calculatecrc.c

Atmega8L控制CC1020 TI的另外一款使用很多的RF ＩＣ。

/* 
demonstrates how the incorrect check value of 0x29B1 may be reported 
for the test string "123456789" when it should be 0xE5CC. 
*/ 
#include <stdio.h> 
#include <string.h>  


#define   poly     0x1021          /* CRC-CCITT mask  x^16 + x^12 +x^5 + 1  */
/* global variables */ 
/*char text[2];*/ 
unsigned int good_crc;  
//unsigned char  text_length =  9 
void update_good_crc(unsigned int ch) 
{ 
    unsigned int  v;
    unsigned char i,xor_flag; 

    /* 
    Align test bit with leftmost bit of the message byte. 
    */ 
    v = 0x80; 

    for (i=0; i<8; i++) 
    { 
        if (good_crc & 0x8000) 
        { 
            xor_flag= 1; 
        } 
        else 
        { 
            xor_flag= 0; 
        } 
        good_crc = good_crc << 1; 

        if (ch & v) 
        { 
            /* 
            Append next bit of message to end of CRC if it is not zero. 
            The zero bit placed there by the shift above need not be 
            changed if the next bit of the message is zero. 
            */ 
            good_crc= good_crc + 1; 
        } 

        if (xor_flag) 
        { 
            good_crc = good_crc ^ poly; 
        } 

        /* 
        Align test bit with next bit of the message byte. 
        */ 
        v = v >> 1; 
    } 
} 

void augment_message_for_good_crc() 
{ 
    unsigned char i, xor_flag; 

    for (i=0; i<16; i++) 
    { 
        if (good_crc & 0x8000) 
        { 
            xor_flag= 1; 
        } 
        else 
        { 
            xor_flag= 0; 
        } 
        good_crc = good_crc << 1; 

        if (xor_flag) 
        { 
            good_crc = good_crc ^ poly; 
        } 
    } 
} 



/*****************************************************************************************
Parameters: text---->point to the start of the string that is going to be CRC calculated.
            Text_length-----> the lenth of the string that you want to calculate

return value:  16 bit CRC value
******************************************************************************************/

 unsigned int Calcu_CRC(unsigned char * text,unsigned char  text_length) 
{ 
   // unsigned int ch;
    unsigned char i; 

    good_crc = 0xffff; 
    
  /*  while((ch=text[i])!=0) */

    for (i=0; i<text_length; i++)
    {     
        update_good_crc(text[i]);     
    }
    
    augment_message_for_good_crc(); 
                                                              
    
    return(good_crc);   
    
}