Backlight

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.

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.

Usage

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:

BACKLIGHT_ENABLE = yes

You should then be able to use the keycodes below to change the backlight level.

Keycodes

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

Caveats

This feature is distinct from both the RGB underglow and RGB matrix 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 supported according to the following table:

Backlight Pin

AT90USB64/128

ATmega16/32U4

ATmega16/32U2

ATmega32A

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 is disabled or only using one timer, the backlight PWM can be triggered by a hardware timer:

Audio Pin

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

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

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 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

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:

#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. 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. 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 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

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