Feeding (kicking) the watchdog in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Watchdog Timer Feeding Frequency

Consider a watchdog timer that resets the system if not fed within 1000 milliseconds. What will be the output of the following code snippet if the feeding interval is set to 1200 milliseconds?

Embedded C

#include <stdio.h>
#include <stdbool.h>

bool watchdog_fed = false;

void feed_watchdog() {
    watchdog_fed = true;
}

void watchdog_check(int interval_ms) {
    if (interval_ms > 1000 && !watchdog_fed) {
        printf("System Reset Triggered\n");
    } else {
        printf("System Running Normally\n");
    }
    watchdog_fed = false;
}

int main() {
    watchdog_check(1200);
    feed_watchdog();
    return 0;
}

A No Output

B System Running Normally

C Compilation Error

D System Reset Triggered

💡 Hint

Think about whether the watchdog is fed within the allowed time interval.

The watchdog expects to be fed within 1000 ms. Feeding at 1200 ms is too late, so the system triggers a reset.

🧠 Conceptual

intermediate

Purpose of Feeding the Watchdog

Why is it important to feed (kick) the watchdog timer regularly in embedded systems?

A To enable debugging mode in the system

B To prevent the system from resetting due to software hang or fault

C To save power by shutting down peripherals

D To increase the CPU clock speed automatically

💡 Hint

Think about what happens if the watchdog is not fed.

Feeding the watchdog prevents it from resetting the system, which happens if the software hangs or malfunctions.

🔧 Debug

advanced

Identify the Bug in Watchdog Feeding Code

What is the bug in the following code that causes the watchdog to reset unexpectedly?

Embedded C

void feed_watchdog() {
    // Missing actual feeding command
}

int main() {
    while(1) {
        // supposed to feed watchdog here
        feed_watchdog();
        // other tasks
    }
    return 0;
}

A The return statement is unreachable

B The main loop is infinite, causing a system hang

C The feed_watchdog function does not actually feed the watchdog hardware

D The watchdog is fed too frequently causing overflow

💡 Hint

Check what the feed_watchdog function does internally.

The function feed_watchdog is empty and does not reset the watchdog timer, so the watchdog triggers a reset.

📝 Syntax

advanced

Correct Feeding Syntax for Watchdog Timer

Which of the following code snippets correctly feeds the watchdog timer assuming the hardware register is WDT_RESET = 1?

A WDT_RESET = 1;

B WDT_RESET == 1;

C if (WDT_RESET = 1) {}

D WDT_RESET := 1;

💡 Hint

Remember the difference between assignment and comparison operators in C.

Assignment uses a single '=', so 'WDT_RESET = 1;' correctly feeds the watchdog.

🚀 Application

expert

Calculate Watchdog Feeding Interval

An embedded system's watchdog timer resets if not fed within 5000 clock cycles. The system clock runs at 1 MHz. What is the maximum time in milliseconds allowed between feeding the watchdog?

A 5 milliseconds

B 0.5 milliseconds

C 5000 milliseconds

D 50 milliseconds

💡 Hint

Calculate time = cycles / clock frequency.

At 1 MHz, 1 cycle = 1 microsecond. 5000 cycles = 5000 microseconds = 5 milliseconds.

Practice

1. What is the main purpose of feeding (kicking) the watchdog in embedded C systems?

easy

A. To start a new program execution

B. To increase the system clock speed

C. To save power by turning off peripherals

D. To reset the watchdog timer and prevent system reset

Solution

1. Step 1: Understand watchdog function

   The watchdog timer resets the system if it is not fed in time, preventing hangs.

2. Step 2: Purpose of feeding watchdog

   Feeding the watchdog resets its timer, showing the system is running fine.

