UC GEE Comments - Answer Overflow

UC GEE

•Created by Sterling on 7/14/2024 in #firmware-and-baremetal

Do I need a mutex to protect 8-bit variables on STM32L476 with ARMv7E-M architecture using FreeRTOS?

Yes @Sterling , multi-byte variables like 16-bit and 32-bit types are not guaranteed to be atomic on ARM Cortex-M4. For these variables, you should use a mutex or other synchronization mechanism to protect access if they are shared between tasks. For example, you can use a FreeRTOS mutex:

uint32_t sharedVar;
SemaphoreHandle_t xMutex;

void main(void) {
    xMutex = xSemaphoreCreateMutex();
    if(xMutex != NULL) {
        if(xSemaphoreTake(xMutex, portMAX_DELAY) == pdTRUE) {
            sharedVar = newValue;
            xSemaphoreGive(xMutex);
        }
    }
}

void ISR_Handler(void) {
    // Ensure no preemption and data corruption
    taskENTER_CRITICAL();
    sharedVar = isrValue;
    taskEXIT_CRITICAL();
}

uint32_t sharedVar;
SemaphoreHandle_t xMutex;

void main(void) {
    xMutex = xSemaphoreCreateMutex();
    if(xMutex != NULL) {
        if(xSemaphoreTake(xMutex, portMAX_DELAY) == pdTRUE) {
            sharedVar = newValue;
            xSemaphoreGive(xMutex);
        }
    }
}

void ISR_Handler(void) {
    // Ensure no preemption and data corruption
    taskENTER_CRITICAL();
    sharedVar = isrValue;
    taskEXIT_CRITICAL();
}

In this case, the mutex ensures that only one task can access the variable at a time. For access from ISRs, you use taskENTER_CRITICAL() and taskEXIT_CRITICAL() to protect the critical section.

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