Often when a microcontroller is being used, we want it to know when certain things occur, and then have something happen in response. A simple example is to have a pushbutton determine when an LED turns on.

One implementation that would satisfy this is called “polling”. Inside of the main while loop we could test the state of an input pin every loop iteration and turn on an LED depending on its state.

int main(void)
{
	initializePins();
	
	while(1)
	{
          if(pushbuttonPinIsHigh)   
	   turnOffLED; 
	  else                   
            turnOnLED;  
	}
	
	return 0;
}

The above psuedocode checks the pin state of an input pin connected to a pushbutton and depending on the state turns on or off an LED.

Polling a pin state becomes tricky when you have to do other things in your event loop, especially things that take a lot of time. If for instance you needed to send or interpret some kind of serial data, doing so would take many clock cycles. This would essentially add latency to the response time of the LED to the pushbutton, as checking the pin state would have the same priority as every other sequential task in the while loop.

Hardware interrupts allow for asynchronous handling of system events. If an interrupt is set for a pin, when the pin state changes the code execution in the main loop halts and the code inside an Interrupt Service Routine (ISR) function is executed.

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