AVR Development Arduino Method (Part 2) Interrupt Subsystem

Before understanding the interrupt subsystem, we must first understand the concept of interrupts. You are reading a book and the phone rings. What will you do? I believe most people will do this: mark the position you are reading first, and then continue reading after answering the phone. This is an example of an interrupt in real life. We call "the phone ringing" an interrupt source. The main processor ATMega328P of Arduino UNO R3 has 26 interrupt sources, as shown in the following table:

Here we take external interrupt 0 as an example to understand the programming of the interrupt subsystem. Using the circuit used for the digital input example in the previous chapter, this example reverses the state of the LED when the key is pressed:

 1 // Interrupt.ino

 2 const byte ledPin = 13;

 3 const byte interruptPin = 2;

 4 volatile byte state = LOW;

 5 

 6 void setup() {

 7   pinMode(ledPin, OUTPUT);

 8   pinMode(interruptPin, INPUT_PULLUP);

 9   attachInterrupt(digitalPinToInterrupt(interruptPin), blink, CHANGE);

10 }

11 

12 void loop() {

13   digitalWrite(ledPin, state);

14 }

15 

16 void blink() {

17   state = !state;

18 }

The Arduino library functions related to external interrupts are:

　　attachInterrupt(digitalPinToInterrupt(pin), ISR, mode): Enables external interrupt for the specified pin and connects it to the specified interrupt service routine

　　pin: specifies the pin for the external interrupt

　　ISR: Specifies the name of the interrupt service routine

　　mode: LOW (low level trigger interrupt), CHANG (logic level change trigger interrupt), RISING (rising edge trigger interrupt) or FALLING (falling edge trigger interrupt)

　　detachInterrupt(digitalPinToInterrupt(pin)): disable the specified interrupt

　　pin: specifies the pin to cancel the external interrupt

　　interrupts(): Enable general interrupts

　　noInterrupts(): disable general interrupts

The external interrupt of ATMega328P is controlled by 2 related registers. The structure of the external interrupt control register EICRA is shown in the figure below:

The ISCx[1:0] (x = 0, 1) bits are used to set the trigger mode of the external interrupt, as shown in the following table:

The external interrupt mask register EIMSK is used to set whether to mask external interrupts. Its structure is shown in the figure below:

If you write 1 to a bit, the external interrupt controlled by that bit is enabled; if you write 0, it is disabled.

Rewrite the above program by directly accessing the registers:

 1 // Interrupt_reg.ino

 2 volatile byte state = LOW;

 3 

 4 void setup() {

 5   DDRB |= (1 << PB5);

 6   

 7   DDRD &= ~(1 << PD2);

 8   PORTD |= (1 << PD2);

 9   EICRA &= ~(1 << ISC01) & ~(1 << ISC00);

10   EIMSK |= (1 << INT0);

11 sei(); // Enable global interrupt

12 }

13 

14 void loop() {

15   if (state == HIGH) {

16     PORTB |= (1 << PB5);

17   } else {

18     PORTB &= ~(1 << PB5);

19   }

20 }

21 

22 // External interrupt 0 interrupt processing function

23 ISR(INT0_vect) {

24   state = !state;

25 }