Interrupts in Arduino - Nuno Alves

Interrupts in ArduinoReference: ArduinoCookbook” (1st ed.)by Michael Margolis.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Sample Code• You have an IR detectorconnected to pin 2.• This program monitors pulseson pin 2 and stores the durationof each pulse in an array.• When the array has been filledeach duration is displayed on theSerial Monitor.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

VolatileKeyword• Its a variable qualifier; it isused before the datatype of avariable, to modify the way inwhich the compiler andsubsequent program treats thevariable.• Compiler will load the variablefrom RAM and not from a storageregister.• Under certain conditions, suchas through interrupts, the valuefor a variable stored in registerscan be inaccurate.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

attachInterruptfunction•The attachInterrupt(0, analyze,CHANGE); call enables theprogram to handle interrupts.• The first number in the callspecifies which interrupt toinitialize.• On a standard Arduino board(such as UNO), two interrupts areavailable: number 0, which uses pin2, and number 1 on pin 3.• Interrupt 0 and interrupt 1 havethe same priorities (with Wiring).CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

A lot moreinterrupts• Yes, there are a lot of otherinterrupts, but they are internal.• We will user the timerinterrupts very soon.• Unsurprisingly the RESETinterrupt has the highestpriority.• This table is from Russell’sbook (listed on syllabus).CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

attachInterruptfunction•The second parameter specifieswhat function to call (interrupthandler) when the interruptevent happens.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering• The final parameter specifieswhat should trigger the interrupt:- CHANGE: whenever the pinlevel changes (low to high orhigh to low).- LOW: when the pin is low.- RISING: when the pin goesfrom low to high.- FALLING: when the pin goesfrom high to low.

Main loop• The main loop just checks theindex variable to see if all theentries have been set by theinterrupt handler.•... And it will print the contents ofthe array results only once.• Nothing in loop changes thevalue of index. The index ischanged inside the analyzefunction when the interruptcondition occurs.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

micros()function• The micros() function returnsthe number of micro-secondssince the Arduino began runningthe current program.•This number will overflow afterThis number will overflow (goback to zero), after approximately70 minutes.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering• On 16 MHz Arduino boards(e.g. UNO), this function has aresolution of four microseconds(i.e. the value returned is always amultiple of four).

Changingvariables• The variables that are changed inan interrupt function are declaredas volatile.• This lets the compiler know thatthe values could change at anytime (by an interrupt handler).• Without using the volatilekeyword, the compiler wouldthink these variables are not beingchanged by any code getting calledand would replace these variableswith constant values.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

What’s thecode doing?• Each time an interrupt istriggered, index is incrementedand the current time is saved.• The time difference is calculatedand saved in the array (except forthe first time the interrupt istriggered, when index is 0).• When the maximum number ofentries has occurred, the innerblock in loop runs, and it printsout all the values to the serialport.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Another sample codeint pin = 13;volatile int state = LOW;void setup(){pinMode(pin, OUTPUT);attachInterrupt(0, blink, CHANGE);}void loop(){digitalWrite(pin, state);}void blink(){state = !state;}• Attach something that willtrigger an interrupt in digitalpin #2 (e.g. resistive sensor,button, ...)• Make sure its either pulled-upor pull-down, as shown on theright.• Attach a LED to digital pin 13. Whenever,the interrupt is triggered (through pin #2changes), the LED will change state.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Arduino wiring C limitationsint pin = 13;volatile int state = LOW;void setup(){pinMode(pin, OUTPUT);attachInterrupt(0, blink, CHANGE);}void loop(){digitalWrite(pin, state);}void blink(){state = !state;}• Inside the interrupt function, delay() won'twork• Values returned by the millis() and micros()functions will not increment.•Serial communications while in thefunction may be lost!• You should declare as volatile any variablesthat you modify within the attachedfunction.• By default, interrupts are atomic.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Atomic sections in Arduino• We can enable/disable interrupts on certainsections of code with interrupts() andnoInterrupts().• Interrupts in Arduino are enabled by default.• Some functions will not work whileinterrupts are disabled, and incomingcommunication may be ignored. Interruptscan slightly disrupt the timing of code,however, and may be disabled for particularlycritical sections of code.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Interrupts default• By default you can not have interrupts inside interrupts.• With C-Wiring both interrupt 0 and interrupt 1 are assigned the samepriority.• If you have a way to have interrupts inside interrupts, then when aninterrupt is issued, you immediately leave the current interrupt andexecute the new interrupt.• Enabling interrupts inside interrupts is a “hack” : Is not recommended asit raises all sorts of issues with preserving the state of the machine beforethe interrupting interrupt is serviced.CPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

Enabling interrupts inside interruptsvolatile int i,j,z;void artificialdelay(){for (i=0; i

Operation scenariosCPE 355 - Real Time Embedded Kernels - Spring ‘12Nuno Alves (nalves@wne.edu), College of Engineering

What happens if you triggerinterrupt0 when inside interrupt0 ?volatile int i,j,z;void artificialdelay(){for (i=0; i

What happens if you triggerinterrupt1 when inside interrupt0 ?volatile int i,j,z;void artificialdelay(){for (i=0; i