Watchdog reset recovery in Embedded C - Step-by-Step Execution

Concept Flow - Watchdog reset recovery

System starts

↓

Initialize watchdog

↓

Main loop starts

↓

Reset watchdog timer periodically?

No→Watchdog timeout

↓

System reset triggered

↓

Continue normal operation

↓

Loop repeats

The system initializes the watchdog timer and enters the main loop. It must reset the watchdog timer regularly to avoid a system reset. If it fails, the watchdog triggers a reset, and the system restarts.

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 prevent system reset.

Execution Table

Step	Action	Watchdog Timer (ticks)	Condition	Result
1	System starts and initializes watchdog	0	N/A	Watchdog timer starts counting
2	Enter main loop	0	N/A	Ready to reset watchdog periodically
3	Reset watchdog timer	0	Timer reset	Watchdog timer cleared
4	Perform normal tasks	1	Timer < timeout	Continue loop
5	Reset watchdog timer again	0	Timer reset	Watchdog timer cleared
6	Perform normal tasks	1	Timer < timeout	Continue loop
7	Watchdog reset missed	5	Timer >= timeout	Watchdog triggers system reset
8	System resets	0	N/A	Restart from step 1

💡 Execution stops because watchdog timer reached timeout without reset, causing system reset.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	Final
Watchdog Timer (ticks)	0	0	0	5	0
System State	Start	Running	Running	Reset triggered	Restarted

Key Moments - 3 Insights

Why does the system reset if the watchdog timer is not reset in time?

Because the watchdog timer counts up and triggers a reset when it reaches the timeout value, as shown in execution_table row 7.

What happens to the watchdog timer when WDT_Reset() is called?

The watchdog timer is cleared to zero, preventing a reset, as seen in execution_table rows 3 and 5.

Why must WDT_Reset() be called regularly inside the main loop?

To keep the watchdog timer from reaching its timeout and causing a system reset, as shown by the repeated resets in the loop (rows 3 and 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the watchdog timer value right after the first WDT_Reset() call?

A 5

B 1

C 0

D Not reset yet

💡 Hint

Check execution_table row 3 where WDT_Reset() is called and timer is cleared.

At which step does the watchdog timer reach the timeout and cause a system reset?

A Step 7

B Step 3

C Step 5

D Step 2

💡 Hint

Look at execution_table row 7 where timer >= timeout triggers reset.

If WDT_Reset() is never called, what will happen according to the execution flow?

A System continues normally

B System resets due to watchdog timeout

C Watchdog timer resets automatically

D Watchdog timer stops counting

💡 Hint

Refer to concept_flow and execution_table rows 6-8 showing reset triggered by timeout.

Concept Snapshot

Watchdog Reset Recovery:
- Initialize watchdog timer at system start
- Regularly call WDT_Reset() in main loop
- Watchdog timer counts up if not reset
- If timer reaches timeout, system resets
- Reset recovery starts system from beginning

Full Transcript

This example shows how a watchdog timer helps recover from system hangs by forcing a reset if the system fails to reset the timer regularly. The system initializes the watchdog, then enters a main loop where it resets the watchdog timer periodically. If the reset is missed, the watchdog timer reaches its timeout and triggers a system reset. This reset restarts the system, allowing recovery from faults. The execution table traces each step, showing the watchdog timer value and system state. Key moments clarify why resetting the watchdog is critical and what happens when it is missed. The quiz tests understanding of timer values and reset conditions.

Practice

1.

What is the main purpose of checking the watchdog reset flag at system startup?

easy

A. To increase the watchdog timer interval

B. To disable the watchdog timer permanently

C. To detect if the system was restarted due to a watchdog timeout

D. To reset all system variables to zero

Solution

1. Step 1: Understand watchdog reset flag role

   The watchdog reset flag indicates if the last reset was caused by the watchdog timer expiring.

2. Step 2: Purpose of checking the flag at startup

   Checking this flag helps the program know if it needs to perform recovery actions after an unexpected reset.

3. Final Answer:

   To detect if the system was restarted due to a watchdog timeout -> Option C

4. Quick Check:

   Watchdog reset flag = detect watchdog reset [OK]

Hint: Watchdog flag shows if reset was watchdog-caused [OK]

Common Mistakes:

• Thinking the flag disables the watchdog
• Assuming it resets variables automatically
• Confusing flag with timer interval setting

2.

