ARM - Initial backlight support (#6487)

* Move AVR backlight to own file, add borrowed ARM implementation

* Tiny fix for backlight custom logic

* Remove duplicate board from rebase

* Fix f303 onekey example

* clang-format

* clang-format

* Remove backlight keymap debug

* Initial pass of ARM backlight docs

* Initial pass of ARM backlight docs - resolve todos

* fix rules validation logic

* Add f072 warning

* Add f072 warning

* tidy up breathing in backlight keymap

* tidy up breathing in backlight keymap

* add missing break to backlight keymap

1 files changed, 37 insertions, 13 deletions

diff --git a/docs/feature_backlight.md b/docs/feature_backlight.md
index 556da73859..6a2946fd6d 100644
--- a/docs/feature_backlight.md
+++ b/docs/feature_backlight.md
@@ -1,6 +1,8 @@
 # Backlighting
 
-Many keyboards support backlit keys by way of individual LEDs placed through or underneath the keyswitches. QMK is able to control the brightness of these LEDs by switching them on and off rapidly in a certain ratio, a technique known as *Pulse Width Modulation*, or PWM. By altering the duty cycle of the PWM signal, it creates the illusion of dimming.
+Many keyboards support backlit keys by way of individual LEDs placed through or underneath the keyswitches. This feature is distinct from both the [RGB underglow](feature_rgblight.md) and [RGB matrix](feature_rgb_matrix.md) features as it usually allows for only a single colour per switch, though you can obviously install multiple different single coloured LEDs on a keyboard.
+
+QMK is able to control the brightness of these LEDs by switching them on and off rapidly in a certain ratio, a technique known as *Pulse Width Modulation*, or PWM. By altering the duty cycle of the PWM signal, it creates the illusion of dimming.
 
 The MCU can only supply so much current to its GPIO pins. Instead of powering the backlight directly from the MCU, the backlight pin is connected to a transistor or MOSFET that switches the power to the LEDs.
 
