Imagine i have a code, that loops, and at one point there is i2c request.

Is loop() paused right away when there is Wire.onReceive() or loop is finished then Wire.onReceive() is called?

  • 2
    Let's not imagine stuff. Let's post an example piece of code.
    – Nick Gammon
    Commented Feb 22, 2018 at 6:57
  • Interrupts happen (almost) immediately if that answers your question.
    – Nick Gammon
    Commented Feb 22, 2018 at 6:59
  • if i have loop of 10 lines, and when loop is on line 4, at that time there is i2c request, will arduino switch to i2c request process it and then countiniue to line 5 of loop or it will finish loop first then go to i2c request?
    – Nicky
    Commented Feb 22, 2018 at 15:21
  • You are basically asking how interrupts work so see my question and answer about interrupts.
    – Nick Gammon
    Commented Feb 22, 2018 at 20:05
  • Thanks Nick, now when i know that receiveEvent is just a classic ISR now its all clear.
    – Nicky
    Commented Feb 23, 2018 at 18:38

1 Answer 1


The onReceive callback is called from an interrupt service routine (ISR). As the name says, interrupts are "interrupting" the currently executing code almost immediately. And "almost immediately" means that it only will take a few clock cycles. (This is the reason why changing data in an ISR can corrupt data, since the ISR may change a variable, when the main code tries to read it)

So the Arduino will not wait for the loop() function to end. It will interrupt in the line, where it was when the interrupt occured, and return to it after the ISR returns.

  • So basically I2C receiveEvent is just normal ISR?? I didnt know that.
    – Nicky
    Commented Feb 23, 2018 at 18:31
  • @Nicky - It is not strictly speaking a normal ISR. For one thing, it takes an argument. However it is called from code in the library function which is called by the TWI_vect (I2C interrupt vector). Therefore the time that the onReceive is called is "any time" (that interrupts are enabled).
    – Nick Gammon
    Commented Feb 23, 2018 at 19:47

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