You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
97 lines
3.7 KiB
97 lines
3.7 KiB
9 years ago
|
# 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.
|