Arduino Interrupts
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1. The 10mS delay just controls how fast we take latency samples and output measurements Since we take 100 measurements and each one is spaced by the 10mS delay we output a result every 1 second Next the latency value from the ISR is accumulated in the latencySum global We just grab the current latency value and add it to what we already have We also increment a counter that keeps track of how many samples we have accumulated Now we check to see if 100 samples have been accumulated If not we skip the rest of the code and return If we have 100 samples then we get an average by dividing the accumulated latency by the sample count and store the result in latencyAverage After we do this we clear the accumulator and the sample count so it can start again Now that we have a good latency measurement we can calculate the processor load estimate We know the timer will timeout every time it counts from our reload value to OxFF then back to 0x00 this is the overflow rate The load percentage should be the latency ticks in the ISR This value is calculated and output as a measurement so we can see the effects of our ISR code http www uchobby com index php 2007 11 24 arduino interrupts Page 8 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM The measurements are output and the loop function returns to be called again and wait 10mS Again point out that the main code has nothing to do with the job of toggling the I O pin It is free to u2. on the sound project but decided it would be best to cover timer interrupts separately The Arduino default processor is an ATMega168 datasheet link This microcontroller has several I O systems that every Arduino user is familiar with because the Arduino library makes using them easy The Digital I O PWM A D Inputs and Serial port for example The ATMega168 also has three internal hardware timers While the Arduino library does make use of some of the timer features it does not directly cover using a timer to generate periodic interrupts Interrupts As the name suggest interrupts are signals that interrupt the normal flow of a program Interrupts are usually used for hardware devices that require immediate attention when events occur For example the serial port system or UART on the microcontroller must be serviced when a new character arrives If it s not handled quickly the new character can be lost Interrupts Links Wikipedia has a good overview about interrupts Interrupts and Exceptions gt Embedded com Introduction to Interrupts When a new character arrives the UART system generates an interrupt The microcontroller stops running the main code your application and jumps to an Interrupt Service Routine ISR for the specific interrupt In this case a received character interrupt This ISR grabs the new character from the UART places it into a buffer then clears the interrupt and returns When the ISR returns the
3. This means we can hijack Timer1 and or Timer2 for our uses The PWM function on some of the I O pins will be affected as a result however If you plan to use PWM you need to know what is affected chose to use timer 2 so PWM pins 3 and 11 will be affected My test code completely disabled the PWM outputs for digital pins controlled from timer 2 suspect that the library PWM functions are expecting the counter to work over a specific range which have overridden The capture compare values loaded just don t make sense with my timer values for PWM David Mellis gave me this link to explore how the library codes uses the timers It is well documented and am sure it will serve me well in future efforts In a future experiment plan to control the PWM generation directly so that it can be done at a much higher frequency then normal High frequency PWM may be great for an Audio DAC function Arduino related interrupt links Here are some links related to interrupts on the Arduino reviewed several of these while playing with interrupts have to give credit for my knowledge to these authors Handling external Interrupts with Arduino http www uchobby com index php 2007 11 24 arduino interrupts Page 3 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM gt will think before code Interrupts vs polling More external interrupts AVR Libc Interrupts The Code The code covered in this article is available in a
