Delta VFD-L series User Manual page 65

CRC (Cyclical Redundancy Check) is calculated by the following steps:
Step 1: Load a 16-bit register (called CRC register) with FFFFH.
Step 2: Exclusive OR the first 8-bit byte of the command message with the low order byte of
the 16-bit CRC register, putting the result in the CRC register.
Step 3: Shift the CRC register one bit to the right with MSB zero filling. Extract and examine
the LSB.
Step 4: If the LSB of CRC register is 0, repeat step 3, else Exclusive or the CRC register
with the polynomial value A001H.
Step 5: Repeat step 3 and 4 until eight shifts have been performed. When this is done, a
complete 8-bit byte will have been processed.
Step 6: Repeat steps 2 to 5 for the next 8-bit byte of the command message. Continue doing
this until all bytes have been processed. The final contents of the CRC register are
the CRC value. When transmitting the CRC value in the message, the upper
and lower bytes of the CRC value must be swapped, i.e. the lower order byte
will be transmitted first.
The following is an example of CRC generation using C language. The function takes two
arguments:
Unsigned char* data
Unsigned char length
The function returns the CRC value as a type of unsigned integer.
Unsigned int crc_chk(unsigned char* data, unsigned char length){
int j;
unsigned int reg_crc=0xFFFF;
while(length--){
reg_crc ^= *data++;
for(j=0;j<8;j++){
}
}
return reg_crc;
}
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a pointer to the message buffer
the quantity of bytes in the message buffer
if(reg_crc & 0x01){ /* LSB(b0)=1 */
reg_crc=(reg_crc>>1) ^ 0xA001;
}else{
reg_crc=reg_crc >>1;
}
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