@@ -12,9 +14,8 @@ Most keyboards have backlighting enabled by default if they support it, but if i
 BACKLIGHT_ENABLE = yes
 ```
 
-You should then be able to use the keycodes below to change the backlight level.
-
 ## Keycodes
+Once enabled the following keycodes below can be used to change the backlight level.
 
 |Key      |Description                               |
 |---------|------------------------------------------|
@@ -26,9 +27,9 @@ You should then be able to use the keycodes below to change the backlight level.
 |`BL_DEC` |Decrease the backlight level              |
 |`BL_BRTG`|Toggle backlight breathing                |
 
-## Caveats
+## AVR driver
 
-This feature is distinct from both the [RGB underglow](feature_rgblight.md) and [RGB matrix](feature_rgb_matrix.md) features as it usually allows for only a single colour per switch, though you can obviously use multiple different coloured LEDs on a keyboard.
+### Caveats
 
 Hardware PWM is supported according to the following table:
 
@@ -58,9 +59,9 @@ All other pins will use software PWM. If the [Audio](feature_audio.md) feature i
 
 When both timers are in use for Audio, the backlight PWM will not use a hardware timer, but will instead be triggered during the matrix scan. In this case, breathing is not supported, and the backlight might flicker, because the PWM computation may not be called with enough timing precision.
 
-## Configuration
+### AVR Configuration
 
-To change the behaviour of the backlighting, `#define` these in your `config.h`:
+To change the behavior of the backlighting, `#define` these in your `config.h`:
 
 |Define               |Default      |Description                                                                                                  |
 |---------------------|-------------|-------------------------------------------------------------------------------------------------------------|
@@ -72,14 +73,14 @@ To change the behaviour of the backlighting, `#define` these in your `config.h`:
 |`BREATHING_PERIOD`   |`6`          |The length of one backlight "breath" in seconds                                                              |
 |`BACKLIGHT_ON_STATE` |`0`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low                        |
 
-## Backlight On State
+### Backlight On State
 
 Most backlight circuits are driven by an N-channel MOSFET or NPN transistor. This means that to turn the transistor *on* and light the LEDs, you must drive the backlight pin, connected to the gate or base, *high*.
 Sometimes, however, a P-channel MOSFET, or a PNP transistor is used. In this case, when the transistor is on, the pin is driven *low* instead.
 
 This functionality is configured at the keyboard level with the `BACKLIGHT_ON_STATE` define.
 
-## Multiple backlight pins
+### Multiple backlight pins
 
 Most keyboards have only one backlight pin which control all backlight LEDs (especially if the backlight is connected to an hardware PWM pin).
 In software PWM, it is possible to define multiple backlight pins. All those pins will be turned on and off at the same time during the PWM duty cycle.
@@ -87,13 +88,13 @@ This feature allows to set for instance the Caps Lock LED (or any other controll
 
 To activate multiple backlight pins, you need to add something like this to your user `config.h`:
 
-~~~c
+```c
 #define BACKLIGHT_LED_COUNT 2
 #undef BACKLIGHT_PIN
 #define BACKLIGHT_PINS { F5, B2 }
-~~~
+```
 
-## Hardware PWM Implementation
+### Hardware PWM Implementation
 
 When using the supported pins for backlighting, QMK will use a hardware timer configured to output a PWM signal. This timer will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
 The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the backlight pin will go low, and is pulled high again when the counter resets.
@@ -102,7 +103,7 @@ In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus th
 The breathing effect is achieved by registering an interrupt handler for `TIMER1_OVF_vect` that is called whenever the counter resets, roughly 244 times per second.
 In this handler, the value of an incrementing counter is mapped onto a precomputed brightness curve. To turn off breathing, the interrupt handler is simply disabled, and the brightness reset to the level stored in EEPROM.
 
-## Software PWM Implementation
+### Software PWM Implementation
 
 When `BACKLIGHT_PIN` is not set to a hardware backlight pin, QMK will use a hardware timer configured to trigger software interrupts. This time will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
 When resetting to 0, the CPU will fire an OVF (overflow) interrupt that will turn the LEDs on, starting the duty cycle.
@@ -111,6 +112,29 @@ In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus th
 
 The breathing effect is the same as in the hardware PWM implementation.
 
+## ARM Driver
+
+### Caveats
+
+Currently only hardware PWM is supported, and does not provide automatic configuration.
+
+?> STMF072 support is being investigated.
+
+### ARM Configuration
+
+To change the behavior of the backlighting, `#define` these in your `config.h`:
+
+|Define                  |Default      |Description                                                                                                  |
+|------------------------|-------------|-------------------------------------------------------------------------------------------------------------|
+|`BACKLIGHT_PIN`         |`B7`         |The pin that controls the LEDs. Unless you are designing your own keyboard, you shouldn't need to change this|
+|`BACKLIGHT_PWM_DRIVER`  |`PWMD4`      |The PWM driver to use, see ST datasheets for pin to PWM timer mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
+|`BACKLIGHT_PWM_CHANNEL` |`3`          |The PWM channel to use, see ST datasheets for pin to PWM channel mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
+|`BACKLIGHT_PAL_MODE`    |`2`          |The pin alternative function to use, see ST datasheets for pin AF mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
+|`BACKLIGHT_LEVELS`      |`3`          |The number of brightness levels (maximum 31 excluding off)                                                   |
+|`BACKLIGHT_CAPS_LOCK`   |*Not defined*|Enable Caps Lock indicator using backlight (for keyboards without dedicated LED)                             |
+|`BACKLIGHT_BREATHING`   |*Not defined*|Enable backlight breathing, if supported                                                                     |
+|`BREATHING_PERIOD`      |`6`          |The length of one backlight "breath" in seconds                                                              |
+
 ## Backlight Functions
 
 |Function  |Description                                                |