4. microcontroller goes back to your main code and continues where it left off This all happens in the background and your main application code is not directly effected If you have lots of interrupts firing or fast timer interrupts your main code will execute slower because the microcontroller is spreading its processing time between your main code and all the ISR functions You may be thinking why not just check for a new character every now and then instead of using this complicated interrupt process Lets work an example to see how important these interrupt processes are Say you have a Serial port with a data rate of 9600 baud This means each bit of a character is sent at a frequency of 9600Hz or about 10KHz Each bit take about 100uS About 10 bits are required to send one character so we receive one complete character every 1mS or so If our UART is buffered we have to pull out the last character before the next one finishes this gives us 1mS to get the job done If our UART is not buffered we have to get the character out in about 1 bit time or 1uS Lets use the buffered example for now We have to check for a received byte faster then once every millisecond to keep from loosing data In the Arduino environment that means our Loop function has to get around to reading the UART status and possibly the data byte 1000 times a second This is easily doable but it would greatly complicate the code you have to write As long as your loop function neve
5. prescaler Mode 0 is a basic timer mode and the 8 prescale is how we get the counter to count at 2MHz or 0 5uS per count Next the overflow interrupt is enabled After this code executes the microcontroller will call the ISR every time the counter rolls over from OxFF to 0x00 This will happen after the counter has stepped from our load value over FF and back to OO Lastly we load the count value into the timer and return this load value so that the ISR can use it later have run the timer as high 50KHz This is very fast and any work done in the ISR will significantly hurt the performance of your main application code recommend that you don t try frequencies above 50KHz unless you are doing almost nothing in the ISR Microcontroller Interrupt load To give you an idea of the effect consider that at 50Khz the timer ISR should be triggered every 20uS The processor running at 16MHz can execute about one machine instruction every 63nS or about 320 machine instructions for every interrupt cycle 20uS Also consider that each line of C code can take many machine instructions to execute Every instruction used in the ISR subtracts from the time available for any other code to execute If our ISR used about 150 machine cycles then we would eat up 1 2 of our available processor time The main code would slow to about 1 2 the time it would have otherwise taken while the interrupt is active 150 machine instructions is not very much C code so you have t
6. thing and the serial port is handled in the background This is one of the great things about interrupt driven programs The functions occur on an event basis and are decoupled from your application code void loop void Accumulate ISR latency every 10ms delay 10 Accumulate the current latency value from the ISR and increment the sample counter latencySum latency sampleCount http www uchobby com index php 2007 11 24 arduino interrupts Page 7 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Once we have 100 samples calculate and output the measurements if sampleCount gt 99 float latencyAverage float loadPercent Calculate the average latency latencyAverage latencySum 1 00 0 zero the accumulator values sampleCount 0 latencySum 0 Calculate the Percentage processor load estimate loadPercent latencyAverage float timerLoadValue loadPercent 100 Scale up from ratio to percentage Output the average Latency Serial print Latency Average Serial print int latencyAverage Serial print latencyAverage int latencyAverage Serial print int latencyAverage 100 Output the load percentage estimate Serial print Load Serial print int loadPercent Serial println Main Code Loop Walk Through The loop function starts by delaying 10mS Note that we could not use a delay like this if we needed to toggle the I O pin at a high rate without an interrupt
7. timerLoadValue If latency is 20 and timerLoadValue is 200 your could would report 10 load when it should be 36 http www uchobby com index php 2007 11 24 arduino interrupts Page 15 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM 27 Jin said on January 1 2009 I ve also instrumented the loop to sample max latency as well as the average Doing the exact same instructions every loop see a max latency of 22 27 with the average between 6 and 9 Why such variability It s doing nothing but reading the PIND register Maybe the main loop s Serial prints are interfering with the register interrupts and that s being recorded If latency is gt 256 timerLoadValue won t the next interrupt happen at a somewhat random interval Subscribe Site RSS feed Tag Cloud Adaptors Arduino Audio AVR Breadboard Contest Development IPhone LED Links Logic Making Microcontroller Mods Nixie Tubes Parts pcs pic Project Prototype Review Robot Scrounging Tips TOO S Tutorial TV http www uchobby com index php 2007 11 24 arduino interrupts Page 16 of 18 Arduino Interrupts uC Hobby Ads by Google Cheapest Arduinos Online Buy the Arduino for 27 And the Mega for 49 www liquidware com Freescale Analog Chips Leading Manufacturers of Analog Processor amp Microcontrollers www Freescale com Analog LOGO Programmable Relay Siemens LOGO logic relay Built in Timers Counters Clock www sea siemens com lo
