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

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