Watchdog task pattern in FreeRTOS - Step-by-Step Execution

Concept Flow - Watchdog task pattern

Start Watchdog Task

↓

Wait for Notification

↓

Check if Task Responded

↓

Reset Timer

↓

Loop to Wait Again

The watchdog task waits for a signal from monitored tasks. If it receives timely signals, it resets its timer. If not, it triggers a system reset.

Execution Sample

FreeRTOS

void WatchdogTask(void *pvParameters) {
  for(;;) {
    if(xTaskNotifyWait(0, 0, NULL, pdMS_TO_TICKS(1000)) == pdTRUE) {
      ResetWatchdogTimer();
    } else {
      SystemReset();
    }
  }
}

This FreeRTOS task waits for a notification every 1000 ms; if received, it resets the watchdog timer, else it resets the system.

Execution Table

Step	Action	Notification Received?	Watchdog Timer Reset?	System Reset Triggered?
1	Wait for notification (timeout 1000ms)	No	No	No
2	Timeout expired without notification	No	No	Yes
3	System reset triggered	-	-	-
4	Restart after reset, begin waiting again	-	-	-
5	Wait for notification (timeout 1000ms)	Yes	No	No
6	Notification received, reset watchdog timer	Yes	Yes	No
7	Loop back to wait for next notification	-	-	-

💡 System reset triggered when no notification is received within 1000 ms timeout

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 5	After Step 6	Final
Notification Received	False	False	False	True	True	True
Watchdog Timer Reset	False	False	False	False	True	True
System Reset Triggered	False	False	True	False	False	False

Key Moments - 3 Insights

Why does the system reset if no notification is received?

Because the watchdog task waits for a notification within a timeout (step 1 and 2). If it times out without receiving it, it triggers a system reset (step 3).

What happens when a notification is received on time?

The watchdog timer is reset (step 6), preventing system reset and allowing the task to wait again (step 7).

Why is the watchdog timer reset important?

Resetting the watchdog timer signals the system is healthy. Without it, the watchdog assumes a fault and triggers a reset (see steps 2 and 3).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2?

A Notification received and watchdog timer reset

B Timeout expired without notification

C System reset completed

D Task waits for notification

💡 Hint

Check the 'Notification Received?' and 'System Reset Triggered?' columns at step 2

At which step does the system reset get triggered?

A Step 1

B Step 5

C Step 3

D Step 7

💡 Hint

Look at the 'System Reset Triggered?' column in the execution table

If the notification is always received on time, what will never happen?

A System reset triggered

B Watchdog timer reset

C Waiting for notification

D Looping back to wait again

💡 Hint

Refer to variable_tracker and execution_table rows where 'System Reset Triggered' is false when notification is true

Concept Snapshot

Watchdog Task Pattern in FreeRTOS:
- Task waits for notification with timeout
- If notification received: reset watchdog timer
- If timeout: trigger system reset
- Loop repeats indefinitely
- Ensures system health by monitoring task responsiveness

Full Transcript

The watchdog task pattern in FreeRTOS involves a dedicated task that waits for notifications from monitored tasks within a set timeout period. If the notification arrives on time, the watchdog timer is reset, indicating the system is healthy. If the notification does not arrive before the timeout expires, the watchdog task triggers a system reset to recover from potential faults. This cycle repeats indefinitely, ensuring the system remains responsive and safe. The execution table shows steps where the task waits, checks notifications, resets timers, or triggers resets. Variables track notification receipt, timer resets, and system resets. Key moments clarify why resets happen and the importance of timely notifications. The visual quiz tests understanding of these steps and outcomes.