TransCore SmartPass AI1611, System Manual

Page: 120 / 179

Communication Protocols
Table 8-1 Example of Routine to Calculate CRC
unsigned short calccrc(char *message)
{
unsigned short crc = 0;
for ( ; *message != (char)0; +)
crc = updcrc(*message & 0xff, crc);
return (crc);
}
Table 8-2 Example of Routine to Calculate CRC-CCITT without Lookup Table
#define BITS_PER_CHAR 8
unsigned short updcrc (unsigned short ch, unsigned short crc)
{
register short counter = BITS_PER_CHAR;
register short temp = crc;
while (--counter >= 0)
if (temp & 0x8000) {
temp <<= 1;
temp += (((ch <<= 1) & 0x0100) != 0);
temp ^= 0x1021;
}
else {
temp <<= 1;
temp += (((ch <<= 1) & 0x0100) != 0);
}
return(temp);
}
8-15