Write the specified byte value to the GPIO registor.  If no value
        specified the current buffered value will be written.

def _write_gpio(self, gpio=None):
        """Write the specified byte value to the GPIO registor.  If no value
        specified the current buffered value will be written.
        """
        if gpio is not None:
            self.gpio = gpio
        self.i2c.write_list(self.GPIO, self.gpio)