CRC8校验方式

1、 CRC8标准生成多项式

CRC-8 x8+x5+x4+1 0x31（0x131）
CRC-8 x8+x2+x1+1 0x07（0x107）
CRC-8 x8+x6+x4+x3+x2+x1 0x5E（0x15E

2.以下都以x8+x5+x4+1为例

先计算第一个字节的crc结果，然后把第一个字节的crc结果与第二个字节进行异或，
异或后的值再进行一次crc计算就可以了，多个字节也是反复这过程就好，如下为多个字节
的crc校验代码：

unsigned char crc_high_first(unsigned char ptr, unsigned char len)
{
unsigned char i;
unsigned char crc=0x00； / 计算的初始crc值 */

while(len--)
{crc ^= *ptr++;  /* 每次先与需要计算的数据异或,计算完指向下一数据 */  for (i=8; i>0; --i)   /* 下面这段计算过程与计算一个字节crc一样 */  { if (crc & 0x80)crc = (crc << 1) ^ 0x31;elsecrc = (crc << 1);}
}return (crc);

}
上面的crc计算是纯采用逻辑运行的方式，可以看到，需要的运行量也是不少的，每一个字节
都需要进行8次判断、移位、或异或操作。可以采用查表法，大大减少计算量，先计算出
0x00~0xFF每一个字节的crc校验结果，后面就可以通过表来查出每个字节的crc结果，大大
减少计算量。
下面是一个表生成程序：
（生成表对应多项式：0x31(多项式：x8+x5+x4+1，100110001)）

void  create_crc_table(void)
{unsigned short i;unsigned char j;for (i=0; i<=0xFF; i++){if (0 == (i%16))printf("\n");j = i&0xFF;printf("0x%.2x, ", cal_table_high_first (j));  /*依次计算每个字节的crc校验值*/}
}

得到的表整理后如下：

static const unsigned char crc_table[] =
{
0x00,0x31,0x62,0x53,0xc4,0xf5,0xa6,0x97,0xb9,0x88,0xdb,0xea,0x7d,0x4c,0x1f,0x2e,
0x43,0x72,0x21,0x10,0x87,0xb6,0xe5,0xd4,0xfa,0xcb,0x98,0xa9,0x3e,0x0f,0x5c,0x6d,
0x86,0xb7,0xe4,0xd5,0x42,0x73,0x20,0x11,0x3f,0x0e,0x5d,0x6c,0xfb,0xca,0x99,0xa8,
0xc5,0xf4,0xa7,0x96,0x01,0x30,0x63,0x52,0x7c,0x4d,0x1e,0x2f,0xb8,0x89,0xda,0xeb,
0x3d,0x0c,0x5f,0x6e,0xf9,0xc8,0x9b,0xaa,0x84,0xb5,0xe6,0xd7,0x40,0x71,0x22,0x13,
0x7e,0x4f,0x1c,0x2d,0xba,0x8b,0xd8,0xe9,0xc7,0xf6,0xa5,0x94,0x03,0x32,0x61,0x50,
0xbb,0x8a,0xd9,0xe8,0x7f,0x4e,0x1d,0x2c,0x02,0x33,0x60,0x51,0xc6,0xf7,0xa4,0x95,
0xf8,0xc9,0x9a,0xab,0x3c,0x0d,0x5e,0x6f,0x41,0x70,0x23,0x12,0x85,0xb4,0xe7,0xd6,
0x7a,0x4b,0x18,0x29,0xbe,0x8f,0xdc,0xed,0xc3,0xf2,0xa1,0x90,0x07,0x36,0x65,0x54,
0x39,0x08,0x5b,0x6a,0xfd,0xcc,0x9f,0xae,0x80,0xb1,0xe2,0xd3,0x44,0x75,0x26,0x17,
0xfc,0xcd,0x9e,0xaf,0x38,0x09,0x5a,0x6b,0x45,0x74,0x27,0x16,0x81,0xb0,0xe3,0xd2,
0xbf,0x8e,0xdd,0xec,0x7b,0x4a,0x19,0x28,0x06,0x37,0x64,0x55,0xc2,0xf3,0xa0,0x91,
0x47,0x76,0x25,0x14,0x83,0xb2,0xe1,0xd0,0xfe,0xcf,0x9c,0xad,0x3a,0x0b,0x58,0x69,
0x04,0x35,0x66,0x57,0xc0,0xf1,0xa2,0x93,0xbd,0x8c,0xdf,0xee,0x79,0x48,0x1b,0x2a,
0xc1,0xf0,0xa3,0x92,0x05,0x34,0x67,0x56,0x78,0x49,0x1a,0x2b,0xbc,0x8d,0xde,0xef,
0x82,0xb3,0xe0,0xd1,0x46,0x77,0x24,0x15,0x3b,0x0a,0x59,0x68,0xff,0xce,0x9d,0xac
}；
采用查表法计算crc代码如下：

unsigned char cal_crc_table(unsigned char *ptr, unsigned char len)
{
unsigned char crc = 0x00;

while (len--)
{crc = crc_table[crc ^ *ptr++];
}
return (crc);

}

反序异或的计算
反序异或与顺序异或差异在数据先判断最低位，并且数据是向右移
的，并且多项式数据位需要高低位反转一下。
还是以多项式：x8+x5+x4+1（二进制为：100110001）为例：
则计算一个字节的crc校验代码如下：

unsigned char cal_table_low_first(unsigned char value)
{
unsigned char i, crc;

crc = value;

/* 同样需要计算8次 /
for (i=8; i>0; --i)
{
if (crc & 0x01) / 反序异或变成判断最低位是否为1 /
/ 数据变成往右移位了 /
/ 计算的多项式从0x31（0011 0001）变成了0x8C (1000 1100) /
/ 多项式值，原来的最高位变成了最低位，原来的最低位变成最高位，8位数据高低位交换一下位置 */
crc = (crc >> 1) ^ 0x8C;
else
crc = (crc >> 1);
}