unsigned char crc8(unsigned char *ptr, unsigned char len)   
{  
  unsigned char i;  
  unsigned char crc=0;  
  while(len--!=0)   
   {  
     for(i=1; i!=0; i*=2)  
      {  
        if((crc&1)!=0) {crc/=2; crc^=0x8C;} 
        else crc/=2;  
        if((*ptr&i)!=0) crc^=0x8C;      
       }   
     ptr++;  
    }  
return(crc);  
}