AVR Timer Interrupts in C

December 1, 2015 Jason Bauer 5 comments

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.

STK500 Setup

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

The Interrupt Header

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

#include &lt;avr/interrupt.h&gt;

1	#include <avr/interrupt.h>

The ISR keyword

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:

<span class="codecomment">// timer1 overflow</span>
ISR(TIMER1_OVF_vect) {
<span class="codecomment">    // process the timer1 overflow here</span>
}

// timer1 overflow

ISR(TIMER1_OVF_vect) {

// process the timer1 overflow here

}

Turning on the Timer Interrupt

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:

<span class="codecomment">// enable timer overflow interrupt for both Timer0 and Timer1</span>
TIMSK=(1&lt;&lt;TOIE0) | (1&lt;&lt;TOIE1);

1 2	<span class="codecomment">// enable timer overflow interrupt for both Timer0 and Timer1</span> TIMSK=(1<<TOIE0) \| (1<<TOIE1);

Setup Your Timer

Once you have your ISR defined, and you have enabled the interrupt in TIMSK, you can setup your timer how ever 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).

<span class="codecomment">// set timer0 counter initial value to 0</span>

1	<span class="codecomment">// set timer0 counter initial value to 0</span>

TCNT0=0x00;

1	TCNT0=0x00;

<span class="codecomment">// start timer0 with /1024 prescaler</span>

1	<span class="codecomment">// start timer0 with /1024 prescaler</span>

TCCR0 = (1&lt;&lt;CS02) | (1&lt;&lt;CS00);

1	TCCR0 = (1<<CS02) \| (1<<CS00);

Enable Interrupts

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

AVR Timer Interrupts Example

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.

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

// ********************************************************************************

// 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);

// lets 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.

Next Up, A More Complicated Example

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.

tagged with AVR

5 comments

Prof. Dattaraj Vidyasagar
April 9, 2016 3:28 am

Wow.
Thank you so much.
You site is just great
Thanks
Tell me how to subscribe to your updates?

Ganesh
April 27, 2016 5:15 am

When I ran this code, I getting an error ? as

In function __vector_16': 49: multiple definition of__vector_16′
/home/circuits/123D-Circuits-arduino-compiler/build/sketch.ino:17: first defined here
error: ld returned 1 exit status

volatile uint16_t tot_overflow;
void timer0_init()
{
TCCR0B |= (1<<CS02);
TCNT0 =0;
TIMSK0 |= (1 << TOIE0);
sei();
tot_overflow =0;
}
void setup()
{
timer0_init();
DDRD |= (1<= 245)
{
if(TCNT0>=25)
{
PORTD ^= (1<<2);
TCNT0=0;
tot_overflow =0;
}
}
}

——————————————————————————————————
but when I do small modification in the code, it gets executed
modifications were instead of void loop – I defined int main() and I called setup function from main function?

Can you tell me what would be the problem?
Ganesh
April 27, 2016 11:36 am

When I ran this code, I getting an error ? as

In function __vector_16′: 49: multiple definition of__vector_16′
/home/circuits/123D-Circuits-arduino-compiler/build/sketch.ino:17: first defined here
error: ld returned 1 exit status

volatile uint16_t tot_overflow;
void timer0_init()
{
TCCR0B |= (1<<CS02);
TCNT0 =0;
TIMSK0 |= (1 << TOIE0);
sei();
tot_overflow =0;
}
void setup()
{
timer0_init();
DDRD |= (1<= 245)
{
if(TCNT0>=25)
{
PORTD ^= (1<<2);
TCNT0=0;
tot_overflow =0;
}
}
}

but when I do small modification in the code, it gets executed
modifications were instead of void loop – I defined int main() and I called setup function from main function?

Can you tell me what would be the problem?
Sundar
July 1, 2016 3:08 am

I got an error

TIMSK is undeclared
TCCR0 is undeclared

can help me

Alex
February 23, 2019 10:38 pm

Hi

Someone kindly can explain me how have been calculated the values below.

– PORTA bit 0 blinking on and off 15 times / second
– PORTA bit 1 blinking on and off every 8.3 seconds

AVR Timer Interrupts in C

STK500 Setup

The Interrupt Header

The ISR keyword

Turning on the Timer Interrupt

Setup Your Timer

Enable Interrupts

AVR Timer Interrupts Example

Next Up, A More Complicated Example

5 comments

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