short crc;

int c, i;

unsigned char byte;
unsigned char crcbit;
unsigned char databit;

// init to all ones
crc = 0xFFFF;

for (c = 0; c < length; c++) 
{
	byte = data[c];
	for (i = 0; i < 8; i++) 
	{
	  crcbit = (crc & 0x8000) ? 1 : 0;
	  databit = (byte & 0x80) ? 1 : 0;
	  crc = crc << 1;
	  byte = byte << 1;
	  
          if (crcbit != databit)
            crc = crc ^ 0x1021;
	}
}