FreeRTOS STM32

in progress

Overview

Multi-task FreeRTOS firmware on STM32F411RE demonstrating production-relevant patterns: task communication via queues, ISR-safe data passing, watchdog supervision, and persistent configuration in flash.

Hardware

Component	Description
Board	STM32 Nucleo-64 (STM32F411RE)
Core	ARM Cortex-M4 @ 100MHz
Sensor	BME280 over I2C (temp/humidity/pressure)
Debug	ST-Link onboard, UART2 virtual COM

Task Architecture

[Sensor P=2] --sensor_queue(4)--> [Processing P=2] --print_queue(8)--> [UART TX P=1]
                                                                              ^
[CLI P=2]    --------------------------------print_queue(8)------------------>|
[UART RX ISR] --rx_char_queue(32)--> [CLI P=2]

[Watchdog P=4] -- supervises all tasks -- feeds IWDG on full checkin

Tasks

Task	Priority	Stack	Description
Sensor	2	512w	BME280 I2C read, 1000ms period via vTaskDelayUntil
Processing	2	512w	Applies flash config offsets, formats output
UART TX	1	256w	Dedicated print task — UART is slow, never block producers
Watchdog	4	256w	Feeds IWDG only when all tasks check in via bitmask
CLI	2	512w	UART shell: status, config get/set, task suspend

Watchdog Pattern

The watchdog supervisor (highest priority) checks a volatile uint32_t bitmask every 500ms. Each task sets its bit atomically. If all bits are set, the supervisor clears them and feeds the IWDG. If any bit is missing at the deadline, the IWDG is not fed and the system resets in ~2 seconds. The UART logs which task failed to check in before reset.

A watchdog fed unconditionally from one place is nearly useless. This pattern catches hangs in any individual task.

Persistent Config

Config struct stored in the last flash page with CRC32 validation:

typedef struct {
    uint32_t magic;           // 0xDEADBEEF
    uint32_t version;
    int32_t  temp_offset_cdeg;
    uint32_t sample_period_ms;
    uint32_t uart_baud;
    uint32_t crc32;
} device_config_t;

On boot: validate magic + CRC, fall back to defaults on corruption.

Build System

CMake + arm-none-eabi-gcc. FreeRTOS as a git submodule. HAL/CMSIS sources from STM32CubeMX (generation only — Makefile discarded). heap_4 allocator: 24KB heap, all allocations at task creation, nothing dynamic at runtime.