Arduino Interrupt. In this post notes abot interrupt and Timer with an example of use of ISR with Arduino timer interrupt (TIMER2OVF). The interrupt mode, which determines what pin action triggers an interrupt. The Arduino Uno supports four interrupt modes: The Arduino Uno supports four interrupt modes:. `RISING`, which activates an interrupt on a rising edge of the interrupt pin.
I have an Arduino Uno (awesome little device!). It has two interrupts; let's call them 0 and 1. I attach a handler to interrupt 0 and a different one to interrupt 1, using
attachInterrupt()
: http://www.arduino.cc/en/Reference/AttachInterrupt.Interrupt 0 is triggered and it calls its handler, which does some number crunching. If interrupt 0's handler is still executing when interrupt 1 is triggered, what will happen?
Will interrupt 1 interrupt interrupt 0, or will interrupt 1 wait until interrupt 0's handler is done executing?
Please note that this question specifically relates to Arduino.
Peter Mortensen14.2k1919 gold badges8888 silver badges115115 bronze badges
Chris LaplanteChris Laplante24.6k1515 gold badges8585 silver badges126126 bronze badges
3 Answers
On Arduino (aka AVR) hardware, nested interrupts don't happen unless you intentionally create the conditions to allow it to happen.
From avr-lib:
The AVR hardware clears the global interrupt flag in SREG before entering an interrupt vector. Thus, normally interrupts will remain disabled inside the handler until the handler exits, where the RETI instruction (that is emitted by the compiler as part of the normal function epilogue for an interrupt handler) will eventually re-enable further interrupts. For that reason, interrupt handlers normally do not nest. For most interrupt handlers, this is the desired behaviour, for some it is even required in order to prevent infinitely recursive interrupts (like UART interrupts, or level-triggered external interrupts). In rare circumstances though it might be desired to re-enable the global interrupt flag as early as possible in the interrupt handler, in order to not defer any other interrupt more than absolutely needed. This could be done using an sei() instruction right at the beginning of the interrupt handler, but this still leaves few instructions inside the compiler-generated function prologue to run with global interrupts disabled.
(source: http://linux.die.net/man/3/avr_interrupts )
gpczgpcz
Will interrupt 1 interrupt interrupt 0, or will interrupt 1 wait until interrupt 0's handler is done executing?
Unless you specifically re-enable interrupts inside an ISR (Interrupt Service Routine) then whatever interrupt is currently running completes, plus one more machine code instruction, before the next interrupt is serviced.
Most interrupts set a flag inside the processor, which is checked between instructions, to see if the interrupt should be serviced. Flags are checked in priority order. On the Uno that is:
(Note that Reset cannot be masked).
Conceivably a low-level interrupt might be in progress (eg. TIMER0_OVF_vect). While that is busy doing its stuff multiple other interrupt events might occur (and set the corresponding bits in the CPU). They will be serviced in the above order, not in the order in which they actually occur in time.
There are hardware registers that can be written to, to cancel a pending interrupt - that is, to clear the flag.
The reason for mentioning 'one more machine code instruction' is that the processor is designed to guarantee that when it transitions from interrupts not enabled, to interrupts enabled, one more instruction is always executed.
This lets you write code like this:
Without that, an interrupt might occur before going to sleep. Which means you never wake, because you were relying upon the interrupt occuring during sleep, not before it.
How wonderfully moronic of Freescale and Atmel to both use the very same instruction names, but with inverted meanings
That is why I prefer the mnemonics of
interrupts
and noInterrupts
because the intent there is very clear. These are implemented by defines in the core include files.Nick GammonNick Gammon
The documentation mention that Arduino interrupts have priority:
![Arduino Arduino](/uploads/1/2/5/7/125771831/316096315.png)
If your sketch uses multiple ISRs, only one can run at a time. Other interrupts will be executed after the current one finishes in an order that depends on the priority they have.
It also provides a link for additional information:
For more information on interrupts, see Nick Gammon's notes.
According to sections What is interrupt priority? and Can interrupts occur while interrupts are disabled?, we can conclude that:
- Interrupts are based on the list of flags. When an event occurs the corresponding flag is set.
- If the ISR can't fire right now, it can be called at any time later because the flag is saved.
- There is a list of all available interrupts, and it generally depends on the chip. The higher up the list, the higher the priority.
So, different interrupts will not interrupt each other. They will be executed according to their priority.
Peter Mortensen14.2k1919 gold badges8888 silver badges115115 bronze badges
kelinkelin6,20855 gold badges4141 silver badges7878 bronze badges
Not the answer you're looking for? Browse other questions tagged embeddedmicrocontrollerarduinointerruptinterrupt-handling or ask your own question.
In a basic Arduino sketch, if you want to take action based on the status of a digital input pin, you’d check the state of the pin repeatedly until a particular condition is met. However, as your sketches become increasingly complex, you’ll find that it’s not always the best way to do this. EngBlaze posted a pairof guides to using interrupts so that you can execute code asynchronously in Arduino sketch. Here’s the analogy they use to illustrate the difference:
Imagine you’re sitting on your couch, enjoying a frosty brew and watching a movie after a long day. Life is good. There’s only one problem: you’re waiting for an incredibly important package to arrive, and you need it as soon as possible. If you were a normal AVR program or Arduino sketch, you’d have to repeatedly stop your movie, get up, and go check the mailbox every 5 minutes to make sure you knew when the package was there.
Instead, imagine if the package was sent Fedex or UPS with delivery confirmation. Now, the delivery man will go to your front door and ring the doorbell as soon as he arrives. That’s your interrupt trigger. Once you get the trigger, you can pause your movie and go deal with the package. That’s your interrupt service routine. As soon as you’re done, you can pick up the film where you left off, with no extra time wasted. That’s the power of interrupts.
The first guide covers using hardware interrupts, such as switches and the second discusses timer interrupts, so that you don’t have to repeatedly check if a certain amount of time has passed when executing code on a particular interval. The guides are complete with code examples to give you a jumpstart on your own interrupt-driven Arduino project. [via Reddit]