Interrupts are exactly what they sound like. They are events that cause a microcontroller’s CPU to stop executing a main program in order to execute a snippet of code on the side. As an example, here I discuss how the analog comparator on the ATmega328 (Arduino Uno) can be configured to generate an interrupt. Let me first show you the code then I will tell you what it does.

void setup()
{
pinMode(7,INPUT);
Serial.begin(9600);
ACSR = B01011010; // comparator interrupt enabled and tripped on falling edge.
}

void loop()
{
}

ISR(ANALOG_COMP_vect)
{
Serial.println("Interrupt Executed!")
}

Now for the explanation. The 8-bit ACSR register (Analog Comparator Control and Status Register) has the following bit structure.

ACSR – [ACD, ACBG, ACO, ACI, ACIE, ACIC, ACIS1, ACIS0]

I won’t explain what all of these bits do, only the ones I am setting in the code. First, seting the ACBG (Analog Comparator Bandgap Select) bit to one tells the ATmega328 to apply its own internal bandgap voltage (about 1.1V) to the positive input of the comparator. The negative input is analog pin 7 on the Arduino. I set the ACI (Analog Comparator Interrupt Flag) bit to one (this clears the interrupt flag) because I am changing other settings and I don’t want these changes to accidently trigger an interrupt. The ACIE (Analog Comparator Interrupt Enable) bit is set so when the analog comparator output changes as specified by the ACISx bits, an interrupt is executed. Setting the ACISx bits to 10 triggers the interrupt on a falling edge occurring at the comparator output. Setting these bits differently will trigger the interrupt on any change of the comparator output (00), or on a rising edge (11).

An aside about how the analog comparator works: The analog comparator output is high if the voltage applied to the positive input is greater then the voltage applied to the negative input. The output is low if the voltage applied to the positive input is less then that applied to the negative input. So, my setup will result in the comparator output going low when the voltage applied to Arduino pin 7 rises above 1.1 V (the bandgap reference voltage applied to the positive input). The falling edge occurring when this happens triggers the interrupt. This allows me to detect analog inputs above 1.1 V on pin 7 and do something when the input is detected. Now back to the explanation of the code above.

When the interrupt is executed, the code contained in the ISR(ANALOG_COMP_vect) function is executed before the CPU returns to the main program. ISR stands for interrupt service request. Here, whenever the interrupt occurs, I am simply printing a message to the serial monitor.