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Utility to detect "bouncing" keyboard keys
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README.md

keydebounce-detect

A Linux utility to detect "bouncing" keys on computer keyboards.

Very small and simple, but could be of use to someone else who also has a keyboards suffering from "bouncing" keys.

Background

Computer keyboards -- at least those with Cherry mechanical keyswitches -- apparently tend to develop keys that "bounce" or "chatter" -- that send two instances of a keystroke.

I've noticed it after about four years of using a Leopold FC200RT, but this is not specific to this keyboard and is common across a number of keyboards using Cherry keyswitches.

It's common for switches to electrically have a certain amount of "bounce". However, normally this is filtered out by the keyboard controller -- I believe that with Cherry keyswitches, bounce is normally expected to be under 5 milliseconds. Whatever mechanical issue arises with older Cherry keyswitches is not being caught by the keyboard controller.

The issue arises on a key-by-key basis. Some keys may not be affected, and some may be.

There are several techniques for diagnosing the problem. One easily-done manual one is to simply hit a key, then the space key. It's reasonably easy for a user to just hit one and then the other fairly quickly, and then a bouncing key becomes easy to see:

Correctly-working keyboard:

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

Keyboard with a bouncing 'l' key:

l l l l l l l l ll l l l l l l l l l l l l l l l l l l l l l

Unfortunately, this is still somewhat-error prone. I also wanted a utility that could pick up on it without as much user effort. Windows has built-in functionality ("Filter Keys") intended for handicapped users that can detect rapid double presses and ignore them; "real" keypresses come in at only a limited rate. Xorg on Linux has similar functionality (accessed via various software packages calling XkbSetBounceKeysDelay()).

keydebounce-detect simply reads keypresses, and waits for rapid duplicate keypresses.

Compile

Type "make".

Usage

Run "./keybounce-detect".

Then start whacking keys on the keyboard. If one bounces, keybounce-detect will provide the name of the key and the delay between the duplicate keypresses.

I saw a delay of ~200000 microseconds on my bouncing key.

The default minimum-time-between-keypresses of 1000000 microseconds worked for me, but if you can hit your key more than ten times a second, you might want to use a different number; pass it as follows:

"./keybounce-detect 50000"

Once you've determined the time on the bounce, you can select an appropriate setting to have the current session of Xorg filter out the keybouncing via "xkbset bouncekeys ".

Caveats

It's a terminal application, so it can't look for modifier keys. Could be possible to make an X11 version.

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