From d9bef1658e062d76f87a752daa30047d865dca0f Mon Sep 17 00:00:00 2001 From: Jiehong Ma Date: Sun, 31 Jul 2016 16:49:31 +0200 Subject: [PATCH] feature: add basic doc about how a keyboard works on USB This comes from the discussion on #520 --- doc/basic_how_keyboards_work.md | 96 +++++++++++++++++++++++++++++++++ 1 file changed, 96 insertions(+) create mode 100644 doc/basic_how_keyboards_work.md diff --git a/doc/basic_how_keyboards_work.md b/doc/basic_how_keyboards_work.md new file mode 100644 index 0000000000..73c3f5c5fc --- /dev/null +++ b/doc/basic_how_keyboards_work.md @@ -0,0 +1,96 @@ +# How keys are registered, and interpreted by computers + +In this file, you can will learn the concepts of how keyboards work over USB, +and you'll be able to better understand what you can expect from changing your +firmware directly. + +## Schematic view + +Whenever you type on 1 particular key, here is the chain of actions taking +place: + +``` text ++------+ +-----+ +----------+ +----------+ +----+ +| User |-------->| Key |------>| Firmware |----->| USB wire |---->| OS | ++------+ +-----+ +----------+ +----------+ |----+ +``` + +This scheme is a very simple view of what's going on, and more details follow +in the next sections. + +## 1. You Press a Key + +Whenever you press a key, the firmware of your keyboard can register this event. +It can register when the key is pressed, held and released. + +This usually happens with a [periodic scan of key presses with a frequency around 100 hz](https://github.com/benblazak/ergodox-firmware/blob/master/references.md#typical-keyboard-information). +This speed often is limited by the mechanical key response time, the protocol +to transfer those key presses (here USB HID), and by the software it is used in. + +## 2. What the Firmware Sends + +The [HID specification](http://www.usb.org/developers/hidpage/Hut1_12v2.pdf) +tells what a keyboard can actually send through USB to have a chance to be +properly recognised. This includes a pre-defined list of keycodes which are +simple numbers from `0x00` to `0xE7`. The firmware assigns a keycode to each +key of the keyboard. + +The firmware does not send actually letters or characters, but only keycodes. +Thus, by modifying the firmware, you only can modify what keycode is sent over +USB for a given key. + +## 3. What the Operating System Does + +Once the keycode reaches the operating system, a piece of software has to have +it match an actual character thanks to a keyboard layout. For example, if your +layout is set to QWERTY, a sample of the matching table is as follow: + +``` text +| keycode | character | +|---------+-----------| +| 0x04 | a/A | +| 0x05 | b/B | +| 0x06 | c/C | +| ... | ... | +| 0x1C | y/Y | +| 0x1D | z/Z | +| ... | ... | +|---------+-----------| +``` + +## Back to the firmware + +As the layout is generally fixed (unless you create your own), the firmware can +actually call a keycode by its layout name directly to ease things for you. + +This is exactly what is done here with `KC_A` actually representing `0x04` in +QWERTY. The full list can be found in `keycode.txt`. + +## List of Characters You Can Send + +Putting aside shortcuts, having a limited set of keycodes mapped to a limited +layout means that **the list of characters you can assign to a given key only +is the ones present in the layout**. + +For example, this means that if you have a QWERTY US layout, and you want to +assign 1 key to produce `€` (euro currency symbol), you are unable to do so, +because the QWERTY US layout does not have such mapping. You could fix that by +using a QWERTY UK layout, or a QWERTY US International. + +You may wonder why a keyboard layout containing all of Unicode is not devised +then? The limited number of keycode available through USB simply disallow such +a thing. + +## How to (Maybe) Enter Unicode Characters + +You can have the firmware send *sequences of keys* to use the [software Unicode +Input +Method](https://en.wikipedia.org/wiki/Unicode_input#Hexadecimal_code_input) of +the target operating system, thus effectively entering characters independently +of the layout defined in the OS. + +Yet, it does come with multiple disadvantages: + + - Tied to a specific OS a a time (need recompilation when changing OS); + - Within a given OS, does not work in all software; + - Limited to a subset of Unicode on some systems.