Fix #3566 use an hardware timer for software PWM stability (#3615)

With my XD60, I noticed that when typing the backlight was flickering.

The XD60 doesn't have the backlight wired to a hardware PWM pin.
I assumed it was a timing issue in the matrix scan that made the PWM
lit the LED a bit too longer. I verified it because the more keys that
were pressed, the more lighting I observed.

This patch makes the software PWM be called during CPU interruptions.
It works almost like the hardware PWM, except instead of using
the CPU waveform generation, the CPU will fire interruption
when the LEDs need be turned on or off.

Using the same timer system as for hardware PWM, when the counter
will reach OCRxx (the current backlight level), an Output Compare
match interrupt will be fired and we'll turn the LEDs off.
When the counter reaches its maximum value, an overflow interrupt
will be triggered in which we turn the LEDs on.
This way we replicate the hardware backlight PWM duty cycle.

This gives a better time stability of the PWM computation than pure
software PWM, leading to a flicker free backlight.

Since this is reusing the hardware PWM code, software PWM also supports
backlight breathing.

Note that if timer1 is used for audio, backlight will use timer3, and if
timer3 is used for audio backlight will use timer1.
If both timers are used for audio, then this feature is disabled and we
revert to the matrix scan based PWM computation.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>

1 files changed, 50 insertions, 2 deletions

diff --git a/docs/feature_backlight.md b/docs/feature_backlight.md
index c7a1f131e..048d75390 100644
--- a/docs/feature_backlight.md
+++ b/docs/feature_backlight.md
@@ -30,7 +30,31 @@ You should then be able to use the keycodes below to change the backlight level.
 
 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.
 
-Hardware PWM is only supported on certain pins of the MCU, so if the backlighting is not connected to one of them, a software implementation will be used, and backlight breathing will not be available. Currently the supported pins are `B5`, `B6`, `B7`, and `C6`.
+Hardware PWM is only supported on certain pins of the MCU, so if the backlighting is not connected to one of them, a software PWM implementation triggered by hardware timer interrupts will be used.
+
+Hardware PWM is supported according to the following table:
+
+| Backlight Pin | Hardware timer |
+|---------------|----------------|
+|`B5`           | Timer 1        |
+|`B6`           | Timer 1        |
+|`B7`           | Timer 1        |
+|`C6`           | Timer 3        |
+| other         | Software PWM   |
+
+The [audio feature](feature_audio.md) also uses hardware timers. Please refer to the following table to know what hardware timer the software PWM will use depending on the audio configuration:
+
+| Audio Pin(s) | Audio Timer | Software PWM Timer |
+|--------------|-------------|--------------------|
+| `C4`         | Timer 3     | Timer 1            |
+| `C5`         | Timer 3     | Timer 1            |
+| `C6`         | Timer 3     | Timer 1            |
+| `B5`         | Timer 1     | Timer 3            |
+| `B6`         | Timer 1     | Timer 3            |
+| `B7`         | Timer 1     | Timer 3            |
+| `Bx` & `Cx`  | Timer 1 & 3 | None               |
+
+When all timers are in use for [audio](feature_audio.md), the backlight software PWM will not use a hardware timer, but instead will be triggered during the matrix scan. In this case the backlight doesn't support breathing and might show lighting artifacts (for instance flickering), because the PWM computation might not be called with enough timing precision.
 
 ## Configuration
 
@@ -39,11 +63,26 @@ To change the behaviour 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 15 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 hardware PWM is used                                                          |
+|`BACKLIGHT_BREATHING`|*Not defined*|Enable backlight breathing, if supported                                                          |
 |`BREATHING_PERIOD`   |`6`          |The length of one backlight "breath" in seconds                                                              |
 
+## 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 }
+~~~
+
 ## 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.
@@ -53,6 +92,15 @@ 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
+
+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.
+The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the CPU will fire a Compare Output match interrupt, which will turn the LEDs off.
+In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus the brightness, where `0x0000` is completely off and `0xFFFF` is completely on.
+
+The breathing effect is the same as in the hardware PWM implementation.
+
 ## Backlight Functions
 
 |Function  |Description                                                |