8. very useful and important and how they affect the main code You also have some example code to build on for your projects Be careful how much work you do in a fast timer ISR If you run the timer at a lower rate then of course your problem is reduced used extreme cases here to give you an idea of what to watch for Comments Please would like to hear your thoughts on interrupts in general and using the timers with the Arduino Did miss something important Was this article useful to you Should do more articles like this Are there some other good links to share gt Have you used interrupts on the Arduino gt Have any links for projects that make use of interrupts Posted in Arduino Discovering Electronics Links Microcontroller Projects By admin November 24 2007 2 comments 2 7 Responses Stay in touch with the conversation subscribe to the RSS feed for comments on this post 1 dfowler said on November 25 2007 http www uchobby com index php 2007 11 24 arduino interrupts Page 9 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Follower found a bunch of typos and a few grammer errors in my post have fixed this and thank him greatly for the thorough review 2 fatlimey said on November 26 2007 Good to see an Arduino article where the Arduino is used as a computing device not just as a simplified programming environment I m a low level software guy who is learning ele
9. Arduino Interrupts uC Hobby Skip to content uC Hobby Microcontrollers Electronics Hobby gt About Discussions Giveaway Program gt Links Make Controller Kit gt Parts Scrounging gt FAQ How To Use a Heat Gun gt How To Work Surface Web Rings Log In Categories Arduino Contest gt Development Tools Discovering Electronics Links Givaways Hacks Ideas Microcontroller Parts Projects gt Review Scrounging Scrounging Parts gt TGIMBOEJ UC Hobby Site Uncategorized Workshop Tips Workshop Tools Arduino Interrupts Often when working on microcontroller projects you need a background function to run at regular intervals This is often done by setting up a hardware timer to generate an interrupt The interrupt triggers an Interrupt Service Routine ISR to handle the periodic interrupt In this article describe setting up the 8 Bit Timer2 to generate interrupts on an Arduino ATMega168 walk through steps required for setup and inside the ISR function http www uchobby com index php 2007 11 24 arduino interrupts 10 19 09 11 29 AM ola Oe Page 1 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM If you are following the Arduino sound articles this one will be important to read as well Future articles will make use of this timer feature to control frequency generation In fact started to do this article as another installment
10. JMG You are correct if the timer autoloaded then you probably would not need to correct for latency There are some cases where the processor could have been delayed getting to the ISR due to interrupts being disabled The correction method above would maintain the average timing where the autoload would tend to have longer times on average The real reason that use the latency trick above is to measure the load my ISR is putting on the uC Once you have the few instructions required to do the load process it really is not a big problem and the benefit is that you can always measure latency and or the ISR load This is especially useful when you are developing the ISR code with a fast timing cycle Steve Hobley said on January 24 2008 Hi lve been trying to set the prescaler to 32 but the timer no longer fires when set the LSB to 0 1 1 when I set them to 1 0 0 64 it works OK Set to 64 TCCR2B I 1 xSmurf said on January 27 2008 Is it possible to change the duty cycle at a specific frequency to use this for fast PWM I m looking into building a switching led driver and need to generate a 33 duty cycle at 13Khz using the internal osc at 8Mhz http www uchobby com index php 2007 11 24 arduino interrupts Page 14 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM 22 23 24 29 26 dfowler said on January 27 2008 xSmurf Yes it is described in the article on PWM sound generation at the
11. ctronics and it s nice to see our side represented for once Keep up the good work 3 dfowler said on November 26 2007 Hey fatlimey am also a low level hardware software guy think the Arduino is a great start for hobbyist but am very intereted in taking the reins of the AVR using the nice interface to GCC that the Arduino IDE provides 4 tat said on November 26 2007 very cool article love the ones on sound as well and can t wait to see where it s going thought the code below could be interesting to all it basically reads analogO in free running mode and left adjusted so i only read the 8 MSBs and reproduce on an 8 bit R2R on pins dO 7 it works but the output is VERY noisy im assuming it comes from the R2R with dv set at 205 it runs at about 40khz for info i send live radio into analogO cheers include byte v dv ISR TIMER2_OVF_vect TCNT2 dv PORTD v void setup UCSRB 0 turns off rx tx DDRD Oxff digital 0 7 as output dv 205 ADMUX B11100000 Vref 2 56v result left adjusted input analogO ADCSRA I 1 lt lt ADFR free running mode ADCSRA I 1 lt lt ADSC start conversions cli http www uchobby com index php 2007 11 24 arduino interrupts Page 10 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM TIMSK B00000000 all interupts OFF TCCR2 B00000010 normal mode prescaler 8 TIMSK B01000000 enable Timer2 overflow i
