83C152.PDF

AP-429

APPLICATION

NOTE

Application Techniques for the

83C152 Global Serial Channel

in CSMA/CD Mode

BOB JOHNSON

Embedded Control Applications Engineering

May 1989

Order Number: 270720-001

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APPLICATION

TECHNIQUES FOR THE

83C152 GLOBAL SERIAL

CHANNEL IN CSMA/CD

MODE

CONTENTS

PAGE

INTRODUCTION ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 1

GSC INITIALIZATION ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 9

INITIALIZATION (PROTOCOL

DEPENDENT) ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 9

Baud Rate ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 9

Preamble Length ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 10

Backoff Mode ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 10

Interframe Space ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 11

Jamming Signal ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 15

Slot Time ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 16

Addressing ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 16

INITIALIZATIONÐPROTOCOL

INDEPENDENT ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 18

Clearing Collision Counter ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 18

Control of the GSC ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 19

Initializing DMA ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 19

Initializing Counters and Pointers ÀÀÀÀÀÀÀÀÀÀ 20

Enabling Receiver and Receiver

Interrupts ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 20

Enabling Transmitter and Transmit

Interrupts ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 21

STARTING, MAINTAINING, AND

ENDING TRANSMISSIONS ÀÀÀÀÀÀÀÀÀÀÀÀ 22

STARTING, MAINTAINING, AND

ENDING RECEPTIONS ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 23

SUMMARY ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 24

SOFTWARE EXAMPLE ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ A-1

CONTROLLING THE BACKOFF

ALGORITHM ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ B-1

REFERENCES ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ C-1

AP-429

INTRODUCTION

application note deals with initializing and running the

GSC in CSMA/CD mode only. Carrier Sense Multiple

Access with Collision Detection (CSMA/CD) is a com-

munication protocol that allows two or more stations to

share a common transmission medium by sensing when

the link is idle or busy (Carrier Sense). While in the

process of transmission, each station monitors its own

transmission to identify if and when a collision occurs.

When a collision occurs, each station involved in the

transmission executes a backoff algorithm and reat-

tempts transmission (Collision Detection). This access

method allows all stations an equal chance to transmit

its own packet and thus is referred to as a ``peer-to-

peer'' type protocol (Multiple Access). Even in

CSMA/CD mode, the user has several variations that

can be implemented. Table 1 summarizes the various

CSMA/CD options available. Most of these variations

will be discussed in this application note.

The 83C152 is an 80C51BH based microcontroller with

DMA capabilities and a high speed, multi-protocol,

synchronous serial communication interface called the

Global Serial Channel (GSC). The GSC uses packe-

tized data frames that consist of a beginning of frame

(BOF) flag, address byte(s), data byte(s), a Cyclic Re-

dundancy Check (CRC), and an End Of Frame (EOF)

flag. An example of this type of packet is shown in

Figure 1. Most 80C152 users will be familiar with

UARTs, another type of serial interface. Figures 1 and

2 compare the two types of frames. The UART uses

start and stop bits with a data byte between as shown in

Figure 2. The 83C152 retains the standard MCS É -51

UART.

The 83C152 will be referred to as the ``C152'' through-

out this application note to refer to the device. This

270720±1

Figure 1. Packetized Frame

270720±2

Figure 2. UART Byte

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