guruthree/pico-model-m

Code for the Pimoroni PGA2040 (a Raspberry Pi RP2040-powered board) to act as a USB keyboard by scanning the membrane of a Model M 122-key keyboard.

pico-model-m's Issues

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

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? 😄