Timer interrupts allow you to perform a task at very specifically timed intervals regardless of what else is going on in your code. In this instructable I'll explain how to setup and execute an interrupt in Clear Timer on Compare Match or CTC Mode. Jump straight to step 2 if you are looking for sample code.

Normally when you write an Arduino sketch the Arduino performs all the commands encapsulated in the loop() {} function in the order that they are written, however, it's difficult to time events in the loop(). Some commands take longer than others to execute, some depend on conditional statements (if, while...) and some Arduino library functions (like digitalWrite or analogRead) are made up of many commands. Arduino timer interrupts allow you to momentarily pause the normal sequence of events taking place in the loop() function at precisely timed intervals, while you execute a separate set of commands. Once these commands are done the Arduino picks up again where it was in the loop().

Interrupts are useful for:

Measuring an incoming signal at equally spaced intervals (constant sampling frequency)
Calculating the time between two events
Sending out a signal of a specific frequency
Periodically checking for incoming serial data
much more...

There are a few ways to do interrupts, for now I'll focus on the type that I find the most useful/flexible, called Clear Timer on Compare Match or CTC Mode. Additionally, in this instructable I'll be writing specifically about the timers to the Arduino Uno (and any other Arduino with ATMEL 328/168... Lilypad, Duemilanove, Diecimila, Nano...). The main ideas presented here apply to the Mega and older boards as well, but the setup is a little different and the table below is specific to ATMEL 328/168.

Step 1: Prescalers and the Compare Match Register

The Uno has three timers called timer0, timer1, and timer2.  Each of the timers has a counter that is incremented on each tick of the timer's clock.  CTC timer interrupts are triggered when the counter reaches a specified value, stored in the compare match register.  Once a timer counter reaches this value it will clear (reset to zero) on the next tick of the timer's clock, then it will continue to count up to the compare match value again.  By choosing the compare match value and setting the speed at which the timer increments the counter, you can control the frequency of timer interrupts.

The first parameter I'll discuss is the speed at which the timer increments the counter.  The Arduino clock runs at 16MHz, this is the fastest speed that the timers can increment their counters.  At 16MHz each tick of the counter represents 1/16,000,000 of a second (~63ns), so a counter will take 10/16,000,000 seconds to reach a value of 9 (counters are 0 indexed), and 100/16,000,000 seconds to reach a value of 99.

In many situations, you will find that setting the counter speed to 16MHz is too fast.  Timer0 and timer2 are 8 bit timers, meaning they can store a maximum counter value of 255.  Timer1 is a 16 bit timer, meaning it can store a maximum counter value of 65535.  Once a counter reaches its maximum, it will tick back to zero (this is called overflow).  This means at 16MHz, even if we set the compare match register to the max counter value, interrupts will occur every 256/16,000,000 seconds (~16us) for the 8 bit counters, and every 65,536/16,000,000 (~4 ms) seconds for the 16 bit counter.  Clearly, this is not very useful if you only want to interrupt once a second.

Instead you can control the speed of the timer counter incrementation by using something called a prescaler.  A prescaler dictates the speed of your timer according the the following equation:

(timer speed (Hz)) = (Arduino clock speed (16MHz)) / prescaler

So a 1 prescaler will increment the counter at 16MHz, an 8 prescaler will increment it at 2MHz, a 64 prescaler = 250kHz, and so on.  As indicated in the tables above, the prescaler can equal 1, 8, 64, 256, and 1024.  (I'll explain the meaning of CS12, CS11, and CS10 in the next step.) 

Now you can calculate the interrupt frequency with the following equation:

interrupt frequency (Hz) = (Arduino clock speed 16,000,000Hz) / (prescaler * (compare match register + 1))
the +1 is in there because the compare match register is zero indexed

rearranging the equation above, you can solve for the compare match register value that will give your desired interrupt frequency:

compare match register = [ 16,000,000Hz/ (prescaler * desired interrupt frequency) ] - 1
remember that when you use timers 0 and 2 this number must be less than 256, and less than 65536 for timer1

so if you wanted an interrupt every second (frequency of 1Hz):
compare match register = [16,000,000 / (prescaler * 1) ] -1
with a prescaler of 1024 you get:
compare match register = [16,000,000 / (1024 * 1) ] -1
= 15,624
since 256 < 15,624 < 65,536, you must use timer1 for this interrupt.

