OWIcrc.c

// This file has been prepared for Doxygen automatic documentation generation.
/*! \file ********************************************************************
*
* Atmel Corporation
*
* \li File:               OWIcrc.c
* \li Compiler:           IAR EWAAVR 3.20a
* \li Support mail:       avr@atmel.com
*
* \li Supported devices:  All AVRs.
*
* \li Application Note:   AVR318 - Dallas 1-Wire(R) master.
*                         
*
* \li Description:        CRC algorithms typically used in a 1-Wire(R) 
*                         environment.
*
*                         $Revision: 1.1.1.1 $
*                         $Date: 2010-01-07 16:39:25 $
****************************************************************************/

#include "OWIcrc.h"
#include "OWIdefs.h"


/*! \brief  Compute the CRC8 value of a data set.
 *
 *  This function will compute the CRC8 or DOW-CRC of inData using seed
 *  as inital value for the CRC.
 *
 *  \param  inData  One byte of data to compute CRC from.
 *
 *  \param  seed    The starting value of the CRC.
 *
 *  \return The CRC8 of inData with seed as initial value.
 *
 *  \note   Setting seed to 0 computes the crc8 of the inData.
 *
 *  \note   Constantly passing the return value of this function 
 *          As the seed argument computes the CRC8 value of a
 *          longer string of data.
 */
unsigned char OWI_ComputeCRC8(unsigned char inData, unsigned char seed)
{
    unsigned char bitsLeft;
    unsigned char temp;

    for (bitsLeft = 8; bitsLeft > 0; bitsLeft--)
    {
        temp = ((seed ^ inData) & 0x01);
        if (temp == 0)
        {
            seed >>= 1;
        }
        else
        {
            seed ^= 0x18;
            seed >>= 1;
            seed |= 0x80;
        }
        inData >>= 1;
    }
    return seed;    
}


/*! \brief  Compute the CRC16 value of a data set.
 *
 *  This function will compute the CRC16 of inData using seed
 *  as inital value for the CRC.
 *
 *  \param  inData  One byte of data to compute CRC from.
 *
 *  \param  seed    The starting value of the CRC.
 *
 *  \return The CRC16 of inData with seed as initial value.
 *
 *  \note   Setting seed to 0 computes the crc16 of the inData.
 *
 *  \note   Constantly passing the return value of this function 
 *          As the seed argument computes the CRC16 value of a
 *          longer string of data.
 */
unsigned int OWI_ComputeCRC16(unsigned char inData, unsigned int seed)
{
    unsigned char bitsLeft;
    unsigned char temp;

    for (bitsLeft = 8; bitsLeft > 0; bitsLeft--)
    {
        temp = ((seed ^ inData) & 0x01);
        if (temp == 0)
        {
            seed >>= 1;
        }
        else
        {
            seed ^= 0x4002;
            seed >>= 1;
            seed |= 0x8000;
        }
        inData >>= 1;
    }
    return seed;    
}


/*! \brief  Calculate and check the CRC of a 64 bit ROM identifier.
 *  
 *  This function computes the CRC8 value of the first 56 bits of a
 *  64 bit identifier. It then checks the calculated value against the
 *  CRC value stored in ROM.
 *
 *  \param  romvalue    A pointer to an array holding a 64 bit identifier.
 *
 *  \retval OWI_CRC_OK      The CRC's matched.
 *  \retval OWI_CRC_ERROR   There was a discrepancy between the calculated and the stored CRC.
 */
unsigned char OWI_CheckRomCRC(unsigned char * romValue)
{
    unsigned char i;
    unsigned char crc8 = 0;
    
    for (i = 0; i < 7; i++)
    {
        crc8 = OWI_ComputeCRC8(*romValue, crc8);
        romValue++;
    }
    if (crc8 == (*romValue))
    {
        return OWI_CRC_OK;
    }
    return OWI_CRC_ERROR;
}