9.5、CRC16校验值计算
/*******CRC16 CCITT-1(LSB)********/
CRC16 CCITT-1(LSB)计算方式有以下三种，每种得出结果是相同的，具体区别见下：
方式一：占用空间最小，计算速度最慢；
unsigned short calc_crc_ccitt_1(unsigned short reg_init, unsigned char *ptr, unsigned long len)
{
unsigned char i;
unsigned short crc = reg_init;
while(len--)
{
crc ^= *ptr;
for (i = 0; i < 8; i++)
{
if(crc & 0x0001)
{
crc = (crc >> 1) ^ 0x8408;
}
else
{
crc >>= 1;
}
}
ptr++;
}
return(crc);
}
(注：初始值是 CRC16_CCITT_INIT 0xffff，后面的值是每次算出的CRC值)
方式二：占用空间居中，计算速度居中；如非特殊要求，建议使用此种，占用空间不算大，而且计算速度也很快；
const unsigned short crc16_ccitt_table_1_4[16] =
{ /* CRC余式表 */
0x0000, 0x1081, 0x2102, 0x3183, 0x4204, 0x5285, 0x6306, 0x7387,
0x8408, 0x9489, 0xa50a, 0xb58b, 0xc60c, 0xd68d, 0xe70e, 0xf78f
};
unsigned short calc_crc_ccitt_1(unsigned short reg_init, unsigned char *ptr, unsigned long len)
{
unsigned short crc = reg_init;
while(len--)
{
/* CRC的高4位和本字节的后半字节相加后查表计算CRC，然后再加上上一次CRC的余数 */
crc = (crc >> 4) ^ crc16_ccitt_table_1_4[(crc ^ (*ptr)) & 0x0F];
/* CRC的高4位和本字节的前半字节相加后查表计算CRC，然后加上上一次CRC的余数 */
crc = (crc >> 4) ^ crc16_ccitt_table_1_4[(crc ^ ((*ptr) >> 4)) & 0x0F];
ptr++;
}
return(crc);
}
方式三：占用空间最大，但计算速度最快。
/****下表是常用ccitt 16,生成式1021反转成8408后的查询表格****/
const unsigned short crc16_ccitt_table_1_8[256] =
{
0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};
/*****CRC计算函数,可将第一个参数reg_init简化掉********/
unsigned short calc_crc_ccitt_1(unsigned short reg_init, unsigned char *ptr, unsigned long len)
{
unsigned short crc = reg_init;
while (len--)
crc = (crc >> 8) ^ crc16_ccitt_table_1_8[(crc ^ *ptr++) & 0xff];
return(crc);
}
示例：
WORD CRC16; //CRC16结果
unsigned char dwDataLength;//待校验的数据长度
BYTE* pData//指向待校验数据内容指针
CRC16 = 0XFFFF;//CRC16初始值
CRC16 = calc_crc_ccitt_1 (CRC16,(BYTE*)(& dwDataLength), 2);//先从数据长度开始计算CRC16值
CRC16 = calc_crc_ccitt_1 (CRC16, pData, dwDataLength);//再从文件数据开始计算CRC16值
CRC16 ~= CRC16;//得到结果按位取反或者与0xFFFF按位异或；
注：CRC校验数据从控制符CTRL开始(包含CTRL)，到命令内容结束(包含命令内容)。