<p>thank u sir </p>
<p>Great instructable! Thank you very much!</p>
<p>Thank you for breaking this down. I've read a lot of articles about timers but only yours makes it clear!</p>
<p>how exactly this is working for you all ?</p><p>For me while compiling in arduino 1.6.10 it is showing </p><p><strong>error: 'TIMSK1' was not declared in this scope</strong> which is same for all above registers. do we need any #include header file here?</p>
<p>Should also mention that the clock prescaler bit names CSX0, CSX1, CSX2 has the first number as 'X'. That number 'X' is linked to the timer number. So if 'X' is 0, then CS00, CS01, CS02 are prescaler bit names for timer 0; and the second value represents the bit position. So C00 means bit 0 (right-most-bit) of the 8-bit register TCCR0B, which is for timer 0.</p><p>The reason why I'm indicating this is because the tables (diagram) that go with &quot;prescalers and compare match register&quot; is confusing - since there's no mention about what CS10, CS11, CS12 etc are actually related to.</p>
<p>&quot;so a counter will take 10/16,000,000 seconds to reach a value of 9 (counters are 0 indexed)&quot;</p><p>Do you actually mean it takes 10/16000000 seconds to begin (initial state) from index 0 and get back to a 0 index again? That is, initial index of 0. Then first increment is 1, second increment is 2, third increment is 3, all the way to 9th increment to index 9, and then one extra increment to get back to index 0 again. Altogether 10 increments to 'get back to 0'.</p>
<p>The comment for the line containing &quot;ISR(TIMER2_COMPA_vect)&quot; is confusing. &quot;<em><strong>timer1</strong></em>&quot; should probably be &quot;<strong><em>timer2</em></strong>&quot;.</p>
<p>my signal is in the frequency range between 20Hz and 500Hz, so i am taking the sampling frequency as 5000Hz (sampling time will be .2ms), after sampling i need to store the sampled values in the SD card and so i need to use spi. i want to know how much time will spi take to store in SD card and will i be able to sample my signal at this rate? kindly help</p>
<p>Can anyone please explain the connections to arduino in example 1 ? <br>I want to know how is the signal generator and Oscilloscope is connected to the Arduino ?</p><p>Thanks</p>
<p>Brilliant! I struggled with the datasheets for a few days before finding this instructable, and now it's all clear.</p><p>Thanks for sharing.</p>
<p>This is the best interrupts explanations I have come across.</p><p>Thanks !</p>
<p>Hi, currently im doing project where it prompt user in serial communication and doing interrupt ISR at the same time. So , when user enter a specific value , it will stop the interrupt for few seconds and then restart the interrupt again. However , when restart the interrupt , the timing is out.(expected interrupt time : 5 microsec per interrupt , actual interrupt time : higher than 5 microsec ) <br><br>my code is here..<br>void setup()</p><p>{</p><p>cli();</p><p> TCCR0A = 0;</p><p> TCCR0B = 0;</p><p> TCNT0 = 0;</p><p> OCR0A = 80;// = (16*10^6) / (200000*1) - 1 (must be &lt;256) //200000hz=5microsec </p><p> TCCR0A |= (1 &lt;&lt; WGM01);</p><p> TCCR0B |= (0 &lt;&lt; CS02)| (0 &lt;&lt; CS01) | (1 &lt;&lt; CS00);</p><p> TIMSK0 |= (1 &lt;&lt; OCIE0A);</p><p> sei();</p><p>}</p><p>void loop()<br>{</p><p>if(serial.available&gt;0)</p><p>{</p><p>//if user press something...</p><p>TIMSK0 &amp;= ~(1 &lt;&lt; OCIE0A);</p><p>delay(10000);</p><p>TIMSK0 |= (1 &lt;&lt; OCIE0A);</p><p>}</p><p>}</p><p>ISR(TIMER0_COMPA_vect)</p><p>{</p><p>//code</p><p>} </p>
<p>Good Work. Thank you! </p>
<p> // turn on CTC mode<br> TCCR0A |= (1 &lt;&lt; WGM01);<br> // Set CS01 and CS00 bits for 64 prescaler<br> TCCR0B |= (1 &lt;&lt; CS01) | (1 &lt;&lt; CS00); </p><p>What is the TCCR0A register? TCCR0B must be the 3-bit one as illustrated (where you set the prescaler) but I don't recall any mention of the other one.</p>
<p>This made it much clearer than other reads I have been to. Based on this explanation, I was able to devise a routine to discipline my software clock based on the 16MHz crystal. One of my units (Uno) is ticking at a very wrong speed. Gaining about 30 seconds per day. That's a LOT! But the software can now take care of it and any other errant crystal it encounters. Thanks!!</p>
