Watchdog timer operation in Embedded C - Mini Project: Build & Apply

Watchdog Timer Operation

📖 Scenario: You are working on a simple embedded system that needs to stay responsive. To avoid the system freezing, you will use a watchdog timer. The watchdog timer resets the system if it is not refreshed regularly.

🎯 Goal: You will write a small program that sets up a watchdog timer, refreshes it in a loop, and prints a message when the watchdog is refreshed.

📋 What You'll Learn

Create a variable to represent the watchdog timer control register

Create a variable for the watchdog timer refresh value

Write a function to refresh the watchdog timer

Use a loop to refresh the watchdog timer multiple times

Print a message each time the watchdog timer is refreshed

💡 Why This Matters

🌍 Real World

Watchdog timers are used in embedded systems like home appliances, cars, and medical devices to ensure the system recovers from unexpected freezes.

💼 Career

Understanding watchdog timer operation is important for embedded software engineers to write reliable and safe firmware.

1

Setup Watchdog Timer Control Register

Create an unsigned int variable called WDT_CTRL and set it to 0x00 to represent the watchdog timer control register.

Embedded C

# Create an unsigned int variable called WDT_CTRL and set it to 0x00
# Your code here

Hint

The watchdog timer control register is usually a hardware register. Here, we simulate it with a variable.

2

Define Watchdog Refresh Value

Create an unsigned int variable called WDT_REFRESH_VALUE and set it to 0xAA. This value will be used to refresh the watchdog timer.

Embedded C

unsigned int WDT_CTRL = 0x00;
# Create an unsigned int variable called WDT_REFRESH_VALUE and set it to 0xAA
# Your code here

Hint

The refresh value is a special code that resets the watchdog timer counter.

3

Write Watchdog Refresh Function

Write a function called refresh_watchdog that takes no parameters and sets WDT_CTRL to WDT_REFRESH_VALUE.

Embedded C

unsigned int WDT_CTRL = 0x00;
unsigned int WDT_REFRESH_VALUE = 0xAA;

// Write a function called refresh_watchdog that sets WDT_CTRL to WDT_REFRESH_VALUE
// Your code here

Hint

This function simulates the action of refreshing the watchdog timer by writing the refresh value to the control register.

4

Refresh Watchdog in a Loop and Print

Write a main function that calls refresh_watchdog() five times in a for loop. After each call, print "Watchdog refreshed" followed by a newline.

Embedded C

unsigned int WDT_CTRL = 0x00;
unsigned int WDT_REFRESH_VALUE = 0xAA;

void refresh_watchdog() {
    WDT_CTRL = WDT_REFRESH_VALUE;
}

// Write main function to refresh watchdog 5 times and print message each time
// Your code here

Hint

The main function runs the refresh five times and prints the message each time.

Practice

1. What is the main purpose of a watchdog timer in embedded systems?

easy

A. To provide power to the microcontroller

B. To increase the processing speed of the CPU

C. To reset the system if the program stops responding

D. To store data temporarily during program execution

Solution

1. Step 1: Understand watchdog timer function

   A watchdog timer monitors the program to detect if it stops responding or freezes.

2. Step 2: Identify watchdog timer action

   If the program fails to reset the watchdog timer in time, the watchdog resets the system to recover from the freeze.

3. Final Answer:

   To reset the system if the program stops responding -> Option C

4. Quick Check:

   Watchdog timer resets system on freeze = D [OK]

Hint: Watchdog resets system if program hangs [OK]

Common Mistakes:

• Thinking watchdog speeds up CPU
• Confusing watchdog with memory storage
• Assuming watchdog provides power

2. Which of the following is the correct way to reset (kick) a watchdog timer in embedded C?

easy

A. watchdog_reset();

B. reset_watchdog();

C. kick_watchdog();

D. wdt_reset();

Solution

1. Step 1: Recall standard watchdog reset function

   In many embedded C libraries, the function to reset the watchdog timer is wdt_reset().

2. Step 2: Check other options

   Other options like watchdog_reset(), reset_watchdog(), and kick_watchdog() are not standard function names and will cause errors.

3. Final Answer:

   wdt_reset(); -> Option D

4. Quick Check:

   Standard watchdog reset function = wdt_reset() [OK]

Hint: Use wdt_reset() to reset watchdog in embedded C [OK]

Common Mistakes:

• Using non-standard function names
• Forgetting parentheses in function call
• Confusing reset with initialization

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

void main() {
  setup_watchdog(5); // 5 seconds timeout
  while(1) {
    // No watchdog reset here
  }
  printf("Running");
}

medium

A. The program prints 'Running' continuously

B. The program resets after 5 seconds and never prints 'Running'

C. The program prints 'Running' once and stops

D. Compilation error due to missing watchdog reset

Solution

1. Step 1: Analyze watchdog setup and loop

   The watchdog is set with a 5-second timeout, but inside the infinite loop, the watchdog reset function is never called.

2. Step 2: Understand watchdog behavior on timeout

   Since the watchdog is not reset, after 5 seconds it triggers a system reset. The printf statement is never reached.

3. Final Answer:

   The program resets after 5 seconds and never prints 'Running' -> Option B

4. Quick Check:

   No watchdog reset causes reset before printf = A [OK]

Hint: No watchdog reset means system resets before print [OK]

Common Mistakes:

• Assuming printf runs before reset
• Thinking program runs normally without reset
• Confusing compile error with runtime reset

4. Identify the error in this watchdog usage code snippet:

void main() {
  setup_watchdog(10);
  while(1) {
    if (some_condition) {
      wdt_reset();
    }
  }
}

medium

A. Watchdog is reset only conditionally, may cause unexpected reset

B. Watchdog timeout is too long for embedded systems

C. Missing initialization of watchdog timer

D. Incorrect function name for watchdog reset

Solution

1. Step 1: Review watchdog reset condition

   The watchdog reset wdt_reset() is called only if some_condition is true, so it may not be called regularly.

2. Step 2: Understand watchdog requirement

   Watchdog must be reset regularly regardless of conditions to prevent system reset. Conditional reset risks unexpected resets.

3. Final Answer:

   Watchdog is reset only conditionally, may cause unexpected reset -> Option A

4. Quick Check:

   Conditional watchdog reset risks reset = C [OK]

Hint: Always reset watchdog regularly, not conditionally [OK]

Common Mistakes:

• Ignoring conditional reset risk
• Thinking timeout length is error
• Assuming missing initialization

5. You want to use a watchdog timer to reset your embedded system if it freezes, but only after 3 seconds of no reset. Which approach correctly implements this behavior?

hard

A. Initialize watchdog with 3 seconds timeout and call wdt_reset() regularly within 3 seconds

B. Initialize watchdog with 10 seconds timeout and call wdt_reset() every 5 seconds

C. Initialize watchdog with 1 second timeout and call wdt_reset() every 4 seconds

D. Do not initialize watchdog and reset system manually every 3 seconds

Solution

1. Step 1: Set watchdog timeout to desired reset delay

   To reset after 3 seconds of no reset, watchdog timeout must be set to 3 seconds.

2. Step 2: Reset watchdog regularly within timeout

   Call wdt_reset() regularly before 3 seconds elapse to prevent reset during normal operation.

3. Final Answer:

   Initialize watchdog with 3 seconds timeout and call wdt_reset() regularly within 3 seconds -> Option A

4. Quick Check:

   Timeout = 3s and regular reset = B [OK]

Hint: Timeout matches reset interval; reset watchdog before timeout [OK]

Common Mistakes:

• Setting timeout longer than reset interval
• Resetting watchdog less frequently than timeout
• Not initializing watchdog at all