feat(kscan): Improve matrix debouncing

Switched the GPIO matrix driver to debouncing using a simple integrator
algorithm. Whenever a key is pressed, we now scan at a rate controlled
by debounce-scan-period-ms (default 1 ms) until all keys are released,
then return to either waiting for an interrupt or polling more slowly.

The timers for key press and release can now be controlled separately,
so debounce-period is deprecated in favor of debounce-press-ms and
debounce-release-ms. Global Kconfig options
ZMK_KSCAN_DEBOUNCE_PRESS_MS and ZMK_KSCAN_DEBOUNCE_RELEASE_MS are also
added to make these easier to set.

Added documentation for debouncing options.

2 files changed, 101 insertions, 0 deletions

diff --git a/docs/docs/features/debouncing.md b/docs/docs/features/debouncing.md
new file mode 100644
index 0000000..f0022a5
--- /dev/null
+++ b/docs/docs/features/debouncing.md
@@ -0,0 +1,100 @@
+---
+title: Debouncing
+sidebar_label: Debouncing
+---
+
+To prevent contact bounce (also known as chatter) and noise spikes from causing
+unwanted key presses, ZMK uses a [cycle-based debounce algorithm](https://www.kennethkuhn.com/electronics/debounce.c),
+with each key debounced independently.
+
+By default the debounce algorithm decides that a key is pressed or released after
+the input is stable for 5 milliseconds. You can decrease this to improve latency
+or increase it to improve reliability.
+
+If you are having problems with a single key press registering multiple inputs,
+you can try increasing the debounce press and/or release times to compensate.
+You should also check for mechanical issues that might be causing the bouncing,
+such as hot swap sockets that are making poor contact. You can try replacing the
+socket or using some sharp tweezers to bend the contacts back together.
+
+## Debounce Configuration
+
+### Global Options
+
+You can set these options in your `.conf` file to control debouncing globally.
+Values must be <= 127.
+
+- `CONFIG_ZMK_KSCAN_DEBOUNCE_PRESS_MS`: Debounce time for key press in milliseconds. Default = 5.
+- `CONFIG_ZMK_KSCAN_DEBOUNCE_RELEASE_MS`: Debounce time for key release in milliseconds. Default = 5.
+
+For example, this would shorten the debounce time for both press and release:
+
+```ini
+CONFIG_ZMK_KSCAN_DEBOUNCE_PRESS_MS=3
+CONFIG_ZMK_KSCAN_DEBOUNCE_RELEASE_MS=3
+```
+
+### Per-driver Options
+
+You can add these Devicetree properties to a kscan node to control debouncing for
+that instance of the driver. Values must be <= 127.
+
+- `debounce-press-ms`: Debounce time for key press in milliseconds. Default = 5.
+- `debounce-release-ms`: Debounce time for key release in milliseconds. Default = 5.
+- ~~`debounce-period`~~: Deprecated. Sets both press and release debounce times.
+- `debounce-scan-period-ms`: Time between reads in milliseconds when any key is pressed. Default = 1.
+
+If one of the global options described above is set, it overrides the corresponding
+per-driver option.
+
+For example, if your board/shield has a kscan driver labeled `kscan0` in its
+`.overlay`, `.dts`, or `.dtsi` files,
+
+```devicetree
+kscan0: kscan {
+    compatible = "zmk,kscan-gpio-matrix";
+    ...
+};
+```
+
+then you could add this to your `.keymap`:
+
+```devicetree
+&kscan0 {
+    debounce-press-ms = <3>;
+    debounce-release-ms = <3>;
+};
+```
+
+This must be placed outside of any blocks surrounded by curly braces (`{...}`).
+
+`debounce-scan-period-ms` determines how often the keyboard scans while debouncing. It defaults to 1 ms, but it can be increased to reduce power use. Note that the debounce press/release timers are rounded up to the next multiple of the scan period. For example, if the scan period is 2 ms and debounce timer is 5 ms, key presses will take 6 ms to register instead of 5.
+
+## Eager Debouncing
+
+Eager debouncing means reporting a key change immediately and then ignoring
+further changes for the debounce time. This eliminates latency but it is not
+noise-resistant.
+
+ZMK does not currently support true eager debouncing, but you can get something
+very close by setting the time to detect a key press to zero and the time to detect
+a key release to a larger number. This will detect a key press immediately, then
+debounce the key release.
+
+```ini
+CONFIG_ZMK_KSCAN_DEBOUNCE_PRESS_MS=0
+CONFIG_ZMK_KSCAN_DEBOUNCE_RELEASE_MS=5
+```
+
+Also consider setting `CONFIG_ZMK_KSCAN_DEBOUNCE_PRESS_MS=1` instead, which adds
+one millisecond of latency but protects against short noise spikes.
+
+## Comparison With QMK
+
+ZMK's default debouncing is similar to QMK's `sym_defer_pk` algorithm.
+
+Setting `CONFIG_ZMK_KSCAN_DEBOUNCE_PRESS_MS=0` for eager debouncing would be similar
+to QMK's (unimplemented as of this writing) `asym_eager_defer_pk`.
+
+See [QMK's Debounce API documentation](https://beta.docs.qmk.fm/using-qmk/software-features/feature_debounce_type)
+for more information.
diff --git a/docs/sidebars.js b/docs/sidebars.js
index 8865b57..2a40658 100644
--- a/docs/sidebars.js
+++ b/docs/sidebars.js
@@ -11,6 +11,7 @@ module.exports = {
     Features: [
       "features/keymaps",
       "features/combos",
+      "features/debouncing",
       "features/displays",
       "features/encoders",
       "features/underglow",