---
title: Backlight
sidebar_label: 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](underglow.md), 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](#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

<Tabs
defaultValue="shield"
values={[
{label: 'Adding to a board', value: 'board'},
{label: 'Adding to a shield', value: 'shield'},
]}>
<TabItem value="board">

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.

:::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](#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;
    };
};
```

</TabItem>
<TabItem value="shield">

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; */
};
```

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 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.

:::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](#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.

</TabItem>
</Tabs>

### 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 */
    };
    ...
};
```