12. go ARM Single Board Computer 29 95 module 21 IOs 7 A Ds and easy to program in BASIC or C www coridiumcorp com Electronic Timers Wireless electronic timing for equestrian events and other sports farmtek net Blogroll gt BricoGeek Circuit Projects gt DIY Live gt DIY4Fun gt EEBeat Electronics Lab gt Electroniq gt Elektronik gt Embedds gt Geeksinside gt GrinanBarrett Hack A Day http www uchobby com index php 2007 11 24 arduino interrupts 10 19 09 11 29 AM Page 17 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Hacked Gadgets gt justDIY Make Blog N5EBW Crazy Man gt Open Circuits Pinotronics Scienceprog gt Solder in the Veins Spark Fun Youritronics SSS ss This work is licensed under a Creative Commons Attribution 2 5 License Attribution with link to www uCHobby com Contact info uCHobby com for site information http www uchobby com index php 2007 11 24 arduino interrupts Page 18 of 18
13. hanks much for the great tutorial It seems it may be what need to communicate with a chip about 100 times sec However using the example when call SetupTimer2 with a an argument such as 100 the load goes way up 45 and don t think I m getting 100Hz put a piezo on pin 9 and also a sound card scope The freq seems to be all over the place even with higher values can t figure out what I m doing wrong any suggestions appreciated John 16 John said on January 15 2008 Well since posted learned more about the prescalers and more Using the 1024 prescaler with a reset of 100 worked out about right Thanks again 17 dfowler said http www uchobby com index php 2007 11 24 arduino interrupts Page 13 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM 18 19 20 21 on January 17 2008 John Glad you found a solution JMG said on January 22 2008 Thanks so much for the fantastic tutorial You do a great service for the hobbyist community not only by posting this information but by responding so diligently to comments One thing I m not sure understand though is the need to correct for the timer load and ISR latency Wouldn t this need be obviated if the timer reset itself automatically Do you simply do the manual measuring in specific example because you want to measure what the load and latency are or is there more to it than that dfowler said on January 22 2008
14. he timer for its first cycle TCNT2 result return result http www uchobby com index php 2007 11 24 arduino interrupts Page 4 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM SetupTimer2 walk through First is a define for the timer clock frequency It shows that the clock frequency is set to 2MHz because we use a divide by 8 prescale from the 16MHz master clock This is hard coded in the function The define just makes the code look better and may be useful in some application At the very least it reminds me how setup the timer The function takes one argument the desired timeout frequency and returns the value needed to re load the timer in the ISR The function does not limit the requested frequency but you should not try going too high discuss this issue later in the article Next the timer reload value is calculated This is a very easy calculation but needs to be done with floating point math Luckily we only need to do this once as floating math is expensive in terms of processor time We assume that the timer will be set to run at 2MHz for each count The reload value is the number of counts we want at 2MHz between interrupts You may notice that used 257 instead of 256 for the number of counts in the equation above know that the correct value should be 256 but get better results with 257 will explain why later in this article The next chunk of cryptic code sets the timer into mode 0 and selects the 8
