Document the ATMegaKeyboard helper a bit

Fixes #463.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

NEWS.md

@@ -85,6 +85,8 @@ Kaleidoscope has been ported to the following devices:
 
 For more information, please see the hardware plugins' documentation.
 
+To make it easier to port Kaleidoscope, we introduced the [ATMegaKeyboard](doc/plugin/ATMegaKeyboard.md) base class. For any board that's based on the ATMega MCU and a simple matrix, this might be a good foundation to develop the hardware plugin upon.
+
 ## New plugins
 
 ### IdleLEDs

doc/plugin/ATMegaKeyboard.md

@@ -0,0 +1,82 @@
+# ATMegaKeyboard
+
+A lot of custom keyboards are built upon the ATMega MCU (most often an
+`atmega32u4`), and the vast majority of them follow a similar architecture, so
+much so that the vast majority of code can be lifted out into a base class,
+making porting to these keyboards trivial. All we have to do is tell it our
+pinout, and we're all done.
+
+## Porting hardware using the ATMegaKeyboard class
+
+The plugin assumes a simple matrix layout, which we can tell it by using the
+`ATMEGA_KEYBOARD_CONFIG` macro, which takes two arguments: a list of row and
+column pins. In the `.cpp`, simply use `ATMEGA_KEYBOARD_DATA`, with the hardware
+object as its argument:
+
+```c++
+// Kaleidoscope-Vendor-ExampleHardware.h
+#define KALEIDOSCOPE_WITH_ATMEGA_KEYBOARD 1
+#include "kaleidoscope/hardware/vendor/ExampleHardware.h"
+
+// kaleidoscope/hardware/vendor/ExampleHardware.h
+namespace kaleidoscope {
+namespace hardware {
+namespace vendor {
+class ExampleKeyboard: public ATMegaKeyboard {
+ public:
+  ExampleKeyboard() {}
+
+  ATMEGA_KEYBOARD_CONFIG(
+    ROW_PIN_LIST({PIN_D1, PIN_D2}),
+    COL_PIN_LIST({PIN_F6, PIN_F7})
+  );
+
+  static constexpr int8_t led_count = 0;
+};
+
+#define KEYMAP(  \
+   r0c0 ,r0c1    \
+  ,r1c0 ,r1c1    \
+)                \
+ { r0c0 ,r0c1 }, \
+ { r1c0 ,r1c1 }
+
+}
+}
+}
+
+// kaleidoscope/hardware/vendor/ExampleHardware.cpp
+namespace kaleidoscope {
+namespace hardware {
+namespace vendor {
+
+ATMEGA_KEYBOARD_DATA(ExampleHardware);
+constexpr int8_t ExampleHardware::led_count;
+
+}
+}
+}
+
+HARDWARE_IMPLEMENTATION KeyboardHardware;
+kaleidoscope::hardware::vendor::ExampleHardware &ExampleHardware = KeyboardHardware;
+```
+
+## Overriding methods
+
+For performance and space-saving reasons, the base class does not use virtual
+methods. Instead, whenever it calls a method of its own, it will call it through
+the `KeyboardHardware` singleton object, which is always an instance of the
+exact hardware plugin. Thus, implementing a function in the subclass will shadow
+the one in the base class.
+
+This can be used to implement more efficient methods, would it be needed. The
+[Atreus][hardware:atreus] port does this, for example, by overriding the generic
+`readCols` with a more specialised, faster implementation.
+
+## Further reading
+
+See the [Planck][hardware:planck] and [Atreus][hardware:atreus] ports for an
+example of how this class can be used in practice.
+
+ [hardware:planck]: ../../src/kaleidoscope/hardware/olkb/Planck.h
+ [hardware:atreus]: ../../src/kaleidoscope/hardware/technomancy/Atreus.h