Timer interrupt for periodic tasks in Embedded C

Introduction

Timer interrupts help run tasks regularly without stopping the main program. They let your device do things on time, like a clock.

Blink an LED every second without stopping other code

Read a sensor value every 100 milliseconds automatically

Send data over communication every fixed time

Update a display regularly without delay

Measure time intervals in a program

Syntax

Embedded C

void Timer_ISR(void) {
    // Code to run when timer interrupt happens
}

// Setup timer with period and enable interrupt
void Timer_Init(void) {
    // Configure timer registers
    // Enable timer interrupt
    // Start timer
}

The function Timer_ISR is called automatically when the timer reaches the set time.

You must enable the timer interrupt and start the timer in your setup function.

Examples

This example toggles an LED every second using the timer interrupt.

Embedded C

void Timer_ISR(void) {
    LED = !LED; // Toggle LED
}

void Timer_Init(void) {
    Timer_Period = 1000; // 1 second
    Enable_Timer_Interrupt();
    Start_Timer();
}

This example reads a sensor every 100 milliseconds automatically.

Embedded C

void Timer_ISR(void) {
    sensor_value = Read_Sensor();
}

void Timer_Init(void) {
    Timer_Period = 100; // 100 ms
    Enable_Timer_Interrupt();
    Start_Timer();
}

Sample Program

This program simulates a timer interrupt that toggles an LED every 1 second. The main loop calls Timer_Tick to simulate time passing. Each time the timer reaches the period, the interrupt toggles the LED state.

Embedded C

#include <stdint.h>
#include <stdbool.h>

volatile bool led_state = false;

// Simulated hardware registers
volatile uint32_t TIMER_COUNT = 0;
volatile uint32_t TIMER_PERIOD = 1000; // 1 second period
volatile bool TIMER_INTERRUPT_FLAG = false;

// Simulated LED output
void Set_LED(bool state) {
    led_state = state;
}

// Timer Interrupt Service Routine
void Timer_ISR(void) {
    // Clear interrupt flag
    TIMER_INTERRUPT_FLAG = false;
    // Toggle LED
    Set_LED(!led_state);
}

// Timer initialization
void Timer_Init(void) {
    TIMER_COUNT = 0;
    TIMER_INTERRUPT_FLAG = false;
    // In real hardware, configure timer registers and enable interrupt here
}

// Simulate timer counting and interrupt triggering
void Timer_Tick(void) {
    TIMER_COUNT++;
    if (TIMER_COUNT >= TIMER_PERIOD) {
        TIMER_COUNT = 0;
        TIMER_INTERRUPT_FLAG = true;
        Timer_ISR();
    }
}

int main(void) {
    Timer_Init();
    for (int i = 0; i < 5; i++) {
        Timer_Tick();
        // Print LED state
        if (led_state) {
            printf("LED ON\n");
        } else {
            printf("LED OFF\n");
        }
    }
    return 0;
}

Important Notes

Timer interrupts run code automatically without waiting.

Always clear the interrupt flag inside the ISR to avoid repeated calls.

Keep ISR code short to avoid slowing the main program.

Summary

Timer interrupts let you run code regularly without stopping other tasks.

Set up the timer period and enable interrupts to use them.

Write your periodic code inside the interrupt service routine.

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