Need some clarification on interrupts

Forum 2005-2010 (read only) General Frequently-Asked Questions
1

The forums I've read (including this one) say that the code is interrupted, the ISR is performed, and control goes back to loop()

What I can't find anywhere is whether the chip starts loop() from the top, or whether it resumes from where it left off..

Can anyone shed some light?

Quick example:

void loop(){
Serial.print("A");
Serial.print("B");
Serial.print("C");
}

void ISR(){
do something
}

Now if ISR is called after the Arduino outputs "A", will it next print "B", or "A" again?

2

An interrupt resumes at the next machine instruction, not at the top of "loop()".
Your example will print "B".

Think of an interrupt as a condition that is tested after every single machine instruction.
If the condition is true, and interrupts are not disabled, then a call to the interrupt service routine is executed, so when the service routine has finished, the next instruction is executed.

3

Crystal clear, thanks!