C语言CRC-8 ITU格式校验函数

CRC校验基于前处理和后处理的不同，由不同的协会标准推出了一些不同格式的版本。这里介绍CRC-8 ITU格式的校验函数。

CRC-8 ITU格式特征

标准CRC-8的校验函数参考： C语言标准CRC-8校验函数
CRC-8 ITU格式和标准CRC-8校验算法比较，增加了输出数据异或0x55的后处理，因此在标准CRC-8校验函数上增加此部分即可实现CRC-8 ITU格式校验函数。

CRC-8 ITU校验函数正向算法

正向算法是符合标准CRC-8的计算理论，从左向右计算，也即计算过程中移位时，向左移出。几种正向算法的实现如下：

CRC-8 ITU格式校验函数一(8位输入数据格式，64位装载计算)：

#include <stdio.h>
#include <stdlib.h>
uint8_t PY_CRC_8_ITU(uint8_t *di, uint32_t len)
{   //Written by Pegasus Yu 2022/04/22
	uint16_t crc_poly = 0x0107; //X^8+X^2+X^1+1 total 9 effective bits. Computed total data shall be compensated 8-bit '0' before CRC computing.
	uint8_t *datain;
	uint64_t cdata = 0; //Computed total data
	uint16_t data_t = 0; //Process data of CRC computing

	uint16_t index_t = 63;  ///bit shifting index for initial '1' searching
	uint16_t index = 63;    //bit shifting index for CRC computing
	uint8_t rec = 0; //bit number needed to be compensated for next CRC computing

	uint32_t cn=(len+1)/7;
	uint32_t cr=(len+1)%7;

	uint32_t j;

	datain = malloc(len+1);
	for(j=0;j<len;j++)
	{
		datain[j]=di[j];
	}
	    datain[len]=0; //Compensate 8-bit '0' for input data

     if(len<=7)
     {
    	 for(j=0;j<=len;j++)
    	 {
    		 cdata = (cdata<<8);
    		 cdata = cdata|datain[j];
    	 }
    	 cn = 1;
     }
     else
     {
    	 if(cr==0)
    	 {
    		 cr=7;
    	 }
         if(cr==1)
    	 {
    		 cr=8;
    		 cn++;
    	 }
    	 else
    	 {
    		 cn++;
    	 }

    	 for(j=0;j<cr;j++)
    	 {
    		 cdata = (cdata<<8);
    		 cdata = cdata|datain[j];
    	 }
     }

     do
     {
 		cn--;

 		while(index_t>0)
 		{
 			if( (cdata>>index_t)&1 )
 			{
 				index = index_t;
 				index_t = 0;

 				data_t |= (cdata>>(index-8));
 				{
 					data_t = data_t ^ crc_poly;
 				}

 	            while((index!=0x5555)&&(index!=0xaaaa))
 	            {
 	            	/*
 	    			if ((data_t>>7)&1) rec = 1;
 	    			else if ((data_t>>6)&1) rec = 2;
 	    			else if ((data_t>>5)&1) rec = 3;
 	    			else if ((data_t>>4)&1) rec = 4;
 	    			else if ((data_t>>3)&1) rec = 5;
 	    			else if ((data_t>>2)&1) rec = 6;
 	    			else if ((data_t>>1)&1) rec = 7;
 	    			else if ((data_t>>0)&1) rec = 8;
 	    			else rec = 9; */

	 	    		for(uint8_t n=1;n<9;n++)
	 	    		{
	 	    			if ((data_t>>(8-n))&1) {rec = n;break;}
	 	    			if (n==8) rec=9;
	 	    		}

 	    			if((index-8)<rec)
 	    			{
 	    				data_t = data_t<<(index-8);
 	    				data_t |=  (uint16_t)((cdata<<(64-(index-8)))>>(64-(index-8)));
 	    				index = 0x5555;
 	    			}
 	    			else
 	    			{
 	        			for(uint8_t i=1;i<=rec;i++)
 	        			{
 	        				data_t = (data_t<<1)|((cdata>>(index-8-i))&1) ;
 	        			}

 	        			if(rec!= 9)
 	        			{
 	        				data_t = data_t ^ crc_poly;
 	        				index -= rec;
 	        			}
 	        			else
 	        			{
 	        				data_t = 0;
 	        				index_t = index-8-1;
 	        				index = 0xaaaa;

 	        			}

 	    			}

 	            }
 				if(index==0x5555) break;
 			}
 			else
 			{
 				index_t--;
 				if(index_t<8) break;
 			}
         }

 		if(cn>0) //next segment
 		{
  			cdata = data_t&0x00ff;

 			for(uint8_t k=0;k<7;k++)
 			{
 	    		 cdata = (cdata<<8);
 	    		 cdata = cdata|datain[j++];

 			}

 	    	data_t = 0;
 	 		index_t = 63;  ///bit shifting index for initial '1' searching
 	 		index = 63;    //bit shifting index for CRC computing
 	 		rec = 0; //bit number needed to be compensated for next CRC computing
 		}

     }
     while(cn>0);

     free(datain);
     return (uint8_t)(data_t ^ 0x55);
}

CRC-8 ITU格式校验函数二（8位输入数据格式）：

#include <stdio.h>
#include <stdlib.h>
uint8_t PY_CRC_8_S_ITU(uint8_t *di, uint32_t len)
{
	uint8_t crc_poly = 0x07; //X^8+X^2+X^1+1 total 8 effective bits without X^8. Computed total data shall be compensated 8-bit '0' before CRC computing.

	uint32_t clen = len+1;
	uint8_t cdata[clen] ;
	memcpy(cdata, di, len); cdata[len]=0;
	uint8_t data_t = cdata[0]; //CRC register

    for (uint32_t i = 1; i < clen; i++)
    {
        for (uint8_t j = 0; j <= 7; j++)
        {
            if(data_t&0x80)
            	data_t = ( (data_t<<1) | ( (cdata[i]>>(7-j))&0x01) ) ^ crc_poly;
            else
            	data_t = ( (data_t<<1) | ( (cdata[i]>>(7-j))&0x01) ) ;
        }
    }
    return (data_t ^ 0x55);
}

CRC-8 ITU格式校验函数三（8位输入数据格式）：

uint8_t PY_CRC_8_T_ITU(uint8_t *di, uint32_t len)
{
	uint8_t crc_poly = 0x07; //X^8+X^2+X^1+1 total 8 effective bits without X^8.
	uint8_t data_t = 0;

    while(len--)
    {
    	data_t ^=  *di++;
        for (int8_t i=8; i>0; --i)
        {
            if (data_t & 0x80)
            	data_t = (data_t<<1) ^ crc_poly;
            else
            	data_t = (data_t<<1);
        }
    }

    return (data_t ^ 0x55);
}