Backlight docs wordsmithing (#9631)

1 files changed, 115 insertions, 144 deletions

diff --git a/docs/feature_backlight.md b/docs/feature_backlight.md
index 7e4401b40..9e467c708 100644
--- a/docs/feature_backlight.md
+++ b/docs/feature_backlight.md
@@ -1,4 +1,4 @@
-# Backlighting
+# Backlighting :id=backlighting
 
 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.
 
@@ -6,103 +6,106 @@ QMK is able to control the brightness of these LEDs by switching them on and off
 
 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.
 
-## Feature Configuration
-
 Most keyboards have backlighting enabled by default if they support it, but if it is not working for you, check that your `rules.mk` includes the following:
 
 ```makefile
 BACKLIGHT_ENABLE = yes
 ```
 
-## Keycodes
-Once enabled the following keycodes below can be used to change the backlight level.
-
-|Key      |Description                               |
-|---------|------------------------------------------|
-|`BL_TOGG`|Turn the backlight on or off              |
-|`BL_STEP`|Cycle through backlight levels            |
-|`BL_ON`  |Set the backlight to max brightness       |
-|`BL_OFF` |Turn the backlight off                    |
-|`BL_INC` |Increase the backlight level              |
-|`BL_DEC` |Decrease the backlight level              |
-|`BL_BRTG`|Toggle backlight breathing                |
-
-## Backlight Functions
-
-|Function  |Description                                                |
-|----------|-----------------------------------------------------------|
-|`backlight_toggle()`    |Turn the backlight on or off                 |
-|`backlight_enable()`    |Turn the backlight on                        |
-|`backlight_disable()`   |Turn the backlight off                       |
-|`backlight_step()`      |Cycle through backlight levels               |
-|`backlight_increase()`  |Increase the backlight level                 |
-|`backlight_decrease()`  |Decrease the backlight level                 |
-|`backlight_level(x)`    |Sets the backlight level to specified level  |
-|`get_backlight_level()` |Return the current backlight level           |
-|`is_backlight_enabled()`|Return whether the backlight is currently on |
-
-### Backlight Breathing Functions
-
-|Function  |Description                                        |
-|----------|---------------------------------------------------|
-|`breathing_toggle()`  |Turn the backlight breathing on or off |
-|`breathing_enable()`  |Turns on backlight breathing           |
-|`breathing_disable()` |Turns off backlight breathing          |
-
-## Driver Configuration
+## Keycodes :id=keycodes
+
+Once enabled, the following keycodes below can be used to change the backlight level.
+
+|Key      |Description                        |
+|---------|-----------------------------------|
+|`BL_TOGG`|Turn the backlight on or off       |
+|`BL_STEP`|Cycle through backlight levels     |
+|`BL_ON`  |Set the backlight to max brightness|
+|`BL_OFF` |Turn the backlight off             |
+|`BL_INC` |Increase the backlight level       |
+|`BL_DEC` |Decrease the backlight level       |
+|`BL_BRTG`|Toggle backlight breathing         |
+
+## Functions :id=functions
+
+These functions can be used to change the backlighting in custom code:
+
+|Function                |Description                                 |
+|------------------------|--------------------------------------------|
+|`backlight_toggle()`    |Turn the backlight on or off                |
+|`backlight_enable()`    |Turn the backlight on                       |
+|`backlight_disable()`   |Turn the backlight off                      |
+|`backlight_step()`      |Cycle through backlight levels              |
+|`backlight_increase()`  |Increase the backlight level                |
+|`backlight_decrease()`  |Decrease the backlight level                |
+|`backlight_level(x)`    |Sets the backlight level to specified level |
+|`get_backlight_level()` |Return the current backlight level          |
+|`is_backlight_enabled()`|Return whether the backlight is currently on|
+
+If backlight breathing is enabled (see below), the following functions are also available:
+
+|Function             |Description                           |
+|---------------------|--------------------------------------|
+|`breathing_toggle()` |Turn the backlight breathing on or off|
+|`breathing_enable()` |Turns on backlight breathing          |
+|`breathing_disable()`|Turns off backlight breathing         |
+
+## Configuration :id=configuration
 
 To select which driver to use, configure your `rules.mk` with the following:
 
 ```makefile
-BACKLIGHT_DRIVER = software # Valid driver values are 'pwm,software,no'
+BACKLIGHT_DRIVER = software
 ```
 
-See below for help on individual drivers.
+Valid driver values are `pwm`, `software`, `custom` or `no`. See below for help on individual drivers.
 
