crc.lst

PIC18读取sd卡的程序

     1: /* crc.c 
     2: /* crc程序，包括两部分，crcInit生成CRC查找表 
     3: /* crcCompute执行后生成crc校验码
     4: /* 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
     5: */
     6: 
     7: #define     POLYNOMIAL    0x1021         /* crc-ccitt mask */
     8: #define     INITIAL_REMAINDER   0xFFFF
     9: #define     FINAL_XOR_VALUE     0x0000
    10: 
    11: typedef unsigned short width;
    12: 
    13: #define WIDTH   (8 * sizeof(width))
    14: #define TOPBIT  (1 << (WIDTH - 1))
    15: 
    16: 
    17: /*
    18:  * An array containing the pre-computed intermediate result for each
    19:  * possible byte of input.  This is used to speed up the computation.
    20:  */
    21: width  crcTable[256];
    22: 
    23: 
    24: /**********************************************************************
    25:  *
    26:  * Function:    crcInit()
    27:  *
    28:  * Description: Initialize the CRC lookup table.  This table is used
    29:  *              by crcCompute() to make CRC computation faster.
    30:  *
    31:  * Notes:       The mod-2 binary long division is implemented here.
    32:  *
    33:  * Returns:     None defined.
    34:  *
    35:  **********************************************************************/
    36:  
    37: void  crcInit(void)
    38: {
    39:     width  remainder;
    40:     width  dividend;
    41:     int    i;
    42: 
    43: 
    44:     /*
    45:      * Perform binary long division, a bit at a time.
    46:      */
    47:     for (dividend = 0; dividend < 256; dividend++)
    48:     {
    49:         /*
    50:          * Initialize the remainder.
    51:          */
    52:         remainder = dividend << (WIDTH - 8);
    53: 
    54:         /*
    55:          * Shift and XOR with the polynomial.
    56:          */
    57:         for (i = 0;i < 8; i++)
    58:         {
    59:             /*
    60:              * Try to divide the current data bit.
    61:              */
    62:             if (remainder & TOPBIT)
    63:             {
    64:                 remainder = (remainder << 1) ^ POLYNOMIAL;
    65:             }
    66:             else
    67:             {
    68:                 remainder = remainder << 1;
    69:             }
    70:         }
    71: 
    72:         /*
    73:          * Save the result in the table.
    74:          */
    75:         crcTable[dividend] = remainder;
    76:     }
    77: 
    78: }   /* crcInit() */
    79: 
    80: 
    81: /**********************************************************************
    82:  *
    83:  * Function:    crcCompute()
    84:  *
    85:  * Description: Compute the CRC checksum of a binary message block.
    86:  *
    87:  * Notes:       This function expects that crcInit() has been called
    88:  *              first to initialize the CRC lookup table.
    89:  *
    90:  * Returns:     The CRC of the data.
    91:  *
    92:  **********************************************************************/
    93: unsigned int crcCompute(unsigned char * message, unsigned int nBytes)
    94: {
    95:     unsigned int   offset;
    96:     unsigned char  byte;
    97:     width          remainder = INITIAL_REMAINDER;
    98: 
    99: 
   100:     /*
   101:      * Divide the message by the polynomial, a byte at time.
   102:      */
   103:     for (offset = 0; offset < nBytes; offset++)
   104:     {
   105:         byte = (remainder >> (WIDTH - 8)) ^ message[offset];
   106:         remainder = crcTable[byte] ^ (remainder << 8);
   107:     }
   108: 
   109:     /*
   110:      * The final remainder is the CRC result.
   111:      */
   112:     return (remainder ^ FINAL_XOR_VALUE);
   113: 
   114: }   /* crcCompute() */
   115: