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feature_backlight.md

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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 and RGB matrix 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.

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.

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

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:

BACKLIGHT_DRIVER = software

Valid driver values are pwm, software, custom or no. See below for help on individual drivers.

To configure 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
BACKLIGHT_LIMIT_VAL 255 The maximum duty cycle of the backlight -- 255 allows for full brightness, any lower will decrease the maximum.

Unless you are designing your own keyboard, you generally should not need to change the BACKLIGHT_PIN or 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 :id=avr-driver

The pwm driver is configured by default, however the equivalent setting within rules.mk would be:

BACKLIGHT_DRIVER = pwm

Caveats :id=avr-caveats

On AVR boards, QMK automatically decides which driver to use according to the following table:

Backlight Pin AT90USB64/128 AT90USB162 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 Timer 1
C4 Timer 3
C5 Timer 3 Timer 1 Timer 1
C6 Timer 3 Timer 1 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
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 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 :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. 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.

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. 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.

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:

BACKLIGHT_DRIVER = pwm

ChibiOS Configuration :id=arm-configuration

The following #defines apply only to ARM-based keyboards:

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

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.

Caveats :id=arm-caveats

Currently only hardware PWM is supported, not timer assisted, and does not provide automatic configuration.

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:

BACKLIGHT_DRIVER = software

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, 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, add something like this to your config.h, instead of BACKLIGHT_PIN:

#define BACKLIGHT_PINS { F5, B2 }

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 this simple API provided by QMK. To enable, add this to your rules.mk:

BACKLIGHT_DRIVER = custom

Then implement any of these hooks:

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
}

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 for more information.

Backlight example circuit