def write_port(self, port, value):
        """ Use a whole port as a bus and write a byte to it.

        :param port: Name of the port ('A' or 'B')
        :param value: Value to write (0-255)
        """
        if port == 'A':
            self.GPIOA = value
        elif port == 'B':
            self.GPIOB = value
        else:
            raise AttributeError('Port {} does not exist, use A or B'.format(port))
        self.sync()