zmk/docs/docs/behaviors/outputs.md

title	sidebar_label
Output Selection Behavior	Output Selection

Summary

The output behavior allows selecting whether keyboard output is sent to the USB or bluetooth connection when both are connected. This allows connecting a keyboard to USB for power but outputting to a different device over bluetooth.

By default, output is sent to USB when both USB and BLE are connected. Once you select a different output, it will be remembered until you change it again.

:::note[Powering the keyboard via USB] ZMK is not always able to detect if the other end of a USB connection accepts keyboard input or not. So if you are using USB only to power your keyboard (for example with a charger or a portable power bank), you will want to select the BLE output through below behavior to be able to send keystrokes to the selected bluetooth profile. :::

Output Command Defines

Output command defines are provided through the dt-bindings/zmk/outputs.h header, which is added at the top of the keymap file:

#include <dt-bindings/zmk/outputs.h>

This allows you to reference the actions defined in this header:

Define	Action
OUT_USB	Prefer sending to USB
OUT_BLE	Prefer sending to the current bluetooth profile
OUT_TOG	Toggle between USB and BLE

Output Selection Behavior

The output selection behavior changes the preferred output on press.

Behavior Binding

• Reference: &out
• Parameter #1: Command, e.g. OUT_BLE

:::note[Output selection persistence] The endpoint that is selected by the &out behavior will be saved to flash storage and hence persist across restarts and firmware flashes. However it will only be saved after CONFIG_ZMK_SETTINGS_SAVE_DEBOUNCE milliseconds in order to reduce potential wear on the flash memory. :::

Examples

1. Behavior binding to prefer sending keyboard output to USB

   &out OUT_USB
   

2. Behavior binding to prefer sending keyboard output to the current bluetooth profile

   &out OUT_BLE
   

3. Behavior binding to toggle between preferring USB and BLE

   &out OUT_TOG