def set_crc(self):
        """Set Userdata[13] and Userdata[14] to the CRC value."""
        data = self.bytes[6:20]
        crc = int(0)
        for b in data:
            # pylint: disable=unused-variable
            for bit in range(0, 8):
                fb = b & 0x01
                fb = fb ^ 0x01 if (crc & 0x8000) else fb
                fb = fb ^ 0x01 if (crc & 0x4000) else fb
                fb = fb ^ 0x01 if (crc & 0x1000) else fb
                fb = fb ^ 0x01 if (crc & 0x0008) else fb
                crc = ((crc << 1) | fb) & 0xffff
                b = b >> 1
        self._userdata['d13'] = (crc >> 8) & 0xff
        self._userdata['d14'] = crc & 0xff
Set Userdata[13] and Userdata[14] to the CRC value.