Morgensen MSI350 Series, Руководство пользователя, Страница: 285 / 388

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is 1, the XOR operation is performed on the current value in the register and the preset value. If LSB
is 0, no operation is performed. This process is repeated 8 times. After the last bit (8
th bit) is detected
and processed, the XOR operation is performed on the next 8-bit byte and the current content in the
register. The final values in the register are the CRC values obtained after operations are performed
on all the bytes in the frame.
The calculation adopts the international standard CRC check rule. You can refer to the related standard
CRC algorithm to compile the CRC calculation program as required.
The following is a simple CRC calculation function for your reference (using the C programming
language):
unsigned int crc_cal_value(unsigned char×data_value,unsigned char data_length)
{
int i;
unsigned int crc_value=0xffff;
while(data_length--)
{
crc_value^=×dat+;
for(i=0;i<8;i++)
{
if(crc_value&0x0001)
crc_value=(crc_value>>1)^0xa001;
else
crc_value=crc_value>>1;
}
}
return(crc_value);
}
In the ladder logic, CKSM uses the table look-up method to calculate the CRC value according to the
content in the frame. The program of this method is simple, and the calculation is fast, but the ROM
space occupied is large. Use this program with caution in scenarios where there are space occupation
limits on programs.