zmk_mf68/docs/docs/features/backlight.md

title	sidebar_label
Backlight	Backlight

Backlight is a feature used to control array of LEDs, usually placed through or under switches. Unlike RGB Underglow, backlight currently allows only one color per LED, also LEDs are not addressable, so you can't control individual LEDs.

Enabling Backlight

To enable backlight on your board or shield, simply enable the CONFIG_ZMK_BACKLIGHT configuration values in the .conf file of your user config directory as such:

CONFIG_ZMK_BACKLIGHT=y

If your board or shield does not have backlight configured, refer to Adding Backlight to a Board.

Configuring Backlight

There are various Kconfig options used to configure the backlight feature. These can all be set in the .conf file.

Option	Description	Default
CONFIG_ZMK_BACKLIGHT_BRT_STEP	Brightness step in percent	20
CONFIG_ZMK_BACKLIGHT_BRT_START	Default brightness in percent	40
CONFIG_ZMK_BACKLIGHT_ON_START	Default backlight state	y
CONFIG_ZMK_BACKLIGHT_AUTO_OFF_IDLE	Turn off backlight when keyboard goes into idle state	y
CONFIG_ZMK_BACKLIGHT_AUTO_OFF_USB	Turn off backlight when USB is disconnected	n

Adding Backlight to a Board

Backlight is always added to a board, not a shield. If you have a shield with backlight, you must add a boards/ directory within your shield folder to define the backlight individually for each board that supports the shield. Inside the boards/ folder, you define a <board>.overlay for each different board.

First, you need to enable PWM by adding the following lines to your .overlay file:

&pwm0 {
	status = "okay";
	ch0-pin = <45>;
	/* ch0-inverted; */
};

The value ch0-pin represents the pin that controls the LEDs. To calculate the value to use, you need a bit of math. You need the hardware port and run it through a function. 32 * X + Y = <Pin number> where X is first part of the hardware port "PX.01" and Y is the second part of the hardware port "P1.Y".

For example, P1.13 would give you 32 * 1 + 13 = <45> and P0.15 would give you 32 * 0 + 15 = <15>.

If your board uses a P-channel MOSFET to control backlight instead of a N-channel MOSFET, you may want to enable ch0-inverted.

Then you have to add the following lines to your .dtsi file inside the root devicetree node:

backlight: pwmleds {
    compatible = "pwm-leds";
    label = "Backlight LEDs";
    pwm_led_0 {
        pwms = <&pwm0 45>;
    };
};

The value inside pwm_led_0 must be the same as you used before.

:::info Note that every LED inside of the backlight node will be treated as a backlight LED, so if you have other PWM LEDs you need to declare them in a separate node. Refer to Multiple backlight LEDs if you have multiple backlight LEDs. :::

Finally you need to add backlight to the chosen element of the root devicetree node:

chosen {
    ...
    zmk,backlight = &backlight;
};

Multiple backlight LEDs

It is possible to control multiple backlight LEDs at the same time. This is useful if, for example, you have a Caps Lock LED connected to a different pin and you want it to be part of the backlight.

In order to do that, first you need to enable PWM for each pin:

&pwm0 {
	status = "okay";
	ch0-pin = <45>; /* LED 0 */
	ch1-pin = <46>; /* LED 1 */
	ch2-pin = <47>; /* LED 2 */
    ...
};

This part may vary based on your MCU as different MCUs may have a different number of modules and channels.

Then you can simply add each of your LED to the backlight node:

backlight: pwmleds {
    compatible = "pwm-leds";
    label = "Backlight LEDs";
    pwm_led_0 {
        pwms = <&pwm0 45>; /* LED 0 */
    };
    pwm_led_1 {
        pwms = <&pwm0 46>; /* LED 1 */
    };
    pwm_led_2 {
        pwms = <&pwm0 47>; /* LED 2 */
    };
    ...
};