Watchdog timer operation in Embedded C - Step-by-Step Execution

Concept Flow - Watchdog timer operation

Start Program

↓

Initialize Watchdog Timer

↓

Enter Main Loop

↓

Reset Watchdog Timer?

No→Watchdog Timer Counts Down

↓

Watchdog Timer Expires

↓

Continue Normal Operation

↓

System Reset

The watchdog timer is set up and regularly reset in the main loop to prevent system reset. If not reset in time, it triggers a system reset.

Execution Sample

Embedded C

void main() {
  WDT_Init();
  while(1) {
    WDT_Reset();
    // normal tasks
  }
}

This code initializes the watchdog timer and resets it continuously inside the main loop to avoid system reset.

Execution Table

Step	Watchdog Timer State	Action	Result	System State
1	Stopped	Call WDT_Init()	Watchdog timer started with timeout	Running
2	Counting down	Enter main loop	Timer counts down	Running
3	Counting down	Call WDT_Reset()	Timer reset to full timeout	Running
4	Counting down	Perform normal tasks	No change to timer	Running
5	Counting down	Call WDT_Reset()	Timer reset again	Running
6	Counting down	Simulate missed reset	Timer continues counting down	Running
7	Near zero	No reset called	Timer expires	System Reset triggered
8	Reset	System restarts	Program restarts from beginning	Running

💡 Execution stops because the watchdog timer expires without reset, causing a system reset.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	After Step 7	Final
Watchdog Timer	Stopped	Full timeout	Full timeout	Counting down	Expired	Reset

Key Moments - 3 Insights

Why does the system reset if WDT_Reset() is not called in time?

Because the watchdog timer counts down and triggers a system reset when it reaches zero without being reset, as shown in step 7 of the execution_table.

What happens when WDT_Reset() is called inside the main loop?

The watchdog timer is reset to its full timeout value, preventing the system reset, as seen in steps 3 and 5.

Does normal task execution affect the watchdog timer?

No, normal tasks do not change the watchdog timer unless WDT_Reset() is called, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the watchdog timer state after step 5?

A Stopped

B Expired

C Full timeout (reset)

D Counting down near zero

💡 Hint

Check the 'Watchdog Timer State' column at step 5 in the execution_table.

At which step does the system reset get triggered?

A Step 4

B Step 7

C Step 3

D Step 2

💡 Hint

Look for 'System Reset triggered' in the 'System State' column in the execution_table.

If WDT_Reset() was never called, how would the timer state change by step 6?

A It would be counting down and near expiration

B It would be reset to full timeout

C It would remain stopped

D It would cause immediate system reset at step 2

💡 Hint

Refer to the 'Watchdog Timer State' progression in variable_tracker and execution_table steps 2 to 6.

Concept Snapshot

Watchdog Timer Operation:
- Initialize watchdog timer at program start
- Regularly call WDT_Reset() inside main loop
- If WDT_Reset() is missed, timer counts down to zero
- Timer expiration triggers system reset
- Prevents system hang by forcing restart

Full Transcript

This visual execution shows how a watchdog timer works in embedded C. First, the watchdog timer is initialized and started. Then the program enters the main loop where it regularly calls WDT_Reset() to reset the timer countdown. If the reset is missed, the timer counts down to zero and triggers a system reset. This mechanism helps recover from software hangs by restarting the system automatically. The execution table traces each step, showing timer state changes and system state. The variable tracker follows the watchdog timer value over time. Key moments clarify why resets are needed and how normal tasks do not affect the timer. The quiz tests understanding of timer states and reset timing.