crc.c

PIC18读取sd卡的程序

/* crc.c 
/* crc程序，包括两部分，crcInit生成CRC查找表 
/* crcCompute执行后生成crc校验码
/* crcCompute两个参数，一个为起始地址，一个为计算个数，返回值为16bit的整型变量
*/

#define     POLYNOMIAL    0x1021         /* crc-ccitt mask */
#define     INITIAL_REMAINDER   0xFFFF
#define     FINAL_XOR_VALUE     0x0000

typedef unsigned short width;

#define WIDTH   (8 * sizeof(width))
#define TOPBIT  (1 << (WIDTH - 1))


/*
 * An array containing the pre-computed intermediate result for each
 * possible byte of input.  This is used to speed up the computation.
 */
width  crcTable[256];


/**********************************************************************
 *
 * Function:    crcInit()
 *
 * Description: Initialize the CRC lookup table.  This table is used
 *              by crcCompute() to make CRC computation faster.
 *
 * Notes:       The mod-2 binary long division is implemented here.
 *
 * Returns:     None defined.
 *
 **********************************************************************/
 
void  crcInit(void)
{
    width  remainder;
    width  dividend;
    int    i;


    /*
     * Perform binary long division, a bit at a time.
     */
    for (dividend = 0; dividend < 256; dividend++)
    {
        /*
         * Initialize the remainder.
         */
        remainder = dividend << (WIDTH - 8);

        /*
         * Shift and XOR with the polynomial.
         */
        for (i = 0;i < 8; i++)
        {
            /*
             * Try to divide the current data bit.
             */
            if (remainder & TOPBIT)
            {
                remainder = (remainder << 1) ^ POLYNOMIAL;
            }
            else
            {
                remainder = remainder << 1;
            }
        }

        /*
         * Save the result in the table.
         */
        crcTable[dividend] = remainder;
    }

}   /* crcInit() */


/**********************************************************************
 *
 * Function:    crcCompute()
 *
 * Description: Compute the CRC checksum of a binary message block.
 *
 * Notes:       This function expects that crcInit() has been called
 *              first to initialize the CRC lookup table.
 *
 * Returns:     The CRC of the data.
 *
 **********************************************************************/
unsigned int crcCompute(unsigned char * message, unsigned int nBytes)
{
    unsigned int   offset;
    unsigned char  byte;
    width          remainder = INITIAL_REMAINDER;


    /*
     * Divide the message by the polynomial, a byte at time.
     */
    for (offset = 0; offset < nBytes; offset++)
    {
        byte = (remainder >> (WIDTH - 8)) ^ message[offset];
        remainder = crcTable[byte] ^ (remainder << 8);
    }

    /*
     * The final remainder is the CRC result.
     */
    return (remainder ^ FINAL_XOR_VALUE);

}   /* crcCompute() */