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.