-## Common Driver Configuration
+To configure the backlighting, `#define` these in your `config.h`:
 
-To change the behavior of the backlighting, `#define` these in your `config.h`:
+|Define               |Default      |Description                                                                          |
+|---------------------|-------------|-------------------------------------------------------------------------------------|
+|`BACKLIGHT_PIN`      |*Not defined*|The pin that controls the LED(s)                                                     |
+|`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_ON_STATE` |`1`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low|
 
-|Define               |Default      |Description                                                                           |
-|---------------------|-------------|--------------------------------------------------------------------------------------|
-|`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_ON_STATE` |`0`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low |
+Unless you are designing your own keyboard, you generally should not need to change the `BACKLIGHT_PIN` or `BACKLIGHT_ON_STATE`.
 
-### Backlight On State
+### Backlight On State :id=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.
 
-## AVR driver
+### AVR Driver :id=avr-driver
+
+The `pwm` driver is configured by default, however the equivalent setting within `rules.mk` would be:
 
-On AVR boards, the default driver currently sniffs the configuration to pick the best scenario. The driver is configured by default, however the equivalent setting within rules.mk would be:
 ```makefile
 BACKLIGHT_DRIVER = pwm
 ```
 
-### Caveats
+#### Caveats :id=avr-caveats
 
-Hardware PWM is supported according to the following table:
+On AVR boards, QMK automatically decides which driver to use according to the following table:
 
 |Backlight Pin|AT90USB64/128|ATmega16/32U4|ATmega16/32U2|ATmega32A|ATmega328/P|
-|-------------|-------------|-------------|-------------|---------|----------|
-|`B1`         |             |             |             |         |Timer 1   |
-|`B2`         |             |             |             |         |Timer 1   |
-|`B5`         |Timer 1      |Timer 1      |             |         |          |
-|`B6`         |Timer 1      |Timer 1      |             |         |          |
-|`B7`         |Timer 1      |Timer 1      |Timer 1      |         |          |
-|`C4`         |Timer 3      |             |             |         |          |
-|`C5`         |Timer 3      |             |Timer 1      |         |          |
-|`C6`         |Timer 3      |Timer 3      |Timer 1      |         |          |
-|`D4`         |             |             |             |Timer 1  |          |
-|`D5`         |             |             |             |Timer 1  |          |
-
-All other pins will use software PWM. If the [Audio](feature_audio.md) feature is disabled or only using one timer, the backlight PWM can be triggered by a hardware timer:
+|-------------|-------------|-------------|-------------|---------|-----------|
+|`B1`         |             |             |             |         |Timer 1    |
+|`B2`         |             |             |             |         |Timer 1    |
+|`B5`         |Timer 1      |Timer 1      |             |         |           |
+|`B6`         |Timer 1      |Timer 1      |             |         |           |
+|`B7`         |Timer 1      |Timer 1      |Timer 1      |         |           |
+|`C4`         |Timer 3      |             |             |         |           |
+|`C5`         |Timer 3      |             |Timer 1      |         |           |
+|`C6`         |Timer 3      |Timer 3      |Timer 1      |         |           |
+|`D4`         |             |             |             |Timer 1  |           |
+|`D5`         |             |             |             |Timer 1  |           |
+
+All other pins will use timer-assisted software PWM:
 
 |Audio Pin|Audio Timer|Software PWM Timer|
 |---------|-----------|------------------|
@@ -113,44 +116,9 @@ All other pins will use software PWM. If the [Audio](feature_audio.md) feature i
 |`B6`     |Timer 1    |Timer 3           |
 |`B7`     |Timer 1    |Timer 3           |
 
-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.
-
-### AVR 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_PINS`     |*Not defined*|experimental: see below for more information                                                                 |
-|`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_ON_STATE` |`1`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low                        |
-
-### 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
-
-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.
-This feature allows to set for instance the Caps Lock LED (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped LCTRL in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on.
-
-To activate multiple backlight pins, you need to add something like this to your user `config.h`:
-
-```c
-#define BACKLIGHT_LED_COUNT 2
-#undef BACKLIGHT_PIN
-#define BACKLIGHT_PINS { F5, B2 }
-```
+When both timers are in use for Audio, the backlight PWM cannot use a hardware timer, and 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.
 
-### Hardware PWM Implementation
+#### Hardware PWM Implementation :id=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.
@@ -159,7 +127,7 @@ 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.
 
