Can I use 3-to-8 demultiplexer to implement 20+8 layout with 24 pins?

[thesmallcreeper]

First of all, great project! I found an old model M 122-keys and I want to restore it 😄

As you mention in README.md, 26 GPIO pins are not enough for driving 20+8 pins.
I looked at .cpp MatrixScanner::scan() and, as far as I understand, you set one column high and the other 7 discharged and floating.

pico-model-m/MatrixScanner.cpp

Lines 77 to 89 in 6fefd26

	void MatrixScanner::scan() {
	uint64_t now;
	bool temp;
	// loop through each send pin and then check each read pin
	for (uint8_t i = 0; i < NUM_ACROSS; i++) {
	gpio_set_dir(across[i], GPIO_OUT);
	gpio_put(across[i], 1);
	sleep_us(30); // delay for changes to GPIO to settle
	uint32_t readings = gpio_get_all();
	setpininput(across[i]); // so that the send pin floats and won't cause a bus conflict

Can I drive the 8 columns with 4 GPIO + multiplexer? 3 GPIO pins select one of 2^3=8 pins and 1 GPIO drives the selected column. What you think? Can I have your opinion? 😄

[guruthree]

Always great to see more old stuff being re-used - I hope your 122 restoration goes well!

I looked at .cpp MatrixScanner::scan() and, as far as I understand, you set one column high and the other 7 discharged and floating.

Yes that's correct.

I've not done much with multiplexers, which is why in favour of keeping the circuit simple I went with a PGA2040. That aside, I think a multiplexer on the 8 output pins should work as long as the pins can still remain floating when not in use. My understanding is that this would be the case with a CD4051BE, which would give 8 outputs for 4, letting the circuit use 24 pins and fit on the Pico. The other alternative I think would be an GPIO expander for some of the 20 input pins, something like a MCP23017, giving 16 inputs for 2. I think the CD4051BE would probably be the more elegant of the two solutions, letting the code still use gpio_get_all() and having a faster response time than the MCP23017.

[thesmallcreeper]

@guruthree Thank you for your reply!
Yes, looking at specs CD4051BE should do the trick.

I will keep you update 😄

[thesmallcreeper]

Hey! Typing you from my Model M, using an Raspberry Pi Pico + your repo with some changes.
I will create an Pull Request soon to add support for using an CD4051BE.
Thank you so much for the time you put in for this script! Also thank you for adding F15+0 hotkey for getting in programming mode.

Some words about my hardware implementation.
I used a classic Pico and a CD4051BE. I got the membrane connector by force from original circuit and soldered them on an solder-able breadboard from behind the holes. Not great idea, because connectors wires where short, after breaking them, and didn't make good contact. I resorted at putting wires directly to some of connector pins. The result is kinda fragile but it works! For how long...? I don't know :P
CD4051BE have no problem working at 3.3V.

[guruthree]

Fantastic it's working! If the connectors go I believe you can get replacement FFC connectors from Te with part numbers 5-520315-8 and 7-520355-0. I looked into doing a PCB but ultimately decided to solder onto the back of the existing one myself.