<p>Sorry. Left out that I am getting NTP time hacks once per day but losing 30 seconds per day is way overboard. Maybe I will replace the crystal and see but as long as I can correct for it, it's not so bad.</p>
<p>In the code you set the timer1 register to CTC mode by &quot;TCCR1B |= (1 &lt;&lt; WGM12);&quot;. However, for all of the examples using timer0 and timer2, you set the CTC mode by &quot;TCCR0B |= (1 &lt;&lt; WGM01);&quot; why do you set bit1 high instead of bit2 for timers 0 and 2 to put them into CTC mode? I found this table online that lists the mode settings(see attached), and it states that (100) is CTC mode. However, it is only for timer1... Is it different for timers 0 and 2?</p>
<p>If the Arduino is executing some code within an interrupt does the counter continue to count up so that the next interrupt is correctly timed? </p>
<p>Yes it does. But you should keep your interupt service routine (ISR) as short as possible. Store a variable, set a flag and that's it. Do time consuming stuff in your main loop. </p>
<p>according to datasheet of atmega328p p106 -108 when we choose values of COM0A0 and COM0BO for timer0</p><p>we set the behavior of pins OC0A OCOB After having config them as output and hardware will proceed as its describe in the datasheet.</p><p>all modes Normal mode ,CTC mode non PWM ..ect ..are described in page 100</p><p>when you'll read:For generating a waveform output in CTC mode the OC0A output can be set to toggle each logical level on each compare match ( you don't have to toggle it by soft ).</p>
<p>you don' need to write a code for toggling because atmega328p has 3 timers</p><p>and every timer has 2 output pins ex: timer2 has OC2A pin 11 and OC2B pin3 and in isr routine u can do other thing.</p>
<p>could you please elaborate more on why the toggle code isn't necessary in this sketch?</p>
<p>so if you wanted an interrupt every second (frequency of 1Hz):</p><p>compare match register = [16,000,000 / (prescaler * 1) ] -1<br>with a prescaler of 1024 you get:</p>
<p>Hi,</p><p>Your project and explanations helped me a lot starting with my project. Thanks!</p><p>I'm using Arduino uno timer 0 and 2 to output 2 distinct (in sync) MIDI Clock signal and it works like charm. I can change BPM with a knob going from 30 to 250.</p><p>I'm struggling on my next step which is introducing an offset (-200ms / +200ms) between the 2 signals using an analogRead on a knob.</p><p>Would you know how to induce such an offset for a Serial output?</p><p>Timr0 and Timer2 doesnt have much room to change start point and end point and the induced delay would only be a few milliseconds maybe.</p><p>Well if this message finds you and you wish to reply, I'm all ears opened.</p><p>Thanks again,</p><p>Stef</p>
<p>Wonderful, I worked with PICs but I see the AVR work slighly different</p>
<p>Hi, great tutorial, thanks heaps.</p><p>What libraries do I need to include, I'm getting compiler errors saying it doesn't know the names of the timing registers.</p>
<p>Hi Amanda,</p><p>I found these article very informative. I'm designing a card reader and i have included these timer interrupts in my programs.I have several confusions to use these timer interrupts on Arduino Leonardo.I have attached pins on pin 3 and pin 2 for D0 and D1 from my Mifare card reader.My card is a Mifare (32bits) type, so when i batch these card,i'm getting the extra bit of 33,so i decided to use timer interrupts just to execute exact 32 bits,but still i'm getting these extra bit.Please help me..Im lost!!</p>
<p>In an Arduino code for pulse sensor 256 prescalar is activated by TCCR2b=0x06 . Why is it so ? Why is it not 0x04 as per the table for Timer0. Is it possible to make an interrupt of 100 Hz using Timer 2. (ocr2a &lt;256, 1024 prescalar is not present). Could you help me with this. I am just a beginner.</p>
<p>How could I make an interrupt of 100Hz with timer 2. Could you share the table for timer2 similar to the one for Timer0 .</p>
<p>Just wondering if it would not be easier and more accurate to generate an interupt when a switch change is registered and use millis() to determine the time since the last switch change? Sampling every 1ms using the method above may miss a switch change?? </p>
