urob's zmk-config

This is my personal ZMK firmware configuration.

Key features

• clean keymap + unicode setup using helper macros from zmk-nodefree-config
• keymap and combo setup are independent of the physical location of keys and are (re)used for multiple keyboards with a varying number of keys
• "timeless" homerow mods on the base layer; sticky mods on other layers
• num-word: a zmk version of smart-layers that automatically de-activate for non-numbers
• combos replacing the symbol layer
• arrow-cluster doubles as home/end/beginning of document/end of document on long-press, backspace/delete delete words on long-press
• sticky shift on right thumb, double-tap (or shift + tap)¹ activates caps-word
• shift + comma morph into semicolon, shift + control + comma morph into less-than, shift + dot morph into colon, shift + control + dot morph into greater-than
• shift + space morphs into dot + space + sticky-shift
• conditional layer: "sys" + "num" layer yield "fun" layer
• "repeat" on right bottom pinky
• "Greek" layer for mathematical typesetting (activated via sticky-layer combo)

Timeless homerow mods

Homerow mods are great. But they can 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 "timeless" HRM setup.²

Here's what I have ended up with: A "timeless" HRM setup with virtually no misfires and yet 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 "timeless". But it also creates two problems: (1) In order to get a "mod" we now have to hold the HRM keys for what feels like eternity. (2) In normal typing, when tapping keys, there can be long delays between the press of a key and the time it appears on the screen. Enter my two favorite configuration options:

• 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 global-quick-tap property, which immediately resolves a HRM as "tap" when it is pressed shortly after another key has been tapped.³ This all but completely eliminates the delay when typing.

This is almost perfect, but there's 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. Given the balanced flavor, this would falsely register key 1 as a mod. To prevent this, 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. The official ZMK version for positional-hold-taps 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 a small patch that delays the positional-hold-tap decision until the next key's release (PR #1423). With the patch, multiple mods can be combined when held, while I still get the benefit from positional-hold-taps 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.

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-nodefree-config/keypos_def/keypos_36keys.h"                // keyposition helpers
#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_BEHAVIOR(hml, hold_tap,
    flavor = "balanced";
    tapping-term-ms = <280>;
    quick-tap-ms = <175>;                // repeat on tap-into-hold
    global-quick-tap-ms = <150>;         // requires PR #1387
    bindings = <&kp>, <&kp>;
    hold-trigger-key-positions = <KEYS_R THUMBS>;
    hold-trigger-on-release;             // requires PR #1423
)

/* right-hand HRMs */
ZMK_BEHAVIOR(hmr, hold_tap,
    flavor = "balanced";
    tapping-term-ms = <280>;
    quick-tap-ms = <175>;                // repeat on tap-into-hold
    global-quick-tap-ms = <150>;         // requires PR #1387
    bindings = <&kp>, <&kp>;
    hold-trigger-key-positions = <KEYS_L THUMBS>;
    hold-trigger-on-release;             // requires PR #1423
)

One last note, the configuration above uses some syntactic sugar introduced in PR #1387, which decouples the quick-tap-ms timeout from the global-quick-tap-ms timeout. Without the PR, one can replace global-quick-tap-ms = <150> with global-quick-tap for a similar effect (global-quick-tap will use the regular quick-tap-ms timeout in this case).

My personal ZMK fork includes both the global-quick-tap-ms PR and the hold-trigger-on-release PR (along with a few other PRs). If you are looking for a ZMK-centric introduction to maintaining your own fork with a custom selection of PRs, you might find my "cookbook approach" helpful.

Combo setup

I make heavy use of combos to replace the usual symbol layer. The combo layout aims to put the most used symbols in easy-to-access locations and also make 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 aims for symmetry with the top row (subscript _ aligns with superscript ^; minus - aligns with +; division / aligns with multiplication *; logical-or | aligns with logical-and &)
• parenthesis, braces, brackets, ! and ? are set up symmetrically in prime locations
• numlock (on W + P), cut (on X + D), copy, and paste are on the left side for one-handed mouse use
• L + Y activates Greek layer for next key, L + U + Y activates shifted Greek layer for next key

Experimental changes

• I recently reduced my core layout to 34 keys. Backspace, Delete and Tap are now all on my Navigation-layer. To make room for these keys, I have added hold-taps to the arrow cluster, which now double as Home/End and Beginning/End of document. I really like the new navigation cluster and will likely keep it in one way or another
• Inspired by Jonas Hietala's Numword for QMK, I implemented my own version of Smart-layers for ZMK. It is triggered via a single tap on my Num-key (holding the key will activate the num layer as usual without triggering Numword). Similar to Capsword, Numword continues to be activated as long as I type numbers, and deactivates automatically on any other keypress. I found that I use Numword for most of my numbers typing. For single digits, it effectively is a sticky-layer, but importantly I can also use it for multiple digits. The only case where it doesn't deactivate automatically is where immediately after a digit I would type any of the letters on which my numpad is located (WFPRSTXCD), which is rare, but does happen. For these cases I have a CANCEL key on my Nav layer that cancels both Numword and Capsword.
• Since the switch to 34 keys, I freed up the tap-position on my left-most thumb key. For now I added a secondary Bspc, but I am still searching for a better use. (I tried adding Repeat here but I found that it doesn't work well adjacent to space)

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 complex user-space implementations in QMK). Below are a few remaining issues:

• ZMK does not yet support tap-only combos (#544). Workaround: pause briefly when chording multiple HRMs together on positions that otherwise would trigger a combo.
• &bootloader doesn't work with Planck_rev6 (#1086). Workaround: Manually press reset-button.
• "sticky-hold" swallows OS shift when typing quickly. Workaround: use sticky-tap for now.
• Sleep is not yet implemented (#1077). Workaround: use sleep-macro instead.
• Invalid DFU suffix signature warning when flashing with dfu-util. No problem for now but may cause issues with future versions of dfu-util.

Footnotes

1. Really what's happening is that Shift + my right home-thumb morph into caps-word. This gives me two separate ways of activating it: (1) Holding the homerow-mod shift on my left index-finger and then pressing my right home-thumb, which is my new preferred way. Or, (2) double-tapping the right home-thumb, which also works because the first tap yields sticky-shift, activating the mod-morph upon the second tap. ↩

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

3. One potential downside of global-quick-tap is that it prevents using modifiers immediately after another key press. Arguably, this is only problematic for shift, which is not a problem for me, because I have a dedicated "sticky shift" on my right thumb. If you rely on homerow mods for regular capitalization, you may want to reduce the global-quick-tap term for just the two shift-mods to about 75-100ms. ↩