Merge remote-tracking branch 'upstream/master'

* upstream/master: (1239 commits)
  Update ez.c
  removes planck/rev3 temporarily
  Move hand_swap_config to ez.c, removes error for infinity
  Update Makefile
  ergodox: Update algernon's keymap to v1.9
  Added VS Code dir to .gitignore
  Support the Pegasus Hoof controller.
  [Jack & Erez] Simplifies and documents TO
  add readme
  use wait_ms instead of _delay_ms
  add messenger
  init keymap
  Add example keymap
  Adding whiskey_tango_foxtrot_capslock ergodox keymap
  Unicode map framework. Allow unicode up to 0xFFFFF using separate mapping table
  CIE 1931 dim curve
  Apply the dim curve to the RGB output
  Update the Cluecard readme files
  Tune snake and knight intervals for Cluecard
  Tunable RGB light intervals
  ...

1 files changed, 96 insertions, 0 deletions

diff --git a/doc/basic_how_keyboards_work.md b/doc/basic_how_keyboards_work.md
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+# 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.