3. Final Answer:

   To reset the watchdog timer and prevent system reset -> Option D

4. Quick Check:

   Feeding watchdog = reset timer [OK]

Hint: Feeding watchdog means resetting its timer regularly [OK]

Common Mistakes:

• Thinking feeding increases clock speed
• Confusing feeding with power saving
• Assuming feeding starts program execution

2. Which of the following is the correct syntax to feed (kick) the watchdog in embedded C if the function is named FeedWatchdog()?

easy

A. FeedWatchdog();

B. feed_watchdog();

C. FeedWatchdog;

D. FeedWatchdog[];

Solution

1. Step 1: Identify correct function call syntax

   In C, calling a function requires parentheses and a semicolon.

2. Step 2: Check each option

   FeedWatchdog(); uses correct capitalization and parentheses; others are incorrect syntax.

3. Final Answer:

   FeedWatchdog(); -> Option A

4. Quick Check:

   Function call syntax = functionName(); [OK]

Hint: Function calls need parentheses and semicolon [OK]

Common Mistakes:

• Omitting parentheses when calling function
• Using wrong capitalization
• Using brackets instead of parentheses

3. What will be the output of the following code snippet if the watchdog is not fed within 5 seconds?

void main() {
  init_watchdog(5); // 5 seconds timeout
  while(1) {
    // no FeedWatchdog() call here
  }
}

medium

A. System runs normally without reset

B. System resets after 5 seconds

C. Compilation error due to missing FeedWatchdog()

D. Watchdog timer disables automatically

Solution

1. Step 1: Understand watchdog timeout behavior

   If the watchdog is not fed within the timeout, it triggers a system reset.

2. Step 2: Analyze code behavior

   No FeedWatchdog() call means timer is not reset, so system resets after 5 seconds.

3. Final Answer:

   System resets after 5 seconds -> Option B

4. Quick Check:

   Not feeding watchdog = system reset [OK]

Hint: No feeding within timeout causes reset [OK]

Common Mistakes:

• Assuming system runs normally without feeding
• Expecting compilation error for missing feed
• Thinking watchdog disables automatically

4. Identify the error in this code snippet that feeds the watchdog incorrectly:

void main() {
  init_watchdog(10);
  while(1) {
    FeedWatchdog;
    delay(1000);
  }
}

medium

A. Missing parentheses in FeedWatchdog call

B. Incorrect delay function usage

C. Watchdog timeout too short

D. No error, code is correct

Solution

1. Step 1: Check FeedWatchdog usage

   FeedWatchdog is a function and must be called with parentheses.

2. Step 2: Analyze code snippet

   FeedWatchdog; is missing parentheses, so it does not call the function.

3. Final Answer:

   Missing parentheses in FeedWatchdog call -> Option A

4. Quick Check:

   Function call needs () [OK]

Hint: Always use () to call functions [OK]

Common Mistakes:

• Forgetting parentheses on function calls
• Assuming delay is incorrect without context
• Thinking timeout value causes error

5. You have a system with a watchdog timeout of 3 seconds. Your main loop takes 5 seconds to complete one iteration. How should you modify your code to prevent the watchdog from resetting the system?

hard

A. Remove the FeedWatchdog() calls entirely

B. Increase the watchdog timeout to 10 seconds

C. Call FeedWatchdog() multiple times inside the loop before 3 seconds pass

D. Call FeedWatchdog() only once after the loop finishes

Solution

1. Step 1: Understand watchdog timeout vs loop time

   The loop takes longer than the watchdog timeout, so feeding once per loop is too late.

2. Step 2: Feeding watchdog multiple times inside loop

   Calling FeedWatchdog() multiple times before 3 seconds resets timer and prevents reset.

3. Final Answer:

   Call FeedWatchdog() multiple times inside the loop before 3 seconds pass -> Option C

4. Quick Check:

   Feed watchdog before timeout expires [OK]

Hint: Feed watchdog often enough within timeout [OK]

Common Mistakes:

• Assuming increasing timeout is always possible
• Feeding watchdog only once after long loop
• Removing feeding calls thinking it helps