urob's zmk-config

This is my personal ZMK firmware configuration. It consists of a 34-keys base layout that is re-used for various boards, including my Corneish Zen and my Planck.

This branch is updated for the latest ZMK using Zephyr 3.5. A legacy version compatible with Zephyr 3.0 is available here.

Highlights

• "Timeless" homerow mods
• Combos replace symbol layer
• Smart numbers and mouse layers auto-toggle off
• Unicode math and international leader key sequences
• Simplified Devicetree syntax using helper macros from zmk-helpers
• Base keymap padded with modular structure of "extra keys" to fit on larger boards
• Arrow-cluster doubles as home, end, begin/end of document on long-press
• More intuitive shift-actions: , ;, . : and ? !
• Fully automated, nix-based local build environment

Timeless homerow mods

Homerow mods (aka "HRMs") can be a game changer -- at least in theory. In practice, they require some finicky timing: In its most naive implementation, in order to produce a "mod", they must be held longer than tapping-term-ms. In order to produce a "tap", they must be held less than tapping-term-ms. This requires very consistent typing speeds that, alas, I do not possess. Hence my quest for a "timer-less" HRM setup.¹

After months of tweaking, I eventually ended up with a HRM setup that is essentially timer-less, resulting in virtually no misfires. Yet it provides a fluent typing experience with mostly no delays.

Let's suppose for a moment we set tapping-term-ms to something ridiculously large, say 5 seconds. This makes the configuration timer-less of sorts. But it has two problems: (1) To activate a mod we will have to hold the HRM keys for what feels like eternity. (2) During regular typing, there are delays between the press of a key and the time it appears on the screen.² Enter two of my favorite ZMK features:

• To address the first problem, I use ZMK's balanced flavor, which produces a "hold" if another key is both pressed and released within the tapping-term. Because that is exactly what I normally do with HRMs, there is virtually never a need to wait past my long tapping term (see below for two exceptions).
• To address the typing delay, I use ZMK's require-prior-idle-ms property, which immediately resolves a HRM as "tap" when it is pressed shortly after another key has been tapped. This all but completely eliminates the delay.

This is great but there are still a few rough edges:

• When rolling keys, I sometimes unintentionally end up with "nested" key sequences: key 1 down, key 2 down and up, key 1 up. Because of the balanced flavor, this would falsely register key 1 as a mod. As a remedy, I use ZMK's positional hold-tap feature to force HRMs to always resolve as "tap" when the next key is on the same side of the keyboard. Problem solved.
• ... or at least almost. By default, positional-hold-tap performs the positional check when the next key is pressed. This is not ideal, because it prevents combining multiple modifiers on the same hand. To fix this, I use the hold-trigger-on-release setting, which delays the positional-hold-tap decision until the next key's release. With the setting, multiple mods can be combined when held, while I still get the benefit from positional-hold-tap when keys are tapped.
• So far, nothing of the configuration depends on the duration of tapping-term-ms. In practice, there are two reasons why I don't set it to infinity:
  1. Sometimes, in rare circumstances, I want to combine a mod with a alpha-key on the same hand (e.g., when using the mouse with the other hand). My positional hold-tap configuration prevents this within the tapping term. By setting the tapping term to something large but not crazy large (I use 280ms), I can still use same-hand mod + alpha shortcuts by holding the mod for just a little while before tapping the alpha-key.
  2. Sometimes, I want to press a modifier without another key (e.g., on Windows, tapping Win opens the search menu). Because the balanced flavour only kicks in when another key is pressed, this also requires waiting past tapping-term-ms.
• Finally, it is worth noting that this setup works best in combination with a dedicated shift for capitalization during normal typing (I like sticky-shift on a home-thumb). This is because shifting alphas is the one scenario where pressing a mod may conflict with require-prior-idle-ms, which may result in false negatives when typing fast.

