NAME¶

<util/crc16.h>: CRC Computations -

Functions¶

static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t __data) static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc, uint8_t __data) static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t __data) static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t __data) static __inline__ 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¶

static __inline__ uint16_t _crc16_update (uint16_t__crc, uint8_t__data) [static]¶

Optimized CRC-16 calculation. Polynomial: x^16 + x^15 + x^2 + 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)
{
    int i;

    crc ^= a;
    for (i = 0; i < 8; ++i)
    {
        if (crc & 1)
        crc = (crc >> 1) ^ 0xA001;
        else
        crc = (crc >> 1);
    }

    return crc;
}

static __inline__ uint8_t _crc8_ccitt_update (uint8_t__crc, uint8_t__data) [static]¶

Optimized CRC-8-CCITT calculation. Polynomial: x^8 + x^2 + 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   i;
    uint8_t   data;

    data = inCrc ^ inData;

    for ( i = 0; i < 8; i++ )
    {
        if (( data & 0x80 ) != 0 )
        {
            data <<= 1;
            data ^= 0x07;
        }
        else
        {
            data <<= 1;
        }
    }
    return data;
}

static __inline__ uint16_t _crc_ccitt_update (uint16_t__crc, uint8_t__data) [static]¶

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 Note: 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));
}

static __inline__ uint8_t _crc_ibutton_update (uint8_t__crc, uint8_t__data) [static]¶

Optimized Dallas (now Maxim) iButton 8-bit CRC calculation. Polynomial: x^8 + x^5 + x^4 + 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)
{
    uint8_t i;

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

    return crc;
}

static __inline__ uint16_t _crc_xmodem_update (uint16_t__crc, uint8_t__data) [static]¶

Optimized CRC-XMODEM calculation. Polynomial: x^16 + x^12 + x^5 + 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)
{
    int i;

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

    return crc;
}

Author¶

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