) - No Acknowledge - If set, indicates that no acknowledgment was received for

the previous frame. Will be set only if HBAEN is set and no acknowledge is received prior to the

end of the IFS. NOACK is not set following a broadcast or multicast packet.

TSTAT.7 (LNI) - Line Idle - If set, indicates that the receive line is idle. In SDLC protocol it is set

if 15 consecutive ones are received. In CSMA/CD protocol, line idle is set if GRxD remains high

for approximately 1.6 bit times. LNI is cleared after a transition on GSxD.

TxD - External Clock input for GSC transmitter.

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UR - Underrun Flag, see TSTAT.

XRCLK - External GSC Receive Clock Enable bit, see PCON.

XTCLK - External GSC Transmit Clock Enable bit, see GMOD.

...............................

PORT 0

Bit:

P0.7

P0.6

P0.5

P0.4

P0.3

P0.2

P0.1

P0.0

Mnemonic: P0

Address: 80h

Port 0 is used as the multiplexed address/data bus for external access.Since the cpu executes all

program code from external memory, the Port P0 should not be used for I/O purposes. Pull-ups

are not required when used as a memory interface.

STACK POINTER

Bit:

SP.7

SP.6

SP.5

SP.4

SP.3

SP.2

SP.1

SP.0

Mnemonic: SP

Address: 81h

The Stack Pointer stores the Scratchpad RAM address where the stack begins. In other words it

always points to the top of the stack.

The SP is set to 07h on any reset.

There is unrestricted read/write access to this SFR.

DATA POINTER LOW

Bit:

DPL.7 DPL.6 DPL5 DPL.4 DPL.3 DPL.2 DPL.1 DPL.0

Mnemonic: DPL

Address: 82h

This is the low byte of the 16-bit data pointer.

The DPL is reset to 00h by a reset.

There is unrestricted read/write access to this SFR.

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DATA POINTER HIGH

Bit:

DPH.7 DPH.6 DPH.5 DPH.4 DPH.3 DPH.2 DPH.1 DPH.0

Mnemonic: DPH

Address: 83h

This is the high byte of the 16-bit data pointer.

The DPH is reset to 00h by a reset.

There is unrestricted read/write access to this SFR.

TIMER CONTROL

Bit:

TF1

TR1

TF0

TR0

IE1

IT1

IE0

IT0

Mnemonic: TCON

Address: 88h

TF1

Timer 1 overﬂow ﬂag: This bit is set when Timer 1 overﬂows. It is cleared automati-

cally when the program does a timer 1 interrupt service routine. Software can also set

or clear this bit.

TR1

TF0

Timer 1 run control: This bit is set or cleared by software to turn timer/counter on or

off.

Timer 0 overﬂow ﬂag: This bit is set when Timer 0 overﬂows. It is cleared automati-

cally when the program does a timer 0 interrupt service routine. Software can also set

or clear this bit.

TR0

IE1

Timer 0 run control: This bit is set or cleared by software to turn timer/counter on or

off.

Interrupt 1 edge detect: Set by hardware when an edge/level is detected on INT1. This

bit is cleared by hardware when the service routine is vectored to only if the interrupt

was edge triggered. Otherwise it follows the pin.

IT1

IE0

Interrupt 1 type control: Set/cleared by software to specify falling edge/ low level trig-

gered external inputs.

Interrupt 0 edge detect: Set by hardware when an edge/level is detected on INT0. This

bit is cleared by hardware when the service routine is vectored to only if the interrupt

was edge triggered. Otherwise it follows the pin.

IT0

Interrupt 0 type control: Set/cleared by software to specify falling edge/ low level trig-

gered external inputs.

The TCON is reset to 00h by a reset.

There is unrestricted read/write access to this SFR.

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TIMER MODE CONTROL

Bit:

GATE

C/T

GATE

C/T

TIMER 1

Mnemonic: TMOD

TIMER 0

Address: 89h

GATE

Gating control: When this bit is set, Timer/counter x is enabled only while INTx pin is

high and TRx control bit is set. When cleared, Timerx is enabled whenever TRx con-

trol bit is set.

C/T

Timer or Counter Select: When cleared, the timer is incremented by internal clocks.

When set, the timer counts high-to-low edges of the Tx pin.

Mode Select bits:

M1 M0

Mode

Mode 0: 8-bits with 5-bit pre-scaler.

Mode 1: 18-bits, no pre-scaler.

Mode 2: 8-bits with auto-reload from THx

Mode 3: (Timer 0) TL0 is an 8-bit timer/counter controlled by the stan-

dard Timer 0 control bits. TH0 is a 8-bit timer only controlled by Timer

1 control bits.

(Timer 1) Timer/counter is stopped.

The TMOD is reset to 00h by a reset.

There is unrestricted read/write access to this SFR.

TIMER 0 LSB

Bit:

TL0.7 TL0.6 TL0.5 TL0.4 TL0.3 TL0.2 TL0.1 TL0.0

Mnemonic: TL0

Address: 8Ah

TL0.7-0 Timer 0 LSB

The TL0 sfr is set to 00h on any reset.

There is unrestricted read/write access to this SFR.

TIMER 1 LSB

Bit:

TL1.7 TL1.6 TL1.5 TL1.4 TL1.3 TL1.2 TL1.1 TL1.0

Mnemonic: TL1

Address: 8Bh

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TL1.7-0 Timer 1 LSB

The TL1 sfr is set to 00h on any reset.

There is unrestricted read/write access to this SFR.

TIMER 0 MSB

Bit:

TH0.7 TH0.6 TH0.5 TH0.4 TH0.3 TH0.2 TH0.1 TH0.0

Mnemonic: TH0

Address: 8Ch

TH0.7-0 Timer 0 MSB

The TH0 sfr is set to 00h on any reset.

