<util/crc16.h>: CRC Computations

Functions
static uint16_t	_crc16_update (uint16_t __crc, uint8_t __data)
static uint16_t	_crc_xmodem_update (uint16_t __crc, uint8_t __data)
static uint16_t	_crc_ccitt_update (uint16_t __crc, uint8_t __data)
static uint8_t	_crc_ibutton_update (uint8_t __crc, uint8_t __data)
static uint8_t	_crc8_ccitt_update (uint8_t __crc, uint8_t __data)

Detailed Description

#include <util/crc16.h>

This header file provides a optimized inline functions for calculating cyclic redundancy checks (CRC) using common polynomials.

References:

See the Dallas Semiconductor app note 27 for 8051 assembler example and general CRC optimization suggestions. The table on the last page of the app note is the key to understanding these implementations.

Jack Crenshaw's "Implementing CRCs" article in the January 1992 isue of Embedded Systems Programming. This may be difficult to find, but it explains CRC's in very clear and concise terms. Well worth the effort to obtain a copy.

A typical application would look like:

// Dallas iButton test vector.
uint8_t serno[] = { 0x02, 0x1c, 0xb8, 0x01, 0, 0, 0, 0xa2 };
 
int
checkcrc (void)
{
    uint8_t crc = 0, i;
 
    for (i = 0; i < sizeof serno / sizeof serno[0]; i++)
        crc = _crc_ibutton_update (crc, serno[i]);
 
    return crc; // must be 0
}

Function Documentation

_crc16_update()

static uint16_t _crc16_update ( uint16_t  __crc, uint8_t  __data  )

inlinestatic

Optimized CRC-16 calculation.

Polynomial: x¹⁶ + x¹⁵ + x² + 1 (0xa001)
Initial value: 0xffff

This CRC is normally used in disk-drive controllers.

The following is the equivalent functionality written in C.

uint16_t
crc16_update (uint16_t crc, uint8_t a)
{
    crc ^= a;
    for (int i = 0; i < 8; ++i)
    {
        if (crc & 1)
            crc = (crc >> 1) ^ 0xA001;
        else
            crc = crc >> 1;
    }
 
    return crc;
}

_crc8_ccitt_update()

static uint8_t _crc8_ccitt_update ( uint8_t  __crc, uint8_t  __data  )

inlinestatic

Optimized CRC-8-CCITT calculation.

Polynomial: x⁸ + x² + x + 1 (0xE0)

For use with simple CRC-8
Initial value: 0x0

For use with CRC-8-ROHC
Initial value: 0xff
Reference: http://tools.ietf.org/html/rfc3095#section-5.9.1

For use with CRC-8-ATM/ITU
Initial value: 0xff
Final XOR value: 0x55
Reference: http://www.itu.int/rec/T-REC-I.432.1-199902-I/en

The C equivalent has been originally written by Dave Hylands. Assembly code is based on _crc_ibutton_update optimization.

The following is the equivalent functionality written in C.

uint8_t
_crc8_ccitt_update (uint8_t inCrc, uint8_t inData)
{
    uint8_t data = inCrc ^ inData;
 
    for (int i = 0; i < 8; i++)
    {
        if ((data & 0x80) != 0)
        {
            data <<= 1;
            data ^= 0x07;
        }
        else
        {
            data <<= 1;
        }
    }
    return data;
}

_crc_ccitt_update()

static uint16_t _crc_ccitt_update ( uint16_t  __crc, uint8_t  __data  )

inlinestatic

Optimized CRC-CCITT calculation.

Polynomial: x¹⁶ + x¹² + x⁵ + 1 (0x8408)
Initial value: 0xffff

This is the CRC used by PPP and IrDA.

See RFC1171 (PPP protocol) and IrDA IrLAP 1.1

Note

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.

uint16_t
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));
}

_crc_ibutton_update()

static uint8_t _crc_ibutton_update ( uint8_t  __crc, uint8_t  __data  )

inlinestatic

Optimized Dallas (now Maxim) iButton 8-bit CRC calculation.

Polynomial: x⁸ + x⁵ + x⁴ + 1 (0x8C)
Initial value: 0x0

See http://www.maxim-ic.com/appnotes.cfm/appnote_number/27

The following is the equivalent functionality written in C.

uint8_t
_crc_ibutton_update (uint8_t crc, uint8_t data)
{
    crc = crc ^ data;
    for (uint8_t i = 0; i < 8; i++)
    {
        if (crc & 0x01)
            crc = (crc >> 1) ^ 0x8C;
        else
            crc >>= 1;
    }
 
    return crc;
}

_crc_xmodem_update()

static uint16_t _crc_xmodem_update ( uint16_t  __crc, uint8_t  __data  )

inlinestatic

Optimized CRC-XMODEM calculation.

Polynomial: x¹⁶ + x¹² + x⁵ + 1 (0x1021)
Initial value: 0x0

This is the CRC used by the Xmodem-CRC protocol.

The following is the equivalent functionality written in C.

uint16_t
crc_xmodem_update (uint16_t crc, uint8_t data)
{
    crc = crc ^ ((uint16_t)data << 8);
    for (int i = 0; i < 8; i++)
    {
        if (crc & 0x8000)
            crc = (crc << 1) ^ 0x1021;
        else
            crc <<= 1;
    }
 
    return crc;
}