OpenPuck — Zephyr RTOS port

This is the Zephyr port of the OpenPuck firmware (originally Arduino / Adafruit-nRF52 / TinyUSB, in ../OpenPuck). It targets mainline Zephyr (v4.4.99) on adafruit_feather_nrf52840/nrf52840/uf2.

Build

./zbuild.sh                 # uses ~/zephyrproject + ~/zephyr-sdk-1.0.1 via the venv
# or, with env set up:
west build -b adafruit_feather_nrf52840/nrf52840/uf2 OpenPuckZephyr

Output build/zephyr/zephyr.uf2 flashes via the existing double-tap-reset DFU procedure (the uf2 board variant reserves the Adafruit bootloader region and uses app base 0x26000).

Migration strategy: compatibility shims

The reverse-engineered radio/protocol/mode logic is the value of this firmware and the part most dangerous to rewrite. So instead of rewriting it, the port keeps those ~8000 lines near-verbatim and supplies a thin compatibility layer in src/compat/:

Shim	Replaces	Backing
`Arduino.h` / `arduino_compat.cpp`	Arduino core	`micros()` via Cortex-M DWT cycle counter; `millis()`/`delay()` via kernel; GPIO via `nrf_gpio`; `Serial` via console; `NRF_*` registers + CMSIS via nrfx MDK (`<nrfx.h>`)
`Adafruit_LittleFS.h` / `InternalFileSystem.h` / `littlefs_shim.cpp`	Adafruit LittleFS	Zephyr `fs` (LittleFS) mounted on `storage_partition`
`Adafruit_TinyUSB.h`	TinyUSB / Adafruit USBD	Zephyr USBD (see USB status below)

The NRF_RADIO/NRF_WDT/NRF_FICR/NRF_CLOCK register code ports essentially unchanged because Zephyr's hal_nordic ships the same Nordic MDK. CONFIG_BT, CONFIG_IEEE802154, and the 802.15.4 radio driver are disabled so the RADIO peripheral is free for the bare-metal protocol.

Status

Compiles + links + produces a UF2, with the real setup()/loop() entry (main.cpp, ported from OpenPuck.ino). Every module is in the build: radio, rf_link, rf_diag, triton, gamepad_util, haptics, controllers, puck_hid, all six modes (lizard, xinput, switch_hori, switch_pro, ps5, hidgyro), webusb_config, serial_console, usb_mount, wake_hid, config, bonds.

Functional / verified-as-compiling

Build pipeline, persistence (LittleFS), config + bond load/save, the radio register layer, the RF protocol state machines, and all mode report-building logic compile under Zephyr.
HFXO is started explicitly in setup() (the Arduino core did it implicitly); the WDT is driven directly via NRF_WDT (Zephyr's watchdog driver is left out to avoid double-ownership).

NOT yet functional (the remaining work, none hardware-verified)

USB is inert. usb_stub.cpp satisfies the TinyUSB shim with safe no-ops (device never mounts, HIDs never ready). The device will not enumerate until the real Zephyr USBD backing is finished. The staged real backing lives in usb_glue.cpp and needs:
- a devicetree overlay declaring N zephyr,hid-device nodes (the HID pool the shim binds to) plus CDC ACM, MSC, and a vendor (WebUSB) interface;
- a usbd_context built per active mode (register exactly the classes that mode needs at init — see the concession below);
- the USBD message callback wired to opk_usb_msg for mount/suspend state.
WebUSB vendor pipe (Adafruit_USBD_WebUSB) + MS OS 2.0 descriptors — needed to preserve Windows/Chrome binding. Currently stubbed inert.
XInput is a custom TinyUSB class driver (mode_xinput.cpp, via the device/usbd_pvt.h shim). It compiles but needs a real Zephyr custom USBD class to enumerate; the report-building logic is intact.
CDC console: serial_console reads via the opk_serial_* weak hooks — wire them to a Zephyr CDC ACM (or RTT) endpoint.
status_led pin mapping: the Arduino code uses logical pin numbers; under the absolute-nRF-pin digitalWrite shim the B-pin (P0.15) needs an override.

Behavioral concession (documented)

Zephyr's USBD descriptor set is fixed once the stack is enabled. The Arduino firmware's dynamic re-enumeration without reboot (usbReenumerate, variable connected-controller count) does not map; it collapses to reboot-to- reenumerate, which was already the firmware's mode-switch path. Per-mode interface sets are achieved by registering only that mode's classes at init.

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