15. int Timer2 Load Serial printIn timerLoadValue HEX Main Code Setup Walk Through Setup starts by setting the toggle pin to output so we can toggle it in the ISR Then it activates the serial port and prints some text to show we are alive Next the SetupTimer2 function is called with the frequency set at 44100Hz a common sound sampling frequency The return value is stored into a global named timerLoadValue for future use by the ISR Lastly Setup prints out the timerLoadValue so we can confirm that it is within reason At this point the timer is running and our ISR function is being called at the specified rate If you hook up an o scope you will see the pin toggling generating a frequency that is 1 2 the timer interval Its 1 2 because we set the pin low on one ISR pass then high on the other Main Code Loop The loop function is called over and over as the program runs Each time loop returns it is called again The code here looks complex but really all we are doing is averaging the latency value from the timer2 ISR and outputting the measurements after we get 100 samples Notice that the loop function needs to do nothing related to the toggling of the I O line This is all handled by the ISR leaving the loop function to do other things without much regard to the processes occurring in the ISR Just like with the serial port you don t have to worry about loading the next character into the UART when it is ready You just do your
16. it is time We only need to setup the timer to signal with an interrupt at the correct times Instead of spinning in a useless loop waiting for the delay to time out our main code can be doing other things like monitoring a motion sensor or doing motor control What ever our project needs we no longer need processor time for polling delays will cover the ISR in general just enough to deal with timer 2 interrupts Study this link from the avr libc user manual for more information about using interrupts on AVR processors You can also look at the timer sections in the ATMega168 data sheet You don t need a complete understanding at this point but ultimately you may want to get up to speed on using interrupts as they are an important tool for microcontroller applications Timers on the Arduino contacted David Mellis of the Arduino development team and learned that the library makes use of all three timers on the ATMega168 gt TimerO System timing PWM 5 and 6 Used to keep track of the time the program has been running The millis function to return the number of milliseconds since the program started using a global incremented in the timer O ISR Timer O is also used for PWM outputs on digital pins 5 and 6 gt Timer1 PWM 9 and 10 Used to drive PWM outputs for digital pins 9 and 10 gt Timer2 PWM 3 and 11 Used to drive PWM outputs for digital pins 3 and 11 While all the timers are used only TimerO has an assigned timer ISR
17. link below http www uchobby com index php 2008 01 01 pwm sound generation The source code is on the Arduino playground and runs the PWM at about 60Khz It should be easy to change this to meet your needs dfowler said on January 27 2008 Steve Are you still having problems Maybe can look at your code to see what s wrong You can send it to dfowler uchobby com It maybe a few days before can look at it David achterwerk said on February 28 2008 Thanks for pointing me in the right direction From your code worked towards a squarewave oscillator running on the compare interrupt of timer1 The difference is that used the compare mode of the Timer instead of resetting the counter by the program It is running stable with a sample rate of 44 1 kHz with the rimer running at 16 MHz driving 6 IO Pins connected to an R2R DAC According to my tuner the frequency of the oscillator is quite exact and stable over a wide range But still some coding to do towards a small synthesizer Thanks a lot Hecke Dane said on November 14 2008 How do use a interrupt on an arduino for converting a pitch into a programmed colored LED am using several different colors Jin said on January 1 2009 don t think your load calculation in the code is correct it is different from the explanation you give in the text You are treating timerLoadValue as if it were the total length of time available between interrupts but shouldn t it be 256
18. nterupt sei void loop v ADCH since R2R is only 8 bits we only read the 8 MSB bits instead of the 10 bits 5 gonium said on November 27 2007 Hey someone actually uses my findings Nice Mathias 6 Mike said on November 29 2007 Code needs include at the top in windows However this does not seem work on my Ubuntu linux system Mike 7 Mike said on November 29 2007 System will not take the greater than and less than characters So this should have said putting in the greater and less than signs include less than avr interrupt h greater than Hope thats clear 8 Ni oScript said on December 18 2007 http www uchobby com index php 2007 11 24 arduino interrupts Page 11 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM 10 11 12 13 maybe using Ni oScript said on December 18 2007 lt gt Ni oScript said on December 18 2007 Sorry for the tripost 2 i was trying to write the greater than and less than symbols you have to use some html here write amp then It for less than or gt for greater than and finish with an without spaces in between then your code will look like the following include lt avr interrupt h gt btw i don t know why it doesn t work in ubuntu i guess it should could you check the avr folder and see if you have the interrupt header Paul said on January 1 2008 Hi David Great a