Which of the following code snippets correctly clears the watchdog reset flag in embedded C?

/* Assume WDT_RESET_FLAG is the flag variable */
// Clear the flag
?

easy

A. WDT_RESET_FLAG = 0;

B. clear(WDT_RESET_FLAG);

C. WDT_RESET_FLAG == 0;

D. reset_flag(WDT_RESET_FLAG);

Solution

1. Step 1: Identify correct syntax to clear a flag

   In embedded C, clearing a flag usually means setting its variable to zero.

2. Step 2: Analyze each option

   WDT_RESET_FLAG = 0; sets the flag to 0, which clears it. clear(WDT_RESET_FLAG); uses a non-standard function. WDT_RESET_FLAG == 0; uses comparison, not assignment. reset_flag(WDT_RESET_FLAG); uses an undefined function.

3. Final Answer:

   WDT_RESET_FLAG = 0; -> Option A

4. Quick Check:

   Clear flag = set to 0 [OK]

Hint: Clear flags by assigning zero directly [OK]

Common Mistakes:

• Using comparison operator '==' instead of assignment
• Calling undefined functions to clear flags
• Confusing clearing with resetting the whole system

3.

Consider the following code snippet that runs at system startup:

if (WDT_RESET_FLAG) {
    recover_from_watchdog();
    WDT_RESET_FLAG = 0;
} else {
    normal_startup();
}

What will happen if the system was reset by the watchdog timer?

medium

A. The system does nothing and hangs

B. The system calls recover_from_watchdog() and clears the flag

C. The system calls normal_startup() only

D. The system resets again immediately

Solution

1. Step 1: Check condition when watchdog reset flag is set

   If WDT_RESET_FLAG is true (non-zero), the if block executes.

2. Step 2: Follow code logic inside the if block

   The function recover_from_watchdog() is called, then the flag is cleared by setting it to zero.

3. Final Answer:

   The system calls recover_from_watchdog() and clears the flag -> Option B

4. Quick Check:

   Flag set = recover and clear flag [OK]

Hint: If flag set, run recovery then clear flag [OK]

Common Mistakes:

• Assuming normal_startup() runs after watchdog reset
• Thinking system hangs without recovery
• Believing system resets again immediately

4.

Identify the error in this watchdog reset recovery code snippet:

if (WDT_RESET_FLAG = 1) {
    recover_from_watchdog();
    WDT_RESET_FLAG = 0;
}

medium

A. Using assignment '=' instead of comparison '==' in the if condition

B. Not clearing the flag after recovery

C. Calling recover_from_watchdog() before checking the flag

D. Missing semicolon after the if statement

Solution

1. Step 1: Analyze the if condition syntax

   The condition uses '=' which assigns 1 to the flag instead of comparing it.

2. Step 2: Understand the impact of assignment in condition

   Assignment always returns true, so the if block runs every time, ignoring actual flag state.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in the if condition -> Option A

4. Quick Check:

   Use '==' for comparison in if conditions [OK]

Hint: Use '==' in if conditions, not '=' [OK]

Common Mistakes:

• Confusing '=' (assign) with '==' (compare)
• Forgetting to clear the flag after recovery
• Misplacing semicolons inside if statements

5.

You want to implement a watchdog reset recovery that logs the reset cause and safely restarts the system only if the reset was caused by the watchdog. Which sequence of actions is correct?

hard

A. Clear watchdog reset flag, restart system, then log reset cause

B. Log reset cause only if watchdog reset flag is not set

C. Restart system immediately, then check and clear watchdog reset flag

D. Check watchdog reset flag, log reset cause, clear flag, then restart system

Solution

1. Step 1: Check the watchdog reset flag at startup

   This tells if the reset was caused by the watchdog timer.

2. Step 2: Log the reset cause and clear the flag before restarting

   Logging helps diagnose issues; clearing the flag prevents repeated recovery loops.

3. Step 3: Restart the system safely after handling the flag

   This ensures the system restarts only after recovery steps are done.

4. Final Answer:

   Check watchdog reset flag, log reset cause, clear flag, then restart system -> Option D

5. Quick Check:

   Check flag -> log -> clear -> restart [OK]

Hint: Always check flag before logging and restarting [OK]

Common Mistakes:

• Restarting before clearing the flag
• Logging after restart (won't execute)
• Ignoring the flag and restarting blindly