Debounce test

This repository provides a test harness for debounce functions. The harness is implemented in C, and produces a digital signal with pseudo-random contact bounces and noise interferences.

I have set up this harness to experiment with debounce functions for keyboard firmware. Check the src directory for examples. In particular, I use the EvenOdd algorithm for the Chrumm keyboard. The lowest latency is achieved by the FlipWait algorithm, at the expense of more complexity.

Signal

The test signal imitates a simple SPST-NO push switch (Single Pole, Single Throw, Normally Open), as commonly used for mechanical keyboards. It is assumed to be connected to a digital input pin with a pull-up resistor. Therefore, the pin reads 1 when the switch is open, and 0 when pressed. It is further assumed that the pin is sampled once per millisecond. Each switch event must be detected within 32 ticks.

      Switch   .--[ R ]-- V+
       __|__   |
GND ---O   O---+--------- Pin

Bounce

Relevant bounce statistics have been published in A Guide to Debouncing by Jack Ganssle, with an average bounce time of 1.6 ms and a maximum of 6.2 ms. Switch Bouncing Around by David Ashton reports reasonably similar results, with a maximum of 3 ms.

Keyboard switch manufacturers Gateron and Kailh specify a maximum bounce time of 5 ms. Cherry used to do the same, but nowadays markets "typical" bounce times of less than 1 ms.

The test signal has a maximum bounce time of 8 ticks, to add a bit of a safety margin.

Noise

In addition to bounces, one must also anticipate random noise, as explained in A Guide to Debouncing by Jack Ganssle, or the Ultimate Guide to Switch Debounce by Max Maxfield. Therefore, the tests are run multiple times with different noise saturations. This is rather arbitrary of course, but should serve as a useful sanity check.

Noise avgPeriod=20 (5%) minGap=10:

-----1----------------------1---
-----------------------1--------
-----------1--------------------
----1-------------------1-------

Noise avgPeriod=10 (10%) minGap=2:

-------1--------------1---------
1--------------1---------1------
-------1--1-----------1-----1---
---------1---------------1------

Noise avgPeriod=5 (20%) minGap=1:

-1-----1-1-1-----1----1----1----
--1---1-----1------1---1---1--1-
----1---1----1--1--1-----1---1-1
------1---1-------1-------1-----

Build

Initialize the build directory and compile everything with:

cmake -B build
cmake --build build

Each function is compiled to a separate binary. To compile a specific function only:

cmake --build build --target test-evenodd