19. o be careful If you take too long in the ISR your main code will slow to a crawl if you take longer then the timer cycle time you will either effectively never execute the main code and eventually suffer a crash of the system stack Measuring the Interrupt load Because did want to have a very fast timer ISR needed a way to measure how much load was placing on the available resources devised a trick that works to estimate the load and lets me output a measurement on the serial port As work on the timer ISR I can keep track of the interrupt load The timer was not placed in a mode where it reloads automatically This means the ISR must reload the timer for the next timeout interval It would be more accurate to have the timer auto reload but using this mode we can measure the time we spend in the ISR and correct the timer load value accordingly The key is that we get reasonable accuracy with this correction but we also get a number that shows us how much time we are spending in the ISR The trick is that the timer keeps ticking even though it s overflowed and interrupted At the end of our ISR we can capture the current http www uchobby com index php 2007 11 24 arduino interrupts Page 5 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM count in the timer This value represents the time it took for us to get to that point in the code It s the sum of the time it took to get into the interrupt routine and to execute
20. r takes more then 1mS to complete you could get away with this But consider that you might have more then one I O device to service or that you need to operate at much higher baud rates Surly you can see how ugly this can get very quick With an interrupt you don t have to keep checking to see if a character has arrived The hardware will signal with an interrupt and the processor will quickly call the ISR to grab the character in time Instead of spending a huge amount of your microprocessor time checking the status of the UART you never have to check the status you just setup the hardware interrupt and do the necessary work in the ISR Your main code is not directly affected and requires no special considerations for the hardware device http www uchobby com index php 2007 11 24 arduino interrupts Page 2 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Timer Interrupt For this article will focus on using the hardware timer 2 for a periodic interrupt The original idea was to use this timer to generate the note frequencies for the Arduino sound projects To bit bang out a tone or frequency we need to toggle an I O pin at a consistent frequency We did this in parts 2 and 3 of the sound articles using delay loops This was easy but means our processor is tied up doing nothing but waiting for the correct time to toggle the pin Using the timer interrupt we can do other things and let the ISR toggle the pin when the timer signals that
21. rticle It s great to have a place to go to get this explained on a fine grained level Now to find the time to rerun your code I m looking at where this is going with a lot of interest Is the Arduino fast enough to really get some sound synthesis done Seems like the answer is going to be a qualified yes Paul dfowler said on January 1 2008 Thanks Paul will be getting back to the audio article serries soon Been tied up with the holidays Yes think we can do some neat sound generation with the Arduino ben said on January 12 2008 Great Article was looking for exactly this you have been immeasurably helpful http www uchobby com index php 2007 11 24 arduino interrupts Page 12 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Know you mentioned that the ISR could be much more efficient if you toggle the pin directly and wanted to add that just a quick optimization can yield some nice results for instance in the ISR instead of doing a digital read which is processor expensive keep track of the pin value in a variable like so digitalWrite TOGGLE_IO flipFlop flipFlop flipFlop 0x01 This brought the latency down to around 14 ticks got it to run up to 90kHz easily after this quick change Cheers and thanks again 14 dfowler said on January 12 2008 Ben Yes You can also get a big boost by replacing the digitalWrite with direct I O David 15 John said on January 13 2008 T
22. se delay calls for its timing and is only affected in terms of how fast it will execute The time it gets is the left over time from all the background ISR processing for this timer ISR and the others that are always active TimerO and serial for example In the example code the timer is loaded with D4 hex or 212 That means it will interrupt every time it ticks 44 times We know that while the processor is executing the ISR code the timer ticks about 20 times so there are only about 24 more ticks before it s back in the ISR again That 24 ticks worth of time is the left over time our main code gets to execute So for a total time of 44 ticks between interrupts we spend 20 ticks in the ISR leaving about 24 ticks worth for the application This works out as about 45 of the processor used up in the ISR We also know that about one tick is used up in all the ISR processing code that is not measured in the latency value This is because needed to add one to the timeout interval to correct for this in the code described above We really only get about 23 out of the 44 ticks for the application ran the same code with the pin toggle commented out the latency averaged at about 3 ticks or a 6 load The ISR itself without serving any useful purpose uses about 6 of the available resource The digital read and write functions eat up the rest of the time Summary If you have read this far you should have a basic understanding of interrupts why they are
