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