FreeRTOS sensor pipeline

The code

Embedded C for Cortex-M. It compiles. There are 5 real bugs in it: the kind that pass a bench test and fail in the field.

QueueHandle_t     sensor_q;
SemaphoreHandle_t i2c_mutex;

/* Sensor data-ready line on PB3 */
void EXTI3_IRQHandler(void)
{
    EXTI->RPR1 = EXTI_RPR1_RPIF3;
    xSemaphoreTake(i2c_mutex, portMAX_DELAY);
    uint16_t raw = i2c_read_reg(SENSOR_ADDR, REG_TEMP);
    xSemaphoreGive(i2c_mutex);
    xQueueSend(sensor_q, &raw, 0);
}

void sensor_task(void *arg)
{
    char log_line[1024];
    uint16_t raw;

    for (;;) {
        if (xQueueReceive(sensor_q, &raw, portMAX_DELAY) == pdTRUE) {
            float temp_c = raw * 0.0625f;
            snprintf(log_line, sizeof(log_line), "T=%.2f C", temp_c);
            log_write(log_line);
        }
    }
}

void app_init(void)
{
    sensor_q  = xQueueCreate(8, sizeof(uint16_t));
    i2c_mutex = xSemaphoreCreateMutex();
    xTaskCreate(sensor_task, "sensor", 256, NULL, 2, NULL);
    NVIC_SetPriority(EXTI3_IRQn, 2);
    NVIC_EnableIRQ(EXTI3_IRQn);
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

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