23. t of work done in the ISR The latency value will help with this as described below My testing shows an average latency of about 20 ticks which works out to about a 45 processor load This means that because of the ISR the main code will execute about 45 slower on average Not a big deal but notice that the only work done in the ISR is to toggle an I O pin This cost us almost half of our processor time The cost is due to the time it takes to service interrupts execute the digitalWrite digitalRead functions and complete the timer reload process The ISR would be much faster if we directly accessed the I O pin with port registers But using the more generic library functions were easy to use and if really did only need to toggle the I O pin the 45 cost would be OK with me Main Code Setup The setup function is called by the Arduino system code once at program start It initializes the I O and the timer It also outputs to the serial port showing that the code is running void setup void Set the pin we want the ISR to toggle for output pinMode TOGGLE_IO OUTPUT http www uchobby com index php 2007 11 24 arduino interrupts Page 6 of 18 Arduino Interrupts uC Hobby 10 19 09 11 29 AM Start up the serial port Serial begin 9600 Signal the program start Serial printIn Timer2 Test Start the timer and get the timer reload value timerLoadValue Setup Timer2 44100 Output the timer reload value Serial pr
24. the code in the ISR There would be some error as the time for the instructions to reload the timer are not accounted for but we could correct for that empirically In fact it s why used 257 in the load value math instead of 256 discovered empirically that this gave me a better result The extra count compensates for the timer reload instructions Timer2 ISR The ISR for the Timer2 overflow interrupt is shown below define TOGGLE_IO 9 Arduino pin to toggle in timer ISR Timer2 overflow interrupt vector handler ISR TIMER2_OVF_vect Toggle the IO pin to the other state digitalWrite TOGGLE_IO digitalRead TOGGLE_IO Capture the current timer value This is how much error we nave due to interrupt latency and the work in this function latency TCNT2 Reload the timer and correct for latency TCNT2 latency timerLoadValue Timer2 ISR walk through The function is short and its primary job is to toggle an I O pin After it does the toggle it captures the current timer count and uses it to correct for latency when the counter is reloaded The latency value is a global which the main application can monitor for the load measurements mentioned above It is the number of counts at 2MHz that it took for this ISR to do its function Remember that the ISR needs to be short as it is called every 20US when we run the timer for 50KHz You can do more in the ISR but you need to find a balance between the interrupt interval and the amoun
25. zip file here Setup Timer2 The code below shows a function created to setup timer2 This function enables the timer2 overflow interrupt sets the prescalar for the timer and calculates the timer load value given the desired timeout frequency This code is based on code found at the links given above do have experience reading AVR datasheets but it is much easier to reuse code if you can find it recommend that you take a look at the timer information in the data sheets if for no other reason then to gain an appreciation for how difficult it can be to figure this out define TIMER_CLOCK_FREQ 2000000 0 2MHz for 8 prescale from 16MHz Setup Timer2 Configures the ATMega168 8 Bit Timer2 to generate an interrupt at the specified frequency Returns the timer load value which must be loaded into TCNT2 inside your ISR routine See the example usage below unsigned char SetupTimer2 float timeoutFrequency unsigned char result The timer load value Calculate the timer load value result int 257 0 TIMER_CLOCK_FREQ timeoutFrequency 0 5 The 257 really should be 256 but get better results with 257 Timer2 Settings Timer Prescaler 8 mode 0 fTimer clock 16MHz 8 2Mhz or 0 5us The 8 prescale gives us a good range to work with So we just hard code this for now TCCR2A 0 TCCR2B 0 lt lt CS22 1 lt lt CS21 0 lt lt CS20 Timer2 Overflow Interrupt Enable TIMSK2 1 lt lt TOIE2 Noad t
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