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.
This usually happens with a periodic scan of key presses. 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 [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 scancodes which are simple numbers from `0x00` to `0xE7`. The firmware assigns a scancode 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
The firmware does not send actually letters or characters, but only scancodes.
Thus, by modifying the firmware, you only can modify what scancode is sent over
USB for a given key.
## 3. What the Operating System Does
@ -45,49 +39,31 @@ 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 |
| ... | ... |
|---------+-----------|
```
|---------|-----------|
| 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`.
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**.
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.
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.
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.
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.
skullY
8 years ago