RC low-pass filter that can expand the output port of microcomputer

It is almost a foregone conclusion of Moore's Law: Next year, the microcomputer will have more features, and the software development team will have bigger ideas. Unfortunately, the number of output pins has not increased. It is difficult to find a spare output port for diagnostics, test or standard I/O. The single-pin "bus" in Figure 1 can provide countless parallel outputs with simple additional hardware. The microcomputer output with an RC low-pass filter controls the serial/parallel converter HC164. To input data to the serial/parallel converter, each bit contains a 1-0-1 transition, which changes the length of the low state. If the low state is longer than the time constant of the low-pass filter, a 0 is transferred to the register. If the low state is shorter, a 1 is transferred to the register. The clock and data signals are thus combined into a single signal. The low-pass filter separates the clock signal from the data signal (Figure 2).

Although the circuit in Figure 1 can control slow-reacting devices such as relays or LCDs, if used with LEDs, the HC164 will produce unpleasant flickering when writing. To solve this problem, the circuit in Figure 3 uses a serial-in/parallel-out register 4094, which has a strobe input that allows all outputs to be updated simultaneously with no intermediate levels. A pair of monostable circuits provide the data and strobe signals, and this circuit should be able to control parallel devices such as a display module based on the HD44780.