Here's my configuration (I use a bunch of helper macros to simplify the syntax, but they are not necessary):

/* use helper macros to define left and right hand keys */
#include "zmk-helpers/key-labels/36.h"                                      // key-position labels
#define KEYS_L LT0 LT1 LT2 LT3 LT4 LM0 LM1 LM2 LM3 LM4 LB0 LB1 LB2 LB3 LB4  // left-hand keys
#define KEYS_R RT0 RT1 RT2 RT3 RT4 RM0 RM1 RM2 RM3 RM4 RB0 RB1 RB2 RB3 RB4  // right-hand keys
#define THUMBS LH2 LH1 LH0 RH0 RH1 RH2                                      // thumb keys

/* left-hand HRMs */
ZMK_HOLD_TAP(hml,
    flavor = "balanced";
    tapping-term-ms = <280>;
    quick-tap-ms = <175>;                // repeat on tap-into-hold
    require-prior-idle-ms = <150>;
    bindings = <&kp>, <&kp>;
    hold-trigger-key-positions = <KEYS_R THUMBS>;
    hold-trigger-on-release;             // delay positional check until key-release
)

/* right-hand HRMs */
ZMK_HOLD_TAP(hmr,
    flavor = "balanced";
    tapping-term-ms = <280>;
    quick-tap-ms = <175>;                // repeat on tap-into-hold
    require-prior-idle-ms = <150>;
    bindings = <&kp>, <&kp>;
    hold-trigger-key-positions = <KEYS_L THUMBS>;
    hold-trigger-on-release;             // delay positional check until key-release
)

Required firmware

After a recent round of patches, the above configuration now works with upstream ZMK.

Other parts of my configuration still require a few PRs that aren't yet in upstream ZMK. My personal ZMK fork includes all PRs needed to compile my configuration. If you prefer to maintain your own fork with a custom selection of PRs, you might find this ZMK-centric introduction to Git helpful.

Troubleshooting

Hopefully, the above configuration "just works". If it doesn't, here's a few smaller (and larger) things to try.

• Noticeable delay when tapping HRMs: Increase require-prior-idle-ms. As a rule of thumb, you want to set it to at least 10500/x where x is your (relaxed) WPM for English prose.³
• False negatives (same-hand): Reduce tapping-term-ms (or disable hold-trigger-key-positions)
• False negatives (cross-hand): Reduce require-prior-idle-ms (or set flavor to hold-preferred -- to continue using hold-trigger-on-release, you must also patch ZMK or use an already patched branch)
• False positives (same-hand): Increase tapping-term-ms
• False positives (cross-hand): Increase require-prior-idle-ms (or set flavor to tap-preferred, which requires holding HRMs past tapping term to activate)

Using combos instead of a symbol layer

I am a big fan of combos for all sort of things. In terms of comfort, I much prefer them over accessing layers that involve lateral thumb movements to be activated, especially when switching between different layers in rapid succession.

One common concern about overloading the layout with combos is that they lead to misfires. Fortunately, the above-mentioned require-prior-idle-ms option also works for combos, which in my experience all but completely eliminates the problem -- even when rolling keys on the home row!

My combo layout aims to place the most used symbols in easy-to-access locations while also making them easy to remember. Specifically:

• the top vertical-combo row matches the symbols on a standard numbers row (except + and & being swapped)
• the bottom vertical-combo row is symmetric to the top row (subscript _ aligns with superscript ^; minus - aligns with +; division / aligns with multiplication *; logical-or | aligns with logical-and &)
• parenthesis, braces, brackets are set up symmetrically as horizontal combos with <, >, { and } being accessed from the Navigation layer (or when combined with Shift)
• left-hand side combos for tap, esc, enter, cut (on X + D), copy and paste that go well with right-handed mouse usage
• L + Y switches to the Greek layer for a single key press, L + U + Y activates one-shot shift in addition
• W + P activates the smart mouse layer

Smart layers and other gimmicks

Numword

Inspired by Jonas Hietala's Numword for QMK, I implemented my own Auto-layer behavior for ZMK to set up Numword. It is triggered via a single tap on "Smart-Num". Numword continues to be activated as long as I type numbers, and deactivates automatically on any other keypress (holding it activates a non-sticky num layer).

