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Combo keys are a way to combine multiple keypresses to output a different key. For example, you can hit the Q and W keys on your keyboard to output escape.


Combos configured in your .keymap file, but are separate from the keymap node found there, since they are processed before the normal keymap. They are specified like this:

/ {    combos {        compatible = "zmk,combos";        combo_esc {            timeout-ms = <50>;            key-positions = <0 1>;            bindings = <&kp ESC>;        };    };};
  • The name of the combo doesn't really matter, but convention is to start the node name with combo_.
  • The compatible property should always be "zmk,combos" for combos.
  • timeout-ms is the number of milliseconds that all keys of the combo must be pressed.
  • key-positions is an array of key positions. See the info section below about how to figure out the positions on your board.
  • layers = <0 1...> will allow limiting a combo to specific layers. This is an optional parameter, when omitted it defaults to global scope.
  • bindings is the behavior that is activated when the behavior is pressed.
  • (advanced) you can specify slow-release if you want the combo binding to be released when all key-positions are released. The default is to release the combo as soon as any of the keys in the combo is released.

Key positions are numbered like the keys in your keymap, starting at 0. So, if the first key in your keymap is Q, this key is in position 0. The next key (possibly W) will have position 1, etcetera.

Advanced usage#

  • Partially overlapping combos like 0 1 and 0 2 are supported.
  • Fully overlapping combos like 0 1 and 0 1 2 are supported.
  • You are not limited to &kp bindings. You can use all ZMK behaviors there, like &mo, &bt, &mt, &lt etc.
Source-specific behaviors on split keyboards

Invoking a source-specific behavior such as one of the reset behaviors using a combo will always trigger it on the central side of the keyboard, regardless of the side that the keys corresponding to key-positions are on.

Advanced configuration#

There are three global combo parameters which are set through KConfig. You can set them in the <boardname>.conf file in the same directory as your keymap file.

  • CONFIG_ZMK_COMBO_MAX_PRESSED_COMBOS is the number of combos that can be active at the same time. Default 4.
  • CONFIG_ZMK_COMBO_MAX_COMBOS_PER_KEY is the maximum number of combos that can be active on a key position. Defaults to 5. (So you can have 5 separate combos that use position 3 for example)
  • CONFIG_ZMK_COMBO_MAX_KEYS_PER_COMBO is the maximum number of keys that need to be pressed to activate a combo. Default 4. If you want a combo that triggers when pressing 5 keys, you'd set this to 5 for example.