zmk/docs/docs/features/backlight.md

title	sidebar_label
Backlight	Backlight

import Tabs from '@theme/Tabs'; import TabItem from '@theme/TabItem';

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

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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

Since the Zephyr 3.2 update the PWM driver has been revised, permitting the use of the pinctrl API to select the pins. For now the old method of pin select is also functional, as long as you don't enable pinctrl for the board at all.

If you do not want to use pinctrl add the following lines to your .dts file:

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

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>.

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 0 10000 PWM_POLARITY_NORMAL>;
            label = "Backlight LED 0";
        };
    };
};

The value inside pwm_led_0 after &pwm0 must be the channel number. Since (<ch0-pin>) is used in the &pwm0 node the channel in this example is 0.

In this example 10000 is the period of the PWM waveform, some drive circuitry might require different values, it could also be altered in the event the drive frequency is audible.

If your board uses a P-channel MOSFET to control backlight instead of a N-channel MOSFET, you may want to change PWM_POLARITY_NORMAL for PWM_POLARITY_INVERTED.

:::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;
    };
};

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

When using the pinctrl API you should create a pinctrl.dtsi file if it's not created already, and include it at the beginning of the board.dts file. You also need to add CONFIG_PINCTRL=y to board_defconfig if it's not already enabled.

A pinctrl file has an &pinctrl node that encompasses all pinctrl settings including for I2C or SPI peripherals (e.g. WS2812 LEDs, Battery fuel gauges):

&pinctrl {
    // Other pinctrl definitions for other hardware
    pwm0_default: pwm0_default {
        group1 {
            psels = <NRF_PSEL(PWM_OUT0, 1, 13)>;
        };
    };
    pwm0_sleep: pwm0_sleep {
        group1 {
            psels = <NRF_PSEL(PWM_OUT0, 1, 13)>;
            low-power-enable;
        };
    };
};

The pin number is handled differently depending on the MCU in question, on nRF boards it is defined as (PWM_OUTX, Y, Z) where X is the PWM channel used (usually 0) Y is first part of the hardware port "PY.01" and Z is the second part of the hardware port "P1.Z".

For example, P1.13 would give you (PWM_OUT0, 1, 13) and P0.15 would give you (PWM_OUT0, 0, 15).

Then you add the PWM device inside the board.dts file and assign the pinctrl definitions to it:

&pwm0 {
    status = "okay";
    pinctrl-0 = <&pwm0_default>;
    pinctrl-1 = <&pwm0_sleep>;
    pinctrl-names = "default", "sleep";
};

Then you have to add the following lines inside the root devicetree node on the same file as before:

/ {
    backlight: pwmleds {
        compatible = "pwm-leds";
        pwm_led_0 {
            pwms = <&pwm0 0 10000 PWM_POLARITY_NORMAL>;
        };
    };
};

The value inside pwm_led_0 after &pwm0 must be the channel number. Since (PWM_OUT0) is defined in the pinctrl node the channel in this example is 0.

In this example 10000 is the period of the PWM waveform, some drive circuitry might require different values, it could also be altered in the event the drive frequency is audible.

If your board uses a P-channel MOSFET to control backlight instead of a N-channel MOSFET, you may want to change PWM_POLARITY_NORMAL for PWM_POLARITY_INVERTED.

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

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

Since the Zephyr 3.2 update the PWM driver has been revised, permitting the use of the pinctrl API to select the pins. For now the old method of pin select is also functional, but note that if you are modifying a shield and the board already has pinctrl enabled then you have to use the pinctrl API as well.

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>;
};

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>.

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 10000 PWM_POLARITY_NORMAL>;
            label = "Backlight LED 0";
        };
    };
};

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

In this example 10000 is the period of the PWM waveform, some drive circuitry might require different values, it could also be altered in the event the drive frequency is audible.

If your board uses a P-channel MOSFET to control backlight instead of a N-channel MOSFET, you may want to change PWM_POLARITY_NORMAL for PWM_POLARITY_INVERTED.

:::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;
    };
}:

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.

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:

&pinctrl {
    // Other pinctrl definitions for other hardware
    pwm0_default: pwm0_default {
        group1 {
            psels = <NRF_PSEL(PWM_OUT0, 1, 13)>;
        };
    };
    pwm0_sleep: pwm0_sleep {
        group1 {
            psels = <NRF_PSEL(PWM_OUT0, 1, 13)>;
            low-power-enable;
        };
    };
};

The pin number is handled differently depending on the MCU in question, on nRF boards it is defined as (PWM_OUTX, Y, Z) where X is the PWM channel used (usually 0) Y is first part of the hardware port "PY.01" and Z is the second part of the hardware port "P1.Z".

For example, P1.13 would give you (PWM_OUT0, 1, 13) and P0.15 would give you (PWM_OUT0, 0, 15).

Then you add the PWM device inside the .overlay file and assign the pinctrl definitions to it:

&pwm0 {
    status = "okay";
    pinctrl-0 = <&pwm0_default>;
    pinctrl-1 = <&pwm0_sleep>;
    pinctrl-names = "default", "sleep";
};

Then you have to add the following lines inside the root devicetree node on the same file as before:

/ {
    backlight: pwmleds {
        compatible = "pwm-leds";
        pwm_led_0 {
            pwms = <&pwm0 0 10000 PWM_POLARITY_NORMAL>;
        };
    };
};

In this example 10000 is the period of the PWM waveform, some drive circuitry might require different values, it could also be altered in the event the drive frequency is audible.

If your board uses a P-channel MOSFET to control backlight instead of a N-channel MOSFET, you may want to change PWM_POLARITY_NORMAL for PWM_POLARITY_INVERTED.

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 */
    };
    ...
};