How to Implement AVR Timer Interrupts in C for Atmel Microcontrollers

Timer interrupts are an excellent way of having your AVR do something at a given interval.

They can fire off and interrupt what ever else the AVR is doing making for very precise timing. They are one of the best ways to implement custom waveforms for things such as positioning robot servos, dimming LED's, and driving speakers at different frequencies.

For this example, make sure that you have your PORTA jumpered to LEDs, as was discussed in our Port Output guide.

In order to use the built in interrupt features in WinAVR you need to include the interrupt header like this:

#include <avr/interrupt.h>

WinAVR uses the keyword ISR to denote an Interrupt Service Routine. We need to define the ISR for timer1 overflow. You do it like this:

// timer1 overflow
ISR(TIMER1_OVF_vect) {
// process the timer1 overflow here
}

In order for the interrupt to fire, you must enable it in the TIMSK register. To enable both timer0 and timer1 interrupts, use the following code in main:

// enable timer overflow interrupt for both Timer0 and Timer1
TIMSK=(1<<TOIE0) | (1<<TOIE1);

Once you have your ISR defined, and you have enabled the interrupt in TIMSK, you can setup your timer however you want. When the timer rolls over, the interrupt will be fired. Here is an example of setting up timer0 to count from 0 to 255, with a divide by 1024 prescaler. This will make the timer0 interrupt fire 30.63 times every second with a frequency of 8.0MHz (8,000,000 / 255 / 1024 = 30.63).

// set timer0 counter initial value to 0
TCNT0=0x00;
// start timer0 with /1024 prescaler
TCCR0 = (1<<CS02) | (1<<CS00);

This step is easy, simply call sei (); to turn on the global interrupt enable flag.

Here is a simple example that turns on both timer0 and timer1. It accomplishes the following:

• Sets up timer0 in divide by 1024 mode, counting from 0 to 255
• Sets up timer1 in divide by 1024 mode, counting from 0 to 65,535
• On timer0 interrupt, toggles PORTA bit 0.
• On timer1 interrupt, toggles PORTA bit 1.

You will see PORTA bit 0 blinking on and off 15 times / second, and PORTA bit 1 blinking on and off every 8.3 seconds.

// ********************************************************************************
// Includes
// ********************************************************************************
#include <avr/io.h>
#include 
#include <avr/interrupt.h>
 
// ********************************************************************************
// Interrupt Routines
// ********************************************************************************
// timer1 overflow
ISR(TIMER1_OVF_vect) {
	// XOR PORTA with 0x02 to toggle the LSB
	PORTA=PORTA ^ 0x02;
}
 
// timer0 overflow
ISR(TIMER0_OVF_vect) {
	// XOR PORTA with 0x01 to toggle the second bit up
	PORTA=PORTA ^ 0x01;
}
 
// ********************************************************************************
// Main
// ********************************************************************************
int main( void ) {
	// Configure PORTA as output
	DDRA = 0xFF;
	PORTA = 0xFF;
	// enable timer overflow interrupt for both Timer0 and Timer1
	TIMSK=(1<<TOIE0) | (1<<TOIE1);
	// set timer0 counter initial value to 0
	TCNT0=0x00;
	// start timer0 with /1024 prescaler
	TCCR0 = (1<<CS02) | (1<<CS00);
	// let's turn on 16 bit timer1 also with /1024
	TCCR1B |= (1 << CS10) | (1 << CS12);
	// enable interrupts
	sei(); 
	while(true) {
	}
}

You can download the complete source code here. This program compiles down to 246 bytes for us.

If you want to see some more timer interrupt examples, then check out our More Complex Timer Interrupt Functions guide. Or head back to our index of AVR Guides here.