CRC-CCITT实施

Question

我使用以下函数生成CRC和,与在线CRC-CCITT计算器相比,它似乎没有返回相同的校验和.

此函数专门使用XMODEM CRC生成,其中0x8408多项式的初始fcs为0xFFFF.

uint16_t crc16(uint8_t byte, uint16_t fcs)
{
    uint8_t bit;

    for(bit=0; bit<8; bit++)
    {
        fcs ^= (byte & 0x01);
        fcs = (fcs & 0x01) ? (fcs >> 1) ^ 0x8408 : (fcs >> 1);
        byte = byte >> 1;
    }
    return fcs;
}

难道我做错了什么？如果我发送0xFF或0x00,我得不到与http://depa.usst.edu.cn/chenjq/www2/SDesign/JavaScript/CRCcalculation.htm相同的校验和

printf("%04X\n", crc16(0x31, 0xFFFF)); //returns 2F8D

Answer 1

看看格雷格库克的出色的CRC目录.有一种变体经常被错误地识别为CCITT CRC,但它并非如此.这就是你的代码,通过0xFFFF初始化,似乎是计算,但反映.Kermit CRC是实际的CCITT CRC.要获得CCITT CRC,您应该从零开始,而不是0xFFFF.XMODEM CRC与Kermit CRC不同,但未反射(因此位在顶部,而您是独占的或与之相对0x1021).

KERMIT
width=16 poly=0x1021 init=0x0000 refin=true refout=true xorout=0x0000 check=0x2189 name="KERMIT"

XMODEM
width=16 poly=0x1021 init=0x0000 refin=false refout=false xorout=0x0000 check=0x31c3 name="XMODEM"

CRC-16/CCITT-FALSE
width=16 poly=0x1021 init=0xffff refin=false refout=false xorout=0x0000 check=0x29b1 name="CRC-16/CCITT-FALSE"

Answer 2

我已经阅读了你的问题,我也有类似你的问题.

我已经解决了在XMODEM中计算CRC-CCITT的这个问题.这里我附加了示例程序来计算CRC-CCITT.

我已经使用在线转换器和此程序尝试了数据.如果您愿意,请使用此功能.

unsigned short crc16(char *ptr, int count)
{
   int  crc;
   char i;
   crc = 0;
   while (--count >= 0)
   {
      crc = crc ^ (int) *ptr++ << 8;
      i = 8;
      do
      {
         if (crc & 0x8000)
            crc = crc << 1 ^ 0x1021;
         else
            crc = crc << 1;
      } while(--i);
   }
   return (crc);
}

CRC应定义为unsigned short,因为该crc16函数返回unsigned short.CRC被定义为一个int,在大多数系统上是4个字节.

Answer 3

static const unsigned short CRC_CCITT_TABLE[256] =
{
    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,
    0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
    0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,
    0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
    0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,
    0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
    0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,
    0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
    0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,
    0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
    0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,
    0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
    0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,
    0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
    0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
    0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
    0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
    0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
    0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
    0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
    0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
    0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
    0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
    0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
    0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
    0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
    0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
    0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
    0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
    0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
    0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
    0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
};

我使用以下代码来计算CRC-CCITT（0xFFFF）：

unsigned short Calculate_CRC_CCITT(const unsigned char* buffer, int size)
{
    unsigned short tmp;
    unsigned short crc = 0xffff;

    for (int i=0; i < size ; i++)
    {
        tmp = (crc >> 8) ^ buffer[i];
        crc = (crc << 8) ^ CRC_CCITT_TABLE[tmp];
    }

    return crc;
}