Software timers let you execute a callback after a timeout — without a dedicated task. FreeRTOS manages them in the timer daemon task. This part covers creating one-shot and auto-reload timers, timer daemon priority, and how to use the idle hook for background work.

✓Create one-shot and auto-reload timers with xTimerCreate()
✓Reset, start, and stop timers safely from tasks and ISRs
✓Understand timer daemon priority and why callbacks must be non-blocking
✓Implement vApplicationIdleHook() for CPU usage measurement

Timer Types

⏱️

One-Shot

Fires once after the period, then stops automatically. Use for timeouts, debounce, and delayed single actions.

pdFALSE → one-shot

🔄

Auto-Reload

Fires repeatedly on every period. Use for heartbeats, periodic status checks, or polling without a full task.

pdTRUE → auto-reload

🛌

Idle Hook

Called whenever the idle task runs. Perfect for CPU usage counters or entering low-power sleep.

vApplicationIdleHook()

Code

Heartbeat + watchdog timer

timers.c

TimerHandle_t xHeartbeat, xWatchdog;

void vHeartbeatCB(TimerHandle_t t) {
    HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_13);
}

void vWatchdogCB(TimerHandle_t t) {
    uart_puts("[WDG] Sensor timeout
");
    Sensor_Restart();
}

void Timers_Init(void) {
    // Auto-reload: LED blinks every 500ms
    xHeartbeat = xTimerCreate("HB",pdMS_TO_TICKS(500),pdTRUE,0,vHeartbeatCB);
    // One-shot: 5s sensor watchdog
    xWatchdog  = xTimerCreate("WDG",pdMS_TO_TICKS(5000),pdFALSE,0,vWatchdogCB);
    xTimerStart(xHeartbeat, 0);
    xTimerStart(xWatchdog,  0);
}

// Reset watchdog on valid data
void OnSensorData(void) { xTimerReset(xWatchdog, 0); }

// Idle hook — runs when nothing else is ready
void vApplicationIdleHook(void) { idle_cycles++; }

💡

Keep callbacks short and non-blocking

Timer callbacks run inside the daemon task. A long or blocking callback delays all other timers. If you need heavy work, use the callback to send a notification to a dedicated task instead.

Continue the Series

Work through all 8 parts of the FreeRTOS Deep Dive to master real-time embedded systems programming.

Part 4: Queues, Semaphores & Mutexes → Related Project →