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// Define Global variables

// message buffer variables

unsigned char msg_buf[20], nbytes, bit_point;
unsigned char * byte_point;

// Switch to select VPW PWM or ISO  0, 1, 2,  respectively

unsigned char protocol;

// values to define protocol variable

#define	vpw	0			
#define	pwm	1
#define     iso	2

/*********************************************************************
CRC CALCULATION SUBROUTINE

Calculates the crc as defined by SAE, or the checksum for ISO
*********************************************************************/
// This routine assumes that all the data bytes are in the array msg_buf[]
//   Starting with msg_buf[0] as the first byte.
// nbytes indicates how many bytes are in the array.
// The subroutine calculates both the checksum and the CRC byte as
//  defined by SAE and ISO specifications.
// Either the CRC or the Checksum is returned, depending on what
//  protocol is selected. 

unsigned char crc(void)
{
	unsigned char crc_reg=0xff,poly,i,j, checksum=0;
	for (i=0, byte_point=msg_buf; i<nbytes; ++i, ++byte_point)
	{
		for (j=0, bit_point=0x80 ; j<8; ++j, 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;
			}
		}

		checksum += *byte_point;			// Calculate checksum
	}
	if (protocol==iso) return checksum;			// Iso uses checksum,
	return ~crc_reg;					// Otherwise, use CRC
}