RGB Underglow

RGB underglow is a feature used to control "strips" of RGB LEDs. Most of the time this is called underglow and creates a glow underneath the board using a ring of LEDs around the edge, hence the name. However, this can be extended to be used to control anything from a single LED to a long string of LEDs anywhere on the keyboard.

ZMK supports all the RGB LEDs supported by Zephyr. Here's the current list supported:

  • WS2812-ish (WS2812B, WS2813, SK6812, or compatible)
  • APA102
  • LPD880x (LPD8803, LPD8806, or compatible)

Of the compatible types, the WS2812 LED family is by far the most popular type. Currently each of these types of LEDs are expected to be run using SPI with a couple of exceptions.

Here you can see the RGB underglow feature in action using WS2812 LEDs.

Enabling RGB Underglow

To enable RGB underglow on your board or shield, simply enable the ZMK_RGB_UNDERGLOW and X_STRIP configuration values in the .conf file of your user config directory as such:

CONFIG_ZMK_RGB_UNDERGLOW=y
# Use the STRIP config specific to the LEDs you're using
CONFIG_WS2812_STRIP=y

If your board or shield does not have RGB underglow configured, refer to Adding RGB Underglow to a Board.

Configuring RGB Underglow

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

OptionDescriptionDefault
ZMK_RGB_UNDERGLOW_HUE_STEPHue step in degrees of 360 used by RGB actions10
ZMK_RGB_UNDERGLOW_SAT_STEPSaturation step in percent used by RGB actions10
ZMK_RGB_UNDERGLOW_BRT_STEPBrightness step in percent used by RGB actions10
ZMK_RGB_UNDERGLOW_HUE_STARTDefault hue 0-359 in degrees0
ZMK_RGB_UNDERGLOW_SAT_STARTDefault saturation 0-100 in percent100
ZMK_RGB_UNDERGLOW_BRT_STARTDefault brightness 0-100 in percent100
ZMK_RGB_UNDERGLOW_SPD_STARTDefault effect speed 1-53
ZMK_RGB_UNDERGLOW_EFF_STARTDefault effect integer from the effect enum0
ZMK_RGB_UNDERGLOW_ON_STARTDefault on statey

Adding RGB Underglow to a Board

RGB underglow is always added to a board, not a shield. This is a consequence of needing to configure SPI to control the LEDs. If you have a shield with RGB underglow, you must add a boards/ directory within your shield folder to define the RGB underglow individually for each board that supports the shield. Inside the boards/ folder, you define a <board>.overlay for each different board. For example, the Kyria shield has a boards/nice_nano.overlay file that defines the RGB underglow for the nice_nano board specifically.

The first step to adding support for underglow is to select you SPI output. With nRF52 boards, you can just use &spi1 and define the pins you want to use. For other boards, you must select an SPI definition that has the MOSI pin as your data pin going to your LED strip.

Here's an example of an nRF52 SPI definition:

&spi1 {
compatible = "nordic,nrf-spi";
status = "okay";
mosi-pin = <6>;
// Unused pins, needed for SPI definition, but not used by the ws2812 driver itself.
sck-pin = <5>;
miso-pin = <7>;
led_strip: ws2812@0 {
compatible = "worldsemi,ws2812-spi";
label = "WS2812";
/* SPI */
reg = <0>; /* ignored, but necessary for SPI bindings */
spi-max-frequency = <4000000>;
/* WS2812 */
chain-length = <10>; /* number of LEDs */
spi-one-frame = <0x70>;
spi-zero-frame = <0x40>;
};
};
info

If you are configuring SPI for an nRF52840 (or other nRF52) based board, double check that you are using pins that aren't restricted to low frequency I/O. Ignoring these restrictions may result in poor wireless performance. You can find the list of low frequency I/O pins here.

Here's another example for a non-nRF52 board on spi1:

&spi1 {
led_strip: ws2812@0 {
compatible = "worldsemi,ws2812-spi";
label = "WS2812";
/* SPI */
reg = <0>;
spi-max-frequency = <5250000>;
/* WS2812 */
chain-length = <10>; /* number of LEDs */
spi-one-frame = <0x70>; /* make sure to configure this properly for your SOC */
spi-zero-frame = <0x40>; /* make sure to configure this properly for your SOC */
};
};

Once you have your led_strip properly defined you need to add it to the root devicetree node chosen element:

/ {
chosen {
zmk,underglow = &led_strip;
};
};

Finally you need to enable the ZMK_RGB_UNDERGLOW and X_STRIP configuration values in the .conf file of your board (or set a default in the Kconfig.defconfig):

CONFIG_ZMK_RGB_UNDERGLOW=y
# Use the STRIP config specific to the LEDs you're using
CONFIG_WS2812_STRIP=y