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