Backlight

Backlight is a feature used to control an array of LEDs, usually placed through or under switches.

info

Unlike RGB Underglow, backlight can only control single color LEDs. Additionally, because backlight LEDs all receive the same power, it's not possible to dim individual LEDs.

Enabling Backlight

To enable backlight on your board or shield, add the following line to your .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 or a shield.

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	n
`CONFIG_ZMK_BACKLIGHT_AUTO_OFF_USB`	Turn off backlight when USB is disconnected	n

Adding Backlight to a board or a shield

Adding to a board
Adding to a shield

First, you must enable PWM by adding the following lines to your Kconfig.defconfig file:

if ZMK_BACKLIGHT

config PWM
    default y

config LED_PWM
    default y

endif # ZMK_BACKLIGHT

Then you have to add the following lines to your .dts file:

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

The value ch0-pin represents the pin that controls the LEDs. With nRF52 boards, you can calculate the value to use in the following way: 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 inside the root devicetree node on the same file as before:

/ {
    backlight: pwmleds {
        compatible = "pwm-leds";
        label = "Backlight LEDs";
        pwm_led_0 {
            pwms = <&pwm0 45>;
            label = "Backlight LED 0";
        };
    };
};

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

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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;
    };
};

You must first add a boards/ directory within your shield folder. For each board that supports the shield you must create a <board>.defconfig file and a <board>.overlay file inside the boards/ folder.

Inside your <board>.defconfig file, add the following lines:

if ZMK_BACKLIGHT

config PWM
    default y

config LED_PWM
    default y

endif # ZMK_BACKLIGHT

Then add the following lines to your .overlay file:

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

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

If your shield 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 inside the root devicetree node on the same file:

/ {
    backlight: pwmleds {
        compatible = "pwm-leds";
        label = "Backlight LEDs";
        pwm_led_0 {
            pwms = <&pwm0 45>;
            label = "Backlight LED 0";
        };
    };
};

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

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

/ {
    chosen {
        zmk,backlight = &backlight;
    };
}:

Optionally, on Pro Micro compatible shields you can add a LED GPIO node to your devicetree, this could be useful if you want your shield to be compatible with newer or untested boards. To do that you have to enable CONFIG_LED_GPIO in your .conf file and then add the following lines inside the root devicetree node of your .dtsi or .dts file:

/ {
    backlight: gpioleds {
        compatible = "gpio-leds";
        label = "Backlight LEDs";
        gpio_led_0 {
            gpios = <&pro_micro 20 GPIO_ACTIVE_HIGH>;
            label = "Backlight LED 0";
        };
    };
};

If no suitable <board>.overlay file is found, this node will act as a fallback, however, without PWM, backlight has limited functionality.

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 */
    };
    ...
};

Enabling Backlight​

Configuring Backlight​

Adding Backlight to a board or a shield​

Multiple backlight LEDs​

Enabling Backlight

Configuring Backlight

Adding Backlight to a board or a shield

Multiple backlight LEDs