A programmable serial input-output controller is provided. A timer circuit performs a timing operation. An input pin is configured to receive an input signal from an external circuit. An output pin is configured to provide an output signal to the external circuit. In an output mode, the sequence controller provides an initial level to the output pin and controls the timer circuit to perform the timing operation. In response to a duration of the timer circuit performing the timing operation reaching first transmission time, the sequence controller inverts the level of the output pin and controls the timer circuit to re-perform the timing operation. In response to the duration of the timer circuit re-performing the timing operation reaching second transmission time, the sequence controller inverts the level of the output pin.