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Example in ANSI C:
The CRC checksum in this software example is implemented as a reverse CRC-
CCITT:
The check sum is always composed out of 4 hex characters (values from 0x0000
to 0x FFFF). These 4 hex characters are transmitted in ASCII code from '0' ..
'9' and 'A' to 'F' .
Example: The check sum 0E2A is transmitted as CHR(0x30), CHR(0x45),
CHR(0x32), CHR(0x41).
// *********************************************************************
// Function to calculate the CRC from a protocol buffer with
// the given length
// *********************************************************************
unsigned int build_crc(unsigned char length, unsigned char* protocol)
{
// the initial CRC value
#define CRC_PRESET
0x0000
// the reverse CRC-CCIT pollynomial
#define CRC_POLYNOM
0x8408
unsigned char i,k;
unsigned int crc;
unsigned char crc_in;
crc CRC_PRESET;
// initial value
for(i=0;i< length;i++)
// loop trough the protocol
{
crc_in = protocol[i]; // get next protocol byte
for (k=0;k<=7;k++)
// loop trough one byte LSB to MSB
{
// test each Bit for CRC calculation
if((((crc_in>>k)&0x01)^(crc&0x0001))==1)
{ crc=crc>>1; crc=crc^0x8408; }
else
{ crc=crc>>1;}
}
}
return(crc);
}
