Mikroelektronika MIKROE-742 데이터 시트

Q: How is CRC checksum calculated on RS485 master side?

START_BYTE := 0x96; // 10010110
STOP_BYTE  := 0xA9; // 10101001

PACKAGE:

-------- 

START_BYTE 0x96

ADDRESS

DATALEN
[DATA1]            // if exists

[DATA2]            // if exists
[DATA3]            // if exists
CRC

STOP_BYTE  0xA9

DATALEN bits
------------
bit7 := 1  MASTER SENDS

0  SLAVE  SENDS

bit6 := 1  ADDRESS WAS XORed with 1, IT WAS EQUAL TO START_BYTE or

STOP_BYTE

0  ADDRESS UNCHANGED

bit5 := 0  FIXED

bit4 := 1  DATA3 (if exists) WAS XORed with 1, IT WAS EQUAL TO

START_BYTE or STOP_BYTE

0  DATA3 (if exists) UNCHANGED

bit3 := 1  DATA2 (if exists) WAS XORed with 1, IT WAS EQUAL TO

START_BYTE or STOP_BYTE

0  DATA2 (if exists) UNCHANGED

bit2 := 1  DATA1 (if exists) WAS XORed with 1, IT WAS EQUAL TO

START_BYTE or STOP_BYTE

0  DATA1 (if exists) UNCHANGED

bit1bit0 := 0 to 3 NUMBER OF DATA BYTES SEND

CRC generation :

----------------
crc_send := datalen 

xor address;

crc_send := crc_send 

xor data[0];    // if exists

crc_send := crc_send 

xor data[1];    // if exists

crc_send := crc_send 

xor data[2];    // if exists

crc_send := not crc_send;

if ((crc_send = START_BYTE) 

or (crc_send = STOP_BYTE)) then

Inc(crc_send);

NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or

STOP_BYTE<4..0> values.

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CHAPTER 6