Fix breathing always on for soft PWM (#5983)

* Fix breathing always on for soft PWM

* Remove reference to hardware PWM pins in BACKLIGHT_BREATHING description

Now, breathing will only be unsupported when Timers 1 and 3 are both used by Audio

* Document BACKLIGHT_ON_STATE and its purpose

1 files changed, 8 insertions, 2 deletions

diff --git a/docs/feature_backlight.md b/docs/feature_backlight.md
index 5a21a6790..b06db89e4 100644
--- a/docs/feature_backlight.md
+++ b/docs/feature_backlight.md
@@ -64,11 +64,17 @@ 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_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 supported                                                          |
+|`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                        |
+
+## 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, *low*.
+Sometimes, however, a P-channel MOSFET, or a PNP transistor is used. In this case you must `#define BACKLIGHT_ON_STATE 1`, so that when the transistor is on, the pin is driven *high* instead.
 
 ## Multiple backlight pins