j1850的crc校验程序

/*
**---------------------------------------------------------------------------
**
** Abstract: Calculate j1850 CRC 
**
** Parameters: Pointer to frame buffer, frame length
**
** Returns: CRC of frame
**
**---------------------------------------------------------------------------
*/
// calculate J1850 message CRC
u8 j1850_crc(u8 *msg_buf, u16 nbytes)
{
 u8 crc_reg=0xff,poly,bit_count;
 u16 byte_count;
 u8 *byte_point;
 u8 bit_point;

 for (byte_count=0, byte_point=msg_buf; byte_count {
  for (bit_count=0, bit_point=0x80 ; bit_count<8; ++bit_count, bit_point>>=1)
  {
   if (bit_point & *byte_point) // case for new bit = 1
   {
    if (crc_reg & 0x80)
     poly=1; // define the polynomial
    else
     poly=0x1c;
    crc_reg= ( (crc_reg << 1) | 1) ^ poly;
   }
   else  // case for new bit = 0
   {
    poly=0;
    if (crc_reg & 0x80)
     poly=0x1d;
    crc_reg= (crc_reg << 1) ^ poly;
   }
  }
 }
 return ~crc_reg; // Return CRC
}