Window watchdog concept in Embedded C - Mini Project: Build & Apply

Window Watchdog Concept in Embedded C

📖 Scenario: You are working on a simple embedded system that needs to monitor if a task is running correctly within a specific time window. If the task does not reset the watchdog timer in time, the system should detect this and take action.

🎯 Goal: Build a simple program that simulates a window watchdog timer. You will create variables to represent the timer, configure the allowed time window, implement the logic to check if the timer is reset within the window, and finally print the status of the watchdog.

📋 What You'll Learn

Create variables to represent the watchdog timer and window limits

Configure the allowed time window for resetting the watchdog

Implement logic to check if the watchdog is reset within the window

Print the watchdog status indicating if it is OK or if a reset is needed

💡 Why This Matters

🌍 Real World

Window watchdog timers are used in embedded systems to ensure tasks run correctly within specific time limits, preventing system crashes.

💼 Career

Understanding watchdog timers is important for embedded software developers working on reliable and safe hardware systems.

1

Create initial watchdog timer variables

Create an integer variable called watchdog_timer and set it to 0. Also create two integer variables called window_start and window_end and set them to 5 and 10 respectively.

Embedded C

# Create variables for watchdog_timer, window_start, and window_end
# Your code here

Hint

Use simple integer variables and assign the exact values as instructed.

2

Configure the reset time for the watchdog

Create an integer variable called reset_time and set it to 7. This represents the time when the watchdog is reset.

Embedded C

int watchdog_timer = 0;
int window_start = 5;
int window_end = 10;

// Create reset_time variable
// Your code here

Hint

Just add one more integer variable with the exact name and value.

3

Implement the window watchdog check logic

Write an if statement that checks if reset_time is greater than or equal to window_start and less than or equal to window_end. If true, set watchdog_timer to 0 to simulate a successful reset. Otherwise, set watchdog_timer to 1 to indicate a reset failure.

Embedded C

int watchdog_timer = 0;
int window_start = 5;
int window_end = 10;
int reset_time = 7;

// Add if statement to check reset_time within window
// Your code here

Hint

Use a simple if-else to check if reset_time is inside the allowed window.

4

Print the watchdog status

Write a printf statement that prints "Watchdog status: OK" if watchdog_timer is 0, otherwise print "Watchdog status: RESET NEEDED".

Embedded C

int watchdog_timer = 0;
int window_start = 5;
int window_end = 10;
int reset_time = 7;

if (reset_time >= window_start && reset_time <= window_end) {
    watchdog_timer = 0;
} else {
    watchdog_timer = 1;
}

// Print watchdog status
// Your code here

Hint

Use printf and check the watchdog_timer value to print the correct message.

Practice

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

easy

A. To increase the processing speed of the microcontroller

B. To reset the system if the software fails to refresh the timer within a specific time window

C. To store data temporarily during program execution

D. To provide power backup during system failure

Solution

1. Step 1: Understand the watchdog timer function

   A watchdog timer monitors if the software is running correctly by expecting periodic refreshes.

2. Step 2: Identify the role of the window watchdog

   The window watchdog resets the system if the refresh happens too early or too late, ensuring software reliability.

3. Final Answer:

   To reset the system if the software fails to refresh the timer within a specific time window -> Option B

4. Quick Check:

   Window watchdog resets system on missed or mistimed refresh [OK]

Hint: Watchdog resets system if refresh is outside allowed time [OK]

Common Mistakes:

• Confusing watchdog with memory storage
• Thinking watchdog speeds up the processor
• Assuming watchdog provides power backup

2. Which of the following is the correct way to refresh a window watchdog timer in embedded C?

easy

A. Call WWDG_Refresh() function within the allowed time window

B. Call WWDG_Refresh() only once at system startup

C. Disable the watchdog timer before refreshing

D. Refresh the watchdog timer continuously without delay

Solution

1. Step 1: Understand watchdog refresh requirements

   The watchdog must be refreshed periodically within a specific time window to avoid reset.

2. Step 2: Identify correct refresh method

   Calling WWDG_Refresh() within the allowed time window is the correct way to refresh.

3. Final Answer:

   Call WWDG_Refresh() function within the allowed time window -> Option A

4. Quick Check:

   Refresh watchdog periodically inside time window [OK]

Hint: Refresh watchdog periodically, not just once or too fast [OK]

Common Mistakes:

• Refreshing only once at startup
• Disabling watchdog before refresh
• Refreshing too frequently causing reset

3. Consider this code snippet using a window watchdog timer:

void main() {
  WWDG_Init();
  while(1) {
    delay(50);  // delay in ms
    WWDG_Refresh();
  }
}

If the allowed refresh window is between 40ms and 60ms, what will happen?

medium

A. The system will reset because refresh is too early

B. The system will reset because refresh is too late

C. The system will run normally without reset

D. The system will freeze due to watchdog error

Solution

1. Step 1: Analyze the refresh timing

   The code delays 50ms before refreshing the watchdog, which is within the 40-60ms window.

2. Step 2: Determine system behavior

   Since refresh happens inside the allowed window, the system will not reset and runs normally.

3. Final Answer:

   The system will run normally without reset -> Option C

4. Quick Check:

   Refresh at 50ms inside 40-60ms window = no reset [OK]

Hint: Check if refresh delay fits inside allowed window [OK]

Common Mistakes:

• Assuming 50ms is too early or too late
• Confusing delay units
• Ignoring the watchdog window limits

4. The following code causes the system to reset unexpectedly. What is the likely error?

void main() {
  WWDG_Init();
  while(1) {
    WWDG_Refresh();
    delay(30);  // delay in ms
  }
}

Assuming the allowed refresh window is 40ms to 60ms.

medium

A. Not calling WWDG_Refresh() frequently enough

B. Delay is too long causing late refresh

C. Missing WWDG_Init() call

D. Refreshing the watchdog too early before the window opens

Solution

1. Step 1: Check refresh timing relative to window

   The code refreshes the watchdog immediately, then delays 30ms, so refresh happens every ~30ms.

2. Step 2: Compare with allowed window

   Since the window starts at 40ms, refreshing at 30ms is too early and causes reset.

3. Final Answer:

   Refreshing the watchdog too early before the window opens -> Option D

4. Quick Check:

   Refresh before 40ms window causes reset [OK]

Hint: Refresh only after window opens, not before [OK]

Common Mistakes:

• Thinking delay is too long
• Ignoring refresh order
• Assuming missing init causes reset here

5. You want to design a system using a window watchdog timer that resets if the refresh happens outside 100ms to 200ms window. Which approach correctly implements this behavior in embedded C?

hard

A. Refresh the watchdog every 150ms

B. Refresh the watchdog every 50ms

C. Refresh the watchdog every 250ms

D. Refresh the watchdog only once at startup

Solution

1. Step 1: Understand the allowed refresh window

   The watchdog must be refreshed between 100ms and 200ms to avoid reset.

2. Step 2: Evaluate each refresh timing option

   Refreshing at 50ms is too early, 150ms is inside window, 250ms is too late, and once at startup is insufficient.

3. Final Answer:

   Refresh the watchdog every 150ms -> Option A

4. Quick Check:

   Refresh inside 100-200ms window avoids reset [OK]

Hint: Refresh watchdog only inside allowed time window [OK]

Common Mistakes:

• Refreshing too early or too late
• Refreshing only once
• Ignoring window timing constraints