static __inline__ uint16_t _crc_ccitt_update

static __inline__ uint16_t _crc_ccitt_update(uint16_t __crc, uint8_t __data)

Optimized CRC-CCITT calculation.

Polynomial: x^16 + x^12 + x^5 + 1 (0x8408) Initial value: 0xffff

This is the CRC used by PPP and IrDA.

See RFC1171 (PPP protocol) and IrDA IrLAP 1.1


Although the CCITT polynomial is the same as that used by the Xmodem protocol, they are quite different. The difference is in how the bits are shifted through the alorgithm. Xmodem shifts the MSB of the CRC and the input first, while CCITT shifts the LSB of the CRC and the input first.

The following is the equivalent functionality written in C.
    crc_ccitt_update (uint16_t crc, uint8_t data)
        data ^= lo8 (crc);
        data ^= data << 4;

        return ((((uint16_t)data << 8) | hi8 (crc)) ^ (uint8_t)(data >> 4) 
                ^ ((uint16_t)data << 3));