Multiplexing

Created: 2023-11-11 06:21:24 -0800 Modified: 2023-11-11 06:27:18 -0800

Background

This is sort of weird background information, but every once in a while, I would try to think of how multiplexing works in a keyboard, but I would end up being confused about something or another.

Overview (reference)

The goal is to minimize the number of I/O lines needed.
If you wanted to have 12 switches without multiplexing, you’d need 12 inputs. With multiplexing, you organize the inputs and outputs in a grid. E.g. in a 3xt grid, you would have 4 outputs leading to your switches, then the switches lead to 3 inputs (for a total of 7 I/O lines instead of 12).
The outputs normally emit logic high, but a logic-low signal is moved through each of them sequentially, then the inputs are inspected in each of these phases. If any switches are pressed, the corresponding input will read zero.
For my Corne, the Pro Micro microcontroller has 18 I/O pins, which is more than enough for the 18-21 keys that it supports for each half. E.g. a 6x4 grid would support 24 keys and only take 10 of the pins.
For a traditional 108-key keyboard, you would typically have a 6x18 grid (or 24 GPIO pins) (reference).
There are other ways to do multiplexing, e.g. with a resistor ladder (reference), but I just wanted to jot down some notes to remember how a single form works.