Timer interrupt for periodic tasks in Embedded C - Time & Space Complexity

Time Complexity: Timer interrupt for periodic tasks

O(n)

Understanding Time Complexity

When using a timer interrupt to run tasks regularly, it is important to understand how the time taken grows as the task or timer settings change.

We want to know how the program's work changes when the timer triggers more often or the task does more work.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

volatile int flag = 0;

void Timer_ISR(void) {
    flag = 1;  // Set flag when timer interrupt occurs
}

int main() {
    while(1) {
        if(flag) {
            flag = 0;
            perform_task();  // Task to run periodically
        }
    }
}
    

This code uses a timer interrupt to set a flag. The main loop checks the flag and runs a task periodically when the flag is set.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The main loop runs continuously, checking the flag.
• How many times: The loop runs forever, but the task runs only when the flag is set by the timer interrupt.

How Execution Grows With Input

As the timer triggers more often, the task runs more times, increasing total work.

Input Size (Timer Triggers)	Approx. Operations (Task Runs)
10	10
100	100
1000	1000

Pattern observation: The total work grows directly with how many times the timer interrupt fires and the task runs.

Final Time Complexity

Time Complexity: O(n)

This means the total work grows linearly with the number of timer interrupts and task executions.

Common Mistake

[X] Wrong: "The main loop only runs once per timer interrupt because the flag controls it."

[OK] Correct: The main loop runs continuously, checking the flag many times even when the task does not run. The task runs only when the flag is set, but the loop itself is always active.

Interview Connect

Understanding how timer interrupts affect program work helps you design efficient embedded systems that respond well without wasting CPU time.

Self-Check

"What if the task inside the interrupt service routine instead of the main loop? How would the time complexity change?"

Practice

1.

What is the main purpose of a timer interrupt in embedded C programming?

easy

A. To stop the main program permanently

B. To run code automatically at regular time intervals

C. To manually start a function when needed

D. To increase the speed of the processor

Solution

1. Step 1: Understand timer interrupt functionality

   Timer interrupts trigger code execution automatically at set time intervals without manual intervention.

2. Step 2: Compare options with timer interrupt purpose

   Only To run code automatically at regular time intervals describes automatic, regular code execution, which matches timer interrupt use.

3. Final Answer:

   To run code automatically at regular time intervals -> Option B

4. Quick Check:

   Timer interrupt = automatic periodic code run [OK]

Hint: Timer interrupts run code regularly without manual calls [OK]

Common Mistakes:

• Thinking timer interrupts stop the main program
• Confusing manual function calls with interrupts
• Assuming timer interrupts speed up the processor

2.

Which of the following is the correct way to enable a timer interrupt in embedded C?

void setup() {
    // Enable timer interrupt here
}

easy

A. TIMSK |= (1 << TOIE0);

B. TIMSK = 0;

C. TCCR0 = 0;

D. PORTB |= (1 << PB0);

Solution

1. Step 1: Identify register for timer interrupt enable

   TIMSK register controls timer interrupt enable bits; setting TOIE0 bit enables Timer0 overflow interrupt.

2. Step 2: Analyze each option

   TIMSK |= (1 << TOIE0); sets TOIE0 bit correctly. TIMSK = 0; clears TIMSK disabling interrupts. TCCR0 = 0; sets timer control register, not interrupt enable. PORTB |= (1 << PB0); sets port pin, unrelated to interrupts.

3. Final Answer:

   TIMSK |= (1 << TOIE0); -> Option A

4. Quick Check:

   Enable timer interrupt = set TIMSK TOIE0 bit [OK]

Hint: Enable timer interrupt by setting TIMSK TOIE0 bit [OK]

Common Mistakes:

• Clearing TIMSK disables interrupts
• Confusing timer control register with interrupt enable
• Setting port pins instead of interrupt bits

3.