<p>I am no expert but to my knowledge the problem with switch state interrupts is that they bounce and your can't denounce a switch using software on an interrupt routine as its already been triggered (as in the interrupt routine is already done before you have a chance to de-bounce.). The other way is to use a hardware based button debounce switch which just uses a capacitor. This will solve the problem. However unless your project is huge, polling a switch every x cycles doesn't really make any difference to any project.</p>
<p>I had the problem with a motor encoder, just put a 10k Ohms resistor to Vcc and a 1&micro;F capacitor to GND around your interrupt pin, this will be ok !!<br>Pin interrupt would be more efficient because you'll never miss a change, and above that, you'll not waste time with useless interruption =)</p>
<p>While I don't think sampling every 1ms may miss a switch change (think about it, 1000 times a second???) I too would like to know if the more efficient way would be to make an interrupt for the switch change. I have a project that would use something similar and I'm trying to determine the most efficient course of action.</p>
<p>thank for Example interrupts that working.</p><p>!!But void loop() has not work</p><p>my increase program</p><p>void setup(){</p><p> //set pins as outputs</p><p> pinMode(7, OUTPUT); //my led</p><p> pinMode(8, OUTPUT);</p><p> pinMode(9, OUTPUT);</p><p> pinMode(13, OUTPUT);</p><p>cli();//stop interrupts</p><p>same example</p><p>sei();//allow interrupts</p><p>}</p><p>void loop(){</p><p> digitalWrite(7, HIGH); // turn the LED on (HIGH is the voltage level)</p><p> delay(100); // wait for a second</p><p> digitalWrite(7, LOW); // turn the LED off by making the voltage LOW</p><p> delay(100); // wait for a second</p><p> //do other things here</p><p>}</p>
<p>I made a python script that walks you through setting up the code for timer interrupts.<br>https://gist.github.com/cltnschlosser/e747efae07a76e117e02</p>
<p>Could this be used for a 2 player reflex timer game? Trying to push your players button on multiples of 5 seconds and getting a point when you do.</p>
<p>Good job, I want to import 200khz frequency, it's about 5us, but I only have 13.6us with every OCRnA that I used to change from 0 to 80. I'm using 2560. How can I do that?</p><p>Thanks</p>
<p>Thank you... Very nice tutorial!</p>
<p>Hei, very usefull application, but i&acute;ve one cuestion, how can i call de interrup for timer 1 inside the loop??Thanks!</p>
<p>I could be wrong. You may want to check</p><p>http://arduino.cc/en/Reference/AttachInterrupt</p>
You don't call interrupts like normal functions. An interrupt service routine (ISR) is &quot;called&quot; or better yet &quot;triggered&quot; at the specified frequency that you determined when you set up the interrupt.
Hello... The code is very useful, however, be aware that the sketch you posted doesn't seem to ever get into the main loop!
<p>I'm kind of a noob about all this, but unless I'm mistaken the point of interupts is that you don't NEED a main loop. The main loop just runs forever and everything is done when interupts happen. It is an extremely efficient way to code....</p>
<p>can we generate square wave with varying pulse width with this interrupt ?</p>
yes it's possible, but probably easier to just use the analogWrite command
<p>sir please send program for generating 15000 sample valuesin 10 sec using timer interrupt values adc values from accelometer sensor</p>
<p>sir please send program for generating 15000 sample values using timer interrupt values adc values from accelometer sensor</p>
<p>i am confused about something in the code, the line</p><p>// turn on CTC mode </p><p> TCCR0A |= (1 &lt;&lt; WGM01);</p><p>shouldnt it be TCCR0A |= (1&lt;&lt;WGM02); or am i understanding this wrong, i am a beginner at arduino so its still very confusing to me. </p>
<p>Thank you for this tutorial, I'm struggling with a project and this helped me stumble at least to the right direction. </p>

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Bio: I'm a grad student at the Center for Bits and Atoms at MIT Media Lab. Before that I worked at Instructables, writing code for ... More »
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