Create README.md

README.md

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+# ZMK trackball interface using HID indicators
+
+This ZMK module allows interfacing with a trackball that can't run ZMK.
+The state of the HID indicator LEDs is shared between input devices so they can be used for communication.
+This module provides macros to send commands to the trackball and implements a listener to enable a mouse layer when movement is detected.
+
+## Usage
+
+To use, first add this module to your `config/west.yml` by adding a new entry to `remotes` and `projects`:
+
+```yaml west.yml
+manifest:
+  remotes:
+    - name: zmkfirmware
+      url-base: https://github.com/zmkfirmware
+    - name: englmaxi
+      url-base: https://github.com/englmaxi
+  projects:
+    - name: zmk
+      remote: zmkfirmware
+      revision: main
+      import: app/west.yml
+    - name: zmk-hid-trackball-interface
+      remote: englmaxi
+      revision: main
+  self:
+    path: config
+```
+
+Then add the following to your keymap:
+```c
+#include <interfaces/hid-trackball.dtsi>
+```
+
+This gives you access to the following predefined behaviors:
+- `&tb_cyc_dpi`: Cycles between the different DPI options of the trackball
+- `&tb_bootloader`: Reboots the trackball into the bootloader
+- `&tb_tg_scroll`: Toggles the trackball between move- and scroll-mode
+- `&tb_mo_scroll`: Toggles the trackball between move- and scroll-mode while the key is held down
+- `&tbs_mt 0 0`: `&tb_tg_scroll` on tap, `&tb_mo_scroll` on hold
+
+If you want to automatically change to a layer or enable scrolling and change DPI on specific layers, add this (with the desired layer inside `<>`) to your keymap:
+```dtsi
+&hid_trackball_interface {
+    automouse-layer = <1>;
+    scroll-layers = <2>;
+    snipe-layers = <3>;
+};
+```
+
+- If a layer is defined in `automouse-layer`, it will be enabled while the mouse is moving
+- If any layers are defined in `scroll-layers`, `&tb_tg_scroll` is executed by default when one of those layers is enabled or disabled.
+- If any layers are defined in `snipe-layers`, `&tb_cyc_dpi` is executed by default when one of those layers is enabled or disabled
+  (this is only usefull if it cycles between two DPI settings only)
+
+If you use a [Ploopy Nano](https://github.com/ploopyco/nano-trackball), you can use the modified firmware in [trackball_firmware](/trackball_firmware).
+Otherwise, you need to modify your firmware to listen to [specific HID indicator changes](#how-does-this-work?).
+
+## How does this work?
+
+The idea is based on the `lkbm` keymap for the Ploopy Nano, [created by @aidalgol](https://github.com/qmk/qmk_firmware/pull/17218).
+It allows sending 2-bit commands by turning NLCK (`0b01`) and CLCK (`0b10`) on and off within a specified short time (<25 ms):
+- `0b01` ... toggle scroll-mode
+- `0b10` ... cycle DPI
+- `0b11` ... bootloader
+
+To enable the `automouse-layer`, the trackball keymap was extended to turn on SLCK while the mouse is moving, which gets detected by this module.
+
+---
+
+### Acknowledgements
+
+Inorichi's [PMW3610 driver](https://github.com/inorichi/zmk-pmw3610-driver) and the original [`lkbm`](https://github.com/qmk/qmk_firmware/pull/17218) firmware 
+were major inspirations for implementing this module.