What will be the output of the following code snippet if the timer interrupt triggers every 1 second and increments count?

volatile int count = 0;

ISR(TIMER0_OVF_vect) {
    count++;
}

int main() {
    setup_timer_interrupt();
    while(count < 3) {}
    printf("Count: %d", count);
    return 0;
}

medium

A. Count: 5

B. Count: 0

C. Count: 1

D. Count: 3

Solution

1. Step 1: Understand interrupt increments

   Each timer interrupt increments count by 1 every 1 second.

2. Step 2: Analyze loop condition and output

   Main loop waits until count reaches 3, then prints count. So output is "Count: 3".

3. Final Answer:

   Count: 3 -> Option D

4. Quick Check:

   count increments 3 times before print [OK]

Hint: Loop waits for count 3, so output is Count: 3 [OK]

Common Mistakes:

• Assuming count stays 0 without interrupt
• Printing before count reaches 3
• Confusing count increments with time

4.

Identify the error in the following code that prevents the timer interrupt from working properly:

volatile int flag = 0;

ISR(TIMER1_COMPA_vect) {
    flag = 1;
}

int main() {
    setup_timer1();
    while(flag == 0) {
        // wait
    }
    // do something
    return 0;
}

medium

A. The ISR vector name is incorrect for Timer1 Compare A interrupt

B. The flag variable should not be volatile

C. The setup_timer1() function is missing enabling the interrupt

D. The while loop condition should be flag != 0

Solution

1. Step 1: Check ISR vector correctness

   ISR vector TIMER1_COMPA_vect is correct for Timer1 Compare A interrupt.

2. Step 2: Verify interrupt enabling in setup

   If setup_timer1() does not enable the interrupt, ISR never triggers, so flag stays 0.

3. Final Answer:

   setup_timer1() function is missing enabling the interrupt -> Option C

4. Quick Check:

   Interrupt must be enabled in setup for ISR to run [OK]

Hint: Always enable interrupt in setup function [OK]

Common Mistakes:

• Assuming ISR vector name is wrong
• Removing volatile from flag
• Changing loop condition incorrectly

5.

You want to toggle an LED every 500ms using a timer interrupt on an 8-bit microcontroller with a 1MHz clock. Which approach correctly sets up the timer interrupt for this periodic task?

Options:

1. Set timer prescaler to 64, preload timer to 131, enable overflow interrupt.
2. Set timer prescaler to 256, preload timer to 0, enable compare match interrupt at 500.
3. Set timer prescaler to 8, preload timer to 0, enable overflow interrupt.
4. Set timer prescaler to 1024, preload timer to 0, enable overflow interrupt.

hard

A. Set timer prescaler to 64, preload timer to 131, enable overflow interrupt.

B. Set timer prescaler to 256, preload timer to 0, enable compare match interrupt at 500.

C. Set timer prescaler to 8, preload timer to 0, enable overflow interrupt.

D. Set timer prescaler to 1024, preload timer to 0, enable overflow interrupt.

Solution

1. Step 1: Calculate timer counts for 500ms with 1MHz clock

   Timer tick = prescaler / clock = 64 / 1,000,000 = 64us. Counts needed = 500ms / 64us = 7812.5 counts.

2. Step 2: Use 8-bit timer overflow with preload

   8-bit timer max count = 256. Number of overflows = 7812.5 / 256 ≈ 30.5. Preload = 256 - (7812.5 % 256) = 256 - 132.5 ≈ 123.5 (131 preload with prescaler 64 is closest practical setup).

3. Step 3: Analyze options

   Set timer prescaler to 64, preload timer to 131, enable overflow interrupt. uses prescaler 64 and preload 131, which is a common approach to adjust overflow timing. Other options either use wrong prescaler or compare match without correct value.

4. Final Answer:

   Set timer prescaler to 64, preload timer to 131, enable overflow interrupt. -> Option A

5. Quick Check:

   Prescaler 64 + preload adjusts overflow for 500ms toggle [OK]

Hint: Use prescaler and preload to match 500ms timer overflow [OK]

Common Mistakes:

• Ignoring timer resolution and prescaler effect
• Using wrong interrupt type for timing
• Not calculating preload for overflow timing