Feeding (kicking) the watchdog in Embedded C - Time & Space Complexity

Time Complexity: Feeding (kicking) the watchdog

O(n)

Understanding Time Complexity

We want to understand how the time to feed the watchdog changes as the program runs.

How often does the feeding action happen as the system runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void feed_watchdog(void) {
    WDT->CLEAR = WDT_CLEAR_KEY;
}

int main(void) {
    while(1) {
        // ... other tasks ...
        feed_watchdog();
    }
    return 0;
}
    

This code repeatedly feeds the watchdog inside an infinite loop to prevent system reset.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The infinite loop calling feed_watchdog().
• How many times: The feeding happens continuously, as many times as the loop runs.

How Execution Grows With Input

Since the loop runs forever, the feeding operation happens repeatedly without end.

Input Size (n)	Approx. Operations
10	10 feedings
100	100 feedings
1000	1000 feedings

Pattern observation: The number of feedings grows directly with the number of loop iterations.

Final Time Complexity

Time Complexity: O(n)

This means the feeding operation happens once per loop iteration, growing linearly with the number of iterations.

Common Mistake

[X] Wrong: "Feeding the watchdog is a one-time setup and does not repeat."

[OK] Correct: The watchdog must be fed regularly inside the loop to prevent reset, so it repeats continuously.

Interview Connect

Understanding how often a watchdog is fed helps show you can reason about repeated operations in embedded systems, a useful skill for real-world programming.

Self-Check

"What if the feeding was done only once outside the loop? How would the time complexity change?"