Watchdog reset recovery in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a watchdog timer in embedded systems?

A watchdog timer is a hardware timer that resets the system if the software becomes unresponsive or stuck, helping to recover from errors automatically.

beginner

How does a watchdog reset recovery work?

When the watchdog timer expires because the software did not reset it in time, it triggers a system reset. After reset, the system can detect the cause and perform recovery steps.

intermediate

What is a common method to detect a watchdog reset after system restart?

Check a special hardware register or flag that indicates the reset source was the watchdog timer, allowing the program to know it recovered from a watchdog reset.

beginner

Why should the watchdog timer be regularly reset in the main program?

To prevent the watchdog from resetting the system during normal operation, the program must reset the watchdog timer regularly, showing it is running correctly.

intermediate

What is a safe practice when recovering from a watchdog reset?

Perform minimal initialization, log the reset cause if possible, and avoid complex operations that might cause another reset immediately.

What triggers a watchdog reset in an embedded system?

A Software fails to reset the watchdog timer in time

B User presses a reset button

C Power supply is disconnected

D Program completes successfully

The watchdog reset happens when the software does not reset the watchdog timer before it expires.

How can a program detect that a reset was caused by the watchdog?

A By checking the system clock

B By reading a file from disk

C By asking the user

D By checking a hardware reset flag or register

Most microcontrollers have a register or flag that indicates the reset source, including watchdog resets.

What should the program do regularly to avoid a watchdog reset?

A Turn off the watchdog

B Reset the watchdog timer

C Increase the watchdog timeout

D Ignore the watchdog

Resetting the watchdog timer regularly tells it the program is running fine and prevents reset.

What is a good practice after recovering from a watchdog reset?

A Disable the watchdog permanently

B Immediately restart the watchdog timer without checks

C Perform minimal initialization and log the reset cause

D Ignore the reset cause and continue normally

Minimal initialization and logging help avoid repeated resets and aid debugging.

Why is a watchdog timer useful in embedded systems?

A It helps recover from software hangs automatically

B It speeds up the processor

C It saves power

D It stores data permanently

The watchdog timer resets the system if software hangs, improving reliability.

Explain how a watchdog timer helps in system reliability and what steps you take after a watchdog reset.

Think about how the watchdog protects the system and what you do when it triggers a reset.

Describe how you would implement watchdog reset recovery in an embedded C program.

Consider initialization, normal operation, and recovery after reset.

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