Timer interrupt for periodic tasks in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a timer interrupt in embedded systems?

A timer interrupt is a signal from a hardware timer that temporarily stops the main program to run a special function, allowing tasks to happen at regular time intervals.

beginner

Why use timer interrupts for periodic tasks?

Timer interrupts let the system run tasks regularly without waiting or blocking the main program, making the system efficient and responsive.

intermediate

In embedded C, what is the typical structure of a timer interrupt service routine (ISR)?

An ISR is a special function marked with an interrupt attribute that runs automatically when the timer interrupt occurs. It usually clears the interrupt flag and performs the periodic task.

intermediate

How do you configure a timer for periodic interrupts?

You set the timer's count value and prescaler to control the interrupt frequency, enable the timer interrupt, and start the timer counting.

intermediate

What is the importance of clearing the interrupt flag inside the ISR?

Clearing the interrupt flag tells the hardware the interrupt was handled, preventing the ISR from running repeatedly for the same event.

What does a timer interrupt do in an embedded system?

A Stops the main program to run a task at set intervals

B Increases the CPU speed

C Saves power by turning off the CPU

D Changes the program code automatically

A timer interrupt pauses the main program briefly to run a task regularly.

Which step is necessary inside a timer ISR to avoid repeated interrupts?

A Restart the main program

B Clear the interrupt flag

C Disable the timer permanently

D Increase the timer count

Clearing the interrupt flag signals the hardware that the interrupt was handled.

How do you control how often a timer interrupt occurs?

A By setting the timer count and prescaler

B By changing the CPU clock speed

C By modifying the main program loop

D By disabling other interrupts

Timer count and prescaler set the timer period, controlling interrupt frequency.

What is the main benefit of using timer interrupts for periodic tasks?

A Uses more power

B Makes the program run slower

C Prevents multitasking

D Allows tasks to run regularly without blocking the main program

Timer interrupts let periodic tasks run smoothly alongside the main program.

Which keyword or attribute is commonly used to define an ISR in embedded C?

A static

B volatile

C __interrupt

D const

The __interrupt keyword marks a function as an interrupt service routine.

Explain how a timer interrupt can be used to blink an LED every second in an embedded system.

Think about setting the timer period and what the ISR should do.

Describe the steps to set up a timer interrupt for periodic tasks in embedded C.

Focus on both hardware setup and software ISR.

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