After using Numword for more than a year now, I have been overall very happy with it. When typing single digits, it effectively is a sticky-layer but with the added advantage that I can also use it to type multiple digits.

The main downside is that if a sequence of numbers is immediately followed by any of the letters on which my numpad is located (WFPRSTXCD), then the automatic deactivation won't work. But this is rare -- most number sequences are terminated by space, return or some form of punctuation/delimination. To deal with the rare cases where they aren't, there is a CANCEL key on the navigation-layer that deactivates Numword, Capsword and Smart-mouse. (It also toggles off when pressing Numword again, but I find it cognitively easier to have a dedicated "off-switch" than keeping track of which modes are currently active.)

Smart-Mouse

Similarly to Numword, I have a smart-mouse layer (activated by comboing W + P), which replaces the navigation cluster with scroll and mouse-movements, and replaces the right thumbs with mouse buttons. Pressing any other key automatically deactivates the layer.

Capsword

My right thumb triggers three variations of shift: Tapping yields sticky-shift (used to capitalize alphas), holding activates a regular shift, and double-tapping (or equivalently shift + tap) activates ZMK's Caps-word behavior.

One minor technical detail: While it would be possible to implement the double-tap functionality as a tap-dance, this would add a delay when using single taps. To avoid the delays, I instead implemented the double-tap functionality as a mod-morph.

Multi-purpose Navigation cluster

To economize on keys, I am using hold-taps on my navigation cluster, which yield home, end, begin/end of document, and delete word forward/backward on long-presses. The exact implementation is tweaked so that Ctrl is silently absorbed in combination with home and end to avoid accidental document-wide operations (which are accessible via the dedicated begin/end document keys.)

Swapper

I am using Nick Conway's fantastic tri-state behavior for a one-handed Alt-Tab switcher (PWin and NWin).

Leader key

I recently started using Nick Conway's Leader key implementation for ZMK. From my limited experience, I really like how it allows making less commonly used behaviors accessible without binding them to a dedicated key. For now I am using it for a variety of Unicode math symbols and international characters. I am planning to extend the use to various firmware interactions once I figure out the technical details.

Local development workspace

I streamline my local build process using nix, direnv and just. This automatically sets up a virtual development environment with west, the zephyr-sdk and all its dependencies when cd-ing into the ZMK-workspace. The environment is completely isolated and won't pollute your system.

Setup

Pre-requisites

1. Install the nix package manager:

   # Install Nix with flake support enabled
   curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix |
      sh -s -- install --no-confirm
   
   # Start the nix daemon without restarting the shell
   . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh

2. Install direnv (and optionally but recommended nix-direnv⁴) using your package manager of choice. E.g., using the nix package manager that we just installed⁵:

   nix profile install nixpkgs#direnv nixpkgs#nix-direnv
   

3. Set up the direnv shell-hook for your shell. E.g., for bash:

   # Install the shell-hook
   echo 'eval "$(direnv hook bash)"' >> ~/.bashrc
   
   # Enable nix-direnv (if installed in the previous step)
   mkdir -p ~/.config/direnv
   echo 'source $HOME/.nix-profile/share/nix-direnv/direnvrc' >> ~/.config/direnv/direnvrc
   
   # Optional: make direnv less verbose
   echo '[global]\nwarn_timeout = "2m"\nhide_env_diff = true' >> ~/.config/direnv/direnv.toml
   
   # Source the bashrc to activate the hook (or start a new shell)
   source ~/.bashrc

Set up the workspace

1. Clone your fork of this repository. I like to name my local clone zmk-workspace as it will be the toplevel of the development environment.