-### Timer Assisted PWM Implementation
+#### Timer Assisted PWM Implementation :id=timer-assisted-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.
@@ -168,81 +136,84 @@ 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
+### ARM Driver :id=arm-configuration
+
+While still in its early stages, ARM backlight support aims to eventually have feature parity with AVR. The `pwm` driver is configured by default, however the equivalent setting within `rules.mk` would be:
 
-While still in its early stages, ARM backlight support aims to eventually have feature parity with AVR. The driver is configured by default, however the equivalent setting within rules.mk would be:
 ```makefile
 BACKLIGHT_DRIVER = pwm
 ```
 
-### Caveats
+#### ChibiOS Configuration :id=arm-configuration
 
-Currently only hardware PWM is supported, not timer assisted, and does not provide automatic configuration.
+The following `#define`s apply only to ARM-based keyboards:
 
-?> Backlight support for STMF072 has had limited testing, YMMV. If unsure, set `BACKLIGHT_ENABLE = no` in your rules.mk.
+|Define                 |Default|Description                        |
+|-----------------------|-------|-----------------------------------|
+|`BACKLIGHT_PWM_DRIVER` |`PWMD4`|The PWM driver to use              |
+|`BACKLIGHT_PWM_CHANNEL`|`3`    |The PWM channel to use             |
+|`BACKLIGHT_PAL_MODE`   |`2`    |The pin alternative function to use|
 
-### ARM Configuration
+See the ST datasheet for your particular MCU to determine these values. Unless you are designing your own keyboard, you generally should not need to change them.
 
-To change the behavior of the backlighting, `#define` these in your `config.h`:
+#### Caveats :id=arm-caveats
 
-|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|
+Currently only hardware PWM is supported, not timer assisted, and does not provide automatic configuration.
+
+?> Backlight support for STM32F072 has had limited testing, so YMMV. If unsure, set `BACKLIGHT_ENABLE = no` in your `rules.mk`.
+
+### Software PWM Driver :id=software-pwm-driver
 
-## Software PWM Driver :id=software-pwm-driver
+In this mode, PWM is "emulated" while running other keyboard tasks. It offers maximum hardware compatibility without extra platform configuration. The tradeoff is the backlight might jitter when the keyboard is busy. To enable, add this to your `rules.mk`:
 
-Emulation of PWM while running other keyboard tasks, it offers maximum hardware compatibility without extra platform configuration. The tradeoff is the backlight might jitter when the keyboard is busy. To enable, add this to your rules.mk:
 ```makefile
 BACKLIGHT_DRIVER = software
 ```
 
-### Software PWM 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_PINS` |*Not defined*|experimental: see below for more information                                                                 |
-
-### Multiple backlight pins
+#### Multiple Backlight Pins :id=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.
-This feature allows to set for instance the Caps Lock LED (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped LCTRL in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on.
+In software PWM, it is possible to define multiple backlight pins, which will be turned on and off at the same time during the PWM duty cycle.
+
+This feature allows to set, for instance, the Caps Lock LED's (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped Control in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on, as it is usually wired to a separate pin from the backlight.
 
-To activate multiple backlight pins, you need to add something like this to your user `config.h`:
+To activate multiple backlight pins, add something like this to your `config.h`, instead of `BACKLIGHT_PIN`:
 
 ```c
-#undef BACKLIGHT_PIN
 #define BACKLIGHT_PINS { F5, B2 }
 ```
 
-## Custom Driver
+### Custom Driver :id=custom-driver
 
-To enable, add this to your rules.mk:
+If none of the above drivers apply to your board (for example, you are using a separate IC to control the backlight), you can implement a custom backlight driver using this simple API provided by QMK. To enable, add this to your `rules.mk`:
 
 ```makefile
 BACKLIGHT_DRIVER = custom
 ```
 
-When implementing the custom driver API, the provided keyboard hooks are as follows:
+Then implement any of these hooks:
 
 ```c
 void backlight_init_ports(void) {
-    // Optional - Run on startup
-    //          - usually you want to configure pins here
+    // Optional - runs on startup
+    //   Usually you want to configure pins here
 }
 void backlight_set(uint8_t level) {
-    // Optional - Run on level change
-    //          - usually you want to respond to the new value
+    // Optional - runs on level change
+    //   Usually you want to respond to the new value
 }
 
 void backlight_task(void) {
-    // Optional - Run periodically
-    //          - long running actions here can cause performance issues
+    // Optional - runs periodically
+    //   Note that this is called in the main keyboard loop,
+    //   so long running actions here can cause performance issues
 }
 ```
+
+## Example Schematic
+
+In this typical example, the backlight LEDs are all connected in parallel towards an N-channel MOSFET. Its gate pin is wired to one of the microcontroller's GPIO pins through a 470Ω resistor to avoid ringing.
+A pulldown resistor is also placed between the gate pin and ground to keep it at a defined state when it is not otherwise being driven by the MCU.
+The values of these resistors are not critical - see [this Electronics StackExchange question](https://electronics.stackexchange.com/q/68748) for more information.
+
+![Backlight example circuit](https://i.imgur.com/BmAvoUC.png)