rtos-patterns

RTOS Patterns for STM32

Provide RTOS integration guidance for STM32 using FreeRTOS (native API and CMSIS-RTOS v2 wrapper).

FreeRTOS + STM32 Integration

HAL Timebase Conflict

STM32 HAL uses SysTick as its timebase by default. FreeRTOS also needs SysTick for its scheduler tick. This creates a conflict.

Solution: Configure HAL to use a different timer (e.g., TIM6 or TIM7) for its timebase:

• In STM32CubeMX: Pinout & Configuration -> SYS -> Timebase Source -> TIM6
• Manually: Implement HAL_InitTick() and HAL_GetTick() using a basic timer

FreeRTOS keeps SysTick for its scheduler tick.

FreeRTOSConfig.h Key Parameters

Extract from documentation or configure based on the target MCU:

#define configCPU_CLOCK_HZ              (SystemCoreClock)  // From RCC config
#define configTICK_RATE_HZ              ((TickType_t)1000) // 1ms tick
#define configMAX_PRIORITIES            (7)                // 0-6 priority levels
#define configMINIMAL_STACK_SIZE        ((uint16_t)128)    // Words (512 bytes)
#define configTOTAL_HEAP_SIZE           ((size_t)15360)    // Bytes, adjust to SRAM
#define configUSE_PREEMPTION            1
#define configUSE_MUTEXES               1
#define configUSE_COUNTING_SEMAPHORES   1
#define configUSE_QUEUE_SETS            0

/* Cortex-M interrupt priority configuration - CRITICAL */
#define configPRIO_BITS                 4  // Check RM for __NVIC_PRIO_BITS
#define configLIBRARY_LOWEST_INTERRUPT_PRIORITY      15
#define configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY  5

Critical: Interrupt Priority Boundary

configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY defines the boundary:

• Interrupts with priority >= this value (lower urgency) CAN use FreeRTOS *FromISR() APIs
• Interrupts with priority < this value (higher urgency) MUST NOT call any FreeRTOS API
• This is the most common source of hard faults in FreeRTOS + STM32 projects

Check the reference manual NVIC section for the number of priority bits (__NVIC_PRIO_BITS).

Task Patterns