There is unrestricted read/write access to this SFR.

TIMER 1 MSB

Bit:

TH1.7 TH1.6 TH1.5 TH1.4 TH1.3 TH1.2 TH1.1 TH1.0

Mnemonic: TH1

Address: 8Dh

TH1.7-0 Timer 1 MSB

The TH1 sfr is set to 00h on any reset.

There is unrestricted read/write access to this SFR.

PORT 1

Bit:

P1.7

P1.6

P1.5

P1.4

P1.3

P1.2

P1.1

P1.0

Mnemonic: P1

Address: 90h

P1.7-0

General purpose I/O port. Most instructions will read the port pins in case of a port

read access, however in case of read-modify-write instructions, the port latch is read.

The P1 sfr is set to FFh by a reset.

There is unrestricted read/write access to this SFR.

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SERIAL PORT CONTROL

Bit:

SM0

SM1

SM2

REN

TB8

RB8

Mnemonic: SCON

Address: 98h

SM0

SM1

Serial port, Mode 0 bit: The operation of SM0 is described below. .

Serial port Mode bit 1:

SM0

SM1

Mode Description

Length

Baud rate

4/12 tclk

variable

64/32 tclk

variable

Synchronous

Asynchronous

SM2

Multiple MCU communication: Setting this bit to 1 enables the multiprocessor com-

munication feature in mode 2 and 3. In mode 2 or 3, if SM2 is set to 1, then RI will not

be activated if the received 9th data bit (RB8) is 0. In mode 1, if SM2 = 1, then RI will

not be activated if a valid stop bit was not received.

REN

TB8

Receive enable: When set to 1 serial reception is enabled, else reception is disabled.

This is the 9th bit to be transmitted in modes 2 and 3. this bit is set and cleared by soft-

ware as desired.

RB8

In modes 2 and 3 this is the received 9th data bit. In mode 1, if SM2 = 0, RB8 is the

stop bit that was received. In mode 0 it has no function.

Transmit interrupt ﬂag: This ﬂag is set by hardware at the end of the 8th bit time in

mode 0, or at the beginning of the stop bit in all other modes during serial transmis-

sion. This bit must be cleared by software.

Receive interrupt ﬂag: This ﬂag is set by hardware at the end of the 8th bit time in

mode 0, or halfway through the stop bits time in the other modes during serial recep-

tion. However the restrictions of SM2 applies on this bit. This bit can be cleared only

by software

The SCON sfr is set to 00h by a reset.

There is unrestricted read/write access to this SFR.

SERIAL DATA BUFFER

Bit:

SBUF.7 SBUF.6 SBUF.5 SBUF.4 SBUF.3 SBUF.2 SBUF.1 SBUF.0

Mnemonic: SBUF Address: 99h

SBUF.7-0 Serial data is read from or written to this location. It actually consists of two separate

8-bit registers. On is the receive resister, and the other is the transmit buffer. Any read

access gets data from the receive data buffer, while write accesses are to the transmit

data buffer.

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The SBUF sfr is set to 00h by a reset.

There is unrestricted read/write access to this SFR.

PORT 2

Bit:

P2.7

P2.6

P2.5

P2.4

P2.3

P2.2

P2.1

P2.0

Mnemonic: P2

Address: A0h

P2.7-0

Non-multiplexed address bus A15-A8: The port latch cannot be used for general I/O

purposes but exists to support the MOVX instructions. Port 2 data will only be

brought out on the P2.7-0 pins during indirect MOVX instructions.

The P2 sfr is set to FFh by a reset.

There is unrestricted read/write access to this SFR.

PORT 3

Bit:

P3.7

P3.6

P3.5

P3.4

P3.3

P3.2

P3.1

P3.0

Mnemonic: P3

Address: B0h

P3.7-0

General purpose I/O port. Each pin also has a alternate input or output function. This

alternate function is enabled if the corresponding port latch bit is set to 1, else the port

pin will remain stuck at 0.

P3.7

P3.6

P3.5

P3.4

P3.3

P3.2

P3.1

P3.0

Strobe for read from external RAM

Strobe for write to external RAM

Timer/counter 1 external count input

Timer/counter 0 external count input

INT0 External interrupt 1

INT1 External interrupt 0

TxD

RxD

Serial port output

Serial port input

The P3 sfr is set to FFh by a reset.

There is unrestricted read/write access to this SFR.

PROGRAM STATUS WORD

Bit:

RS1

RS0

Mnemonic: PSW

Address: D0h

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Carry ﬂag: Set for an arithmetic operation which results in a carry being generated

from the ALU. It is also used as the accumulator for the bit operations.

Auxiliary carry: Set when the previous operation resulted in a carry (during addition0

or a borrow (during subtraction) from the high order nibble.

RS.1-0

User ﬂag 0: General purpose ﬂag that can be set or cleared by the user by software.

RS1

RS0

Address

00-07h

08-0Fh

10-17h

18-1Fh

Overﬂow ﬂag: Set when a carry was generated from the seventh bit but not from the

8th bit as a result of the previous operation or vice-versa.

User Flag 1: General purpose ﬂag that can be set or cleared by the user by software

Parity ﬂag: Set/cleared by hardware to indicate odd/even number of 1’s in the accumu-

lator.

The PSW sfr is set to 00h by a reset.

There is unrestricted read/write access to this SFR.

ACCUMULATOR

Bit:

ACC.7 ACC.6 ACC.5 ACC.4 ACC.3 ACC.2 ACC.1 ACC.0

Mnemonic: ACC

Address: E0h

ACC.7-0 The A or ACC register is the accumulator

The ACC is reset to 00h on any reset

This sfr has unrestricted read/write access.

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