celrs

A C23 project for interfacing with ELRS TX modules (e.g., BAYCK Nano Dual Band) via serial USB using the CRSF (Crossfire Serial) protocol.

Built on the same TDD foundation as ctdd using Unity and CMock.

All dependencies are fetched automatically via CMake FetchContent — no manual installation required beyond the tools listed below.

Requirements

Tool	Purpose
CMake ≥ 3.21	Build system
Ninja	Build backend
C23 compiler	GCC 14+, Clang 18+
Ruby ≥ 3.0	CMock mock generation
gcovr ≥ 6.0	Coverage reports — optional (uv tool install gcovr)

Build

Configure:

cmake -S . -B build -G Ninja

Build:

ninja -C build

Test

Full Unity output with colors:

ninja -C build check

CTest summary only:

ninja -C build test

check builds all suites and runs CTest with --output-on-failure, so assertion-level detail appears on any failure without running binaries by hand.

Coverage

Configure with coverage instrumentation:

cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON

Generate the HTML report:

ninja -C build-cov coverage

Open build-cov/coverage/index.html in a browser to view results.

Only celrs/ source files are measured. Unity, CMock, and generated mock files are excluded. Requires GCC or Clang with gcov support, and gcovr on PATH.

Windows note: requires GCC (e.g. scoop install gcc) or a Clang build that includes compiler-rt. A custom Clang without compiler-rt will fail at link time.

Architecture

celrs/
  crsf.h / crsf.c         CRSF protocol: CRC8/DVB-S2, frame build/parse
  crsf_telemetry.h/.c     Telemetry frame decoders (GPS, battery, link..)
  crsf_stream.h/.c        Incremental streaming frame reader
  crsf_param.h/.c         Parameter protocol (read/write/set_power)
  serial.h / serial.c     Serial port abstraction (Win/POSIX)
  logger.h / logger.c     Level-filtering logger
  log_write.h/.c          stdout log sink
tools/
  telemetry.c             Telemetry read tool
tests/
  test_crsf.c             CRSF CRC, parse, build tests
  test_serial.c           Serial open/close/stub tests
  test_logger.c           Logger level-filtering tests
deps/
  FindUnity.cmake         Fetches Unity v2.6.1 via ZIP
  FindCMock.cmake         Fetches CMock v2.6.0 via ZIP

CRSF Protocol

CRSF (Crossfire Serial Protocol) is the serial protocol used by ELRS for communication between ground station and TX/RX modules.

Frame format (ELRS USB CRSF)

+--------+----------+--------+----------+-----+
| addr   | length   | type   | payload  | CRC |
+--------+----------+--------+----------+-----+
  1 byte   1 byte    1 byte    N bytes   1B

• Address: Frame sync byte (0xC8 for host, 0xEE for module, etc.)
• Length: Total bytes after this field (type + payload + CRC)
• Type: Frame type (RC channels, telemetry, parameter, etc.)
• Payload: Frame-specific data
• CRC: CRC8/DVB-S2 (poly 0xD5) over type + payload

Common device addresses

Address	Device
0x00	FC Broadcast
0x10	Flight Controller
0x80	TBS Ground Station
0xEA	Custom Module (Radio)
0xEE	ELRS TX Module
0xEF	ELRS Lua (host script)

Common frame types

Type	Name
0x01	RC Channels Packed
0x02	GPS
0x08	Battery
0x0B	Heartbeat
0x14	Link Stats
0x16	RC Channels
0x28	Device Ping
0x29	Device Info
0x2B	Parameter Entry
0x2C	Parameter Read
0x2D	Parameter Write
0x2E	ELRS Status