rtos-sensor-node

FreeRTOS sensor node on RP2040. Reads temperature over I2C, samples the supply rail through the ADC, logs to UART, runs a supervised watchdog, and blinks a status LED.

Covers

Embedded Firmware Engineer (entry-level) · RTOS Software Engineer (entry-level) · Microcontroller Firmware Engineer · Firmware Design Engineer

Output

See docs/sensor-log.csv. The sensor column reports ok or fault per sample, so a missing TMP102 is visible in the log rather than silently producing zeros.

Quickstart

# copy build/sensor_node.uf2 onto the Pico (BOOTSEL drag-and-drop)
pip install -r host/requirements.txt
python host/monitor.py --port /dev/ttyACM0 --seconds 30

Exits nonzero if fewer than --min-samples rows were captured, and reports a port error instead of raising.

Task layout

Task	Priority	Period	Responsibility
watchdog	5	1s	feed HW watchdog only when all tasks checked in
sensor	3	1s	read TMP102 over I2C, sample VREF via ADC
log	2	2s	UART log of a mutex-protected snapshot
heartbeat	1	500ms	status LED on GP25

The watchdog task does not unconditionally kick the hardware watchdog. Each supervised task sets a flag each cycle, and the watchdog is fed only when every flag is set — so one hung task causes a reset instead of being masked by an independent feeder.

Build

Needs both the pico-sdk and a FreeRTOS kernel tree:

export PICO_SDK_PATH=/path/to/pico-sdk
export FREERTOS_KERNEL_PATH=/path/to/FreeRTOS-Kernel   # V11.1.0 tested
cmake -G Ninja -S firmware -B build -DPICO_BOARD=pico -DCMAKE_BUILD_TYPE=Release
cmake --build build          # -> build/sensor_node.uf2

firmware/FreeRTOSConfig.h is checked in and tuned for the Cortex-M0+ port (configPRIO_BITS=2, kernel at the lowest priority, configTICK_RATE_HZ=1000, 80 KB heap — task stacks total ~16.3 KB).

Tests

pip install pytest
python -m pytest host/test_monitor.py -q

Drives the real script over a pty: CSV capture, negative temperatures, legacy lines without the sensor field, noise rejection, dead port, --min-samples.

What works

TMP102 I2C read with bounded timeouts, real ADC sampling of the supply rail, mutex-protected consistent snapshots, supervised watchdog, LED heartbeat, pty-based test suite, CI that builds the firmware.

What doesn't

No CAN bus yet. No SD card logging yet. Task behaviour has not been observed on real silicon — the firmware is CI-built, not bench-run.

License

MIT