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.
Most keyboards have backlighting enabled by default if they support it, but if it is not working for you (or you have added support), check that your `rules.mk` includes the following:
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.
Most keyboards have only one backlight pin which controls all backlight LEDs (especially if the backlight is connected to a hardware PWM pin).
The `timer` and `software` drivers allow you 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 configure multiple backlight pins, add something like this to your `config.h`, instead of `BACKLIGHT_PIN`:
```c
#define BACKLIGHT_PINS { F5, B2 }
```
## Driver Configuration :id=driver-configuration
Backlight driver selection is configured in `rules.mk`. Valid drivers are `pwm` (default), `timer`, `software`, or `custom`. See below for information on individual drivers.
This driver is similar to the PWM driver, but instead of directly configuring the pin to output a PWM signal, an interrupt handler is attached to the timer to turn the pin on and off as appropriate.
```make
BACKLIGHT_DRIVER = timer
```
### Software Driver :id=software-driver
In this mode, PWM is "emulated" while running other keyboard tasks. It offers maximum hardware compatibility without extra platform configuration. However, breathing is not supported, and the backlight can flicker when the keyboard is busy.
```make
BACKLIGHT_DRIVER = software
```
### Custom Driver :id=custom-driver
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 a simple API.
```make
BACKLIGHT_DRIVER = custom
```
```c
void backlight_init_ports(void) {
// Optional - runs on startup
// Usually you want to configure pins here
}
void backlight_set(uint8_t level) {
// Optional - runs on level change
// Usually you want to respond to the new value
}
void backlight_task(void) {
// Optional - runs periodically
// Note that this is called in the main keyboard loop,
// so long running actions here can cause performance issues
The following table describes the supported pins for the PWM driver. Only cells marked with a timer number are capable of hardware PWM output; any others must use the `timer` driver.
|`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|
Refer to the ST datasheet for your particular MCU to determine these values. For example, these defaults are set up for pin `B8` on a Proton-C (STM32F303) using `TIM4_CH3` on AF2. Unless you are designing your own keyboard, you generally should not need to change them.
### Timer Driver :id=arm-timer-driver
Depending on the ChibiOS board configuration, you may need to enable general-purpose timers at the keyboard level. For STM32, this would look like:
Since 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.
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.