   # Replace `urob` with your username
   git clone https://github.com/urob/zmk-config zmk-workspace

2. Enter the workspace and set up the environment.

   # The first time you enter the workspace, you will be prompted to allow direnv
   cd zmk-workspace
   
   # Allow direnv for the workspace, which will set up the environment
   direnv allow
   
   # Initialize the Zephyr workspace and pull in the ZMK dependencies
   # (same as `west init -l config && west update && west zephyr-export`)
   just init

Usage

After following the steps above your workspace should look like this:

zmk-workspace
├── config
├── firmware (created after building)
├── modules
│   ├── auto-layer
│   ├── helpers
│   └── tri-state
└── zmk
    └── ...

Building the firmware

To build the firmware, simply type just build all from anywhere within the workspace. This will parse build.yaml and build the firmware for all board and shield combinations listed there.

To only build the firmware for a specific target, use just build <target>. This will build the firmware for all matching board and shield combinations. For instance, to build the firmware for my Corneish Zen, I can type just build zen, which builds both corneish_zen_v2_left and corneish_zen_v2_right. (just list shows all valid build targets.)

Additional arguments to just build are passed on to west. For instance, a pristine build can be triggered with just build all -p.

(For this particular example, there is also a just clean recipe, which clears the build cache. To list all available recipes, type just. Bonus tip: just provides completion scripts for many shells.)

Drawing the keymap

The build environment packages keymap-drawer. just draw parses base.keymap and draws it to draw/base.svg. I haven't gotten around to tweaking the output yet, so for now this is just a demonstration of how to set things up.

Hacking the firmware

To make changes to the ZMK source or any of the modules, simply edit the files or use git to pull in changes.

To switch to any remote branches or tags, use git fetch inside a module directory to make the remote refs locally available. Then switch to the desired branch with git checkout <branch> as usual. You may also want to register additional remotes to work with or consider making them the default in config/west.yml.

Updating the build environment

To update the ZMK dependencies, use just update. This will pull in the latest version of ZMK and all modules specified in config/west.yml. Make sure to commit and push all local changes you have made to ZMK and the modules before running this command, as this will overwrite them.

To upgrade the Zephyr SDK and Python build dependencies, use just upgrade-sdk.

Issues and workarounds

Since I switched from QMK to ZMK I have been very impressed with how easy it is to set up relatively complex layouts in ZMK. For the most parts I don't miss any functionality (to the contrary, I found that ZMK supports many features natively that would require complex user-space implementations in QMK). Below are a few remaining issues:

• ZMK does not yet support "tap-only" combos (#544), requiring a brief pause when wanting to chord HRMs that overlap with combo positions. As a workaround, I implemented all homerow combos as homerow-mod-combos. This is good enough for day-to-day, but does not address all edge cases (eg changing active mods).
• Very minor: &bootloader doesn't work with stm32 boards like the Planck (#1086)

Footnotes

1. I call it "timer-less", because the large tapping-term makes the behavior insensitive to the precise timings. One may say that there is still the require-prior-idle timeout. However, with both a large tapping-term and positional-hold-taps, the behavior is not actually sensitive to the require-prior-idle timing: All it does is reduce the delay in typing; i.e., variations in typing speed won't affect what is being typed but merely how fast it appears on the screen. ↩

2. The delay is determined by how quickly a key is released and is not directly related to the tapping-term. But regardless of its length, most people still find it noticable and disruptive. ↩

3. E.g, if your WPM is 70 or larger, then the default of 150ms (=10500/70) should work well. The rule of thumb is based on an average character length of 4.7 for English words. Taking into account 1 extra tap for space, this yields a minimum require-prior-idle-ms of (60 _ 1000) / (5.7 _ x) ≈ 10500 / x milliseconds. The approximation errs on the safe side, as in practice home row taps tend to be faster than average. ↩

4. nix-direnv provides a vastly improved caching experience compared to only having direnv, making entering and exiting the workspace instantaneous after the first time. ↩

5. This will permanently install the packages into your local profile, forgoing many of the benefits that make Nix uniquely powerful. A better approach, though beyond the scope of this document, is to use home-manager to maintain your user environment. ↩