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README.md

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+# ptt-fhss
+
+Bare-metal PTT (Push-to-Talk) firmware for the **Seeed XIAO BLE (nRF52840)** with
+frequency-hopping spread spectrum (FHSS) on the 2.4 GHz band.
+
+Designed to be low-power, hard to detect, and built with a fully open toolchain —
+no Zephyr, no Nordic SDK, no SoftDevice.
+
+## Hardware
+
+| Component | Part |
+|-----------|------|
+| Target | Seeed XIAO BLE (nRF52840) |
+| Programmer | RP2040 running DAPLink |
+
+## How it works
+
+### FHSS
+
+The radio hops across 40 channels (2402–2480 MHz, 2 MHz steps) every 2 ms.
+The hopping sequence is generated by AES-128-ECB keyed with a shared secret —
+both devices derive identical sequences independently without any synchronisation
+traffic. To an observer with an SDR the transmissions appear as short, scattered
+impulses with no identifiable pattern.
+
+### Low power
+
+The nRF52840 stays in SYSTEM_ON sleep (`WFI`) with the DC/DC converter active
+(~1.5 µA quiescent). A GPIOTE edge event on the PTT button wakes the CPU;
+the radio is active only during the burst.
+
+### Register access
+
+All hardware register writes use typed bitfield unions defined in `include/regs.h`
+rather than raw bit-shift expressions. The union layout is guaranteed correct with
+`arm-none-eabi-gcc` (LSB-first bitfields on Cortex-M).
+
+## Toolchain
+
+Everything that produces the binary runs inside a container. The host only needs
+tools that talk to hardware (pyocd) or manage the build (just).
+
+| Tool | Purpose | Install |
+|------|---------|---------|
+| podman or docker | Container runtime | distro package |
+| just | Task runner | `cargo install just` or distro package |
+| pyocd | Flash / debug via DAPLink | `pip install pyocd` |
+| git | Source control + submodules | distro package |
+
+Inside the container: `debian:bookworm-slim` + `gcc-arm-none-eabi` from apt +
+`cmake` + `ninja`.
+
+## Quick start
+
+```sh
+git clone <repo-url> ptt
+cd ptt
+git submodule update --init --depth=1
+
+just build    # build firmware.hex inside container
+just flash    # flash via DAPLink (host pyocd)
+```
+
+On first run `just build` pulls the container image and compiles. Subsequent
+builds reuse the cached image and only recompile changed files.
+
+## Tasks
+
+```
+just build       compile firmware inside the container
+just flash       build + flash via DAPLink
+just gdbserver   start pyocd GDB server on port 3333
+just clean       remove build/
+just clean-all   remove build/ and the container image
+```
+
+## Project structure
+
+```
+.
+├── Dockerfile               container image (debian + arm-gcc from apt)
+├── justfile                 build / flash / debug tasks
+├── CMakeLists.txt
+├── cmake/
+│   └── arm-none-eabi.cmake  CMake toolchain file
+├── link/
+│   └── nrf52840.ld          linker script (no SoftDevice, Flash @ 0x00000000)
+├── include/
+│   ├── regs.h               hardware register bitfield unions
+│   ├── radio.h
+│   ├── fhss.h
+│   └── power.h
+├── src/
+│   ├── startup.c            vector table (64 entries) + Reset_Handler
+│   ├── main.c
+│   ├── radio.c              RADIO peripheral driver (stub)
+│   ├── fhss.c               AES-ECB hopping sequence generator
+│   └── power.c              DC/DC, GPIOTE wakeup, WFI sleep
+└── vendor/
+    ├── nrfx/                Nordic HAL headers, pinned to v2.9.0 (includes mdk/)
+    ├── CMSIS_5/             ARM core headers (core_cm4.h etc.)
+    └── tiny-aes-c/          Public domain AES-128 implementation
+```
+
+## Before first flash
+
+The XIAO BLE ships with a SoftDevice which occupies the start of Flash.
+Erase it before flashing bare-metal firmware:
+
+```sh
+pyocd erase --target nrf52840 --chip
+```
+
+## Status
+
+| Module | State |
+|--------|-------|
+| Startup / vector table | done |
+| Linker script | done |
+| Power management (sleep + wakeup) | done |
+| FHSS sequence generator | done |
+| RADIO driver | stub — TX loop not yet implemented |
+| Sync protocol | not started |