/*
ATtiny85 External Interrupt Request 0, on INT0 vector 2.
🧠 Register Overview
Register	  Purpose
GIMSK	      Enable pin change interrupt (PCIE)
PCMSK	      Choose which pin(s) can trigger interrupt
GIFR	      Flag register
sei()	      Enable global interrupts

📘 Example: Blink LED on INT0 Trigger (PD2)
Trigger on falling edge (button press to GND)

For the ISR, note:
1. You do not pass any parameters to the ISR and do not let it return anything.
2. The ISR should always be kept as short as possible.
3. For some boards, the ISR must be placed before setup(). That’s why I’m doing it this way throughout this post.
4. Global variables that you change in the ISR must be defined with the keyword volatile.
5. Within the ISR, all further interrupts are suspended.
*/

#define F_CPU 8000000UL
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

#define LED_PIN PB0     
#define BUTTON_PIN PB2 // (INT0)
#define DEBOUNCE_TIME 50 // Debounce time in milliseconds

// ==== START FUNCTIONS =====
void setup_external_interrupt() {
  // GIMSK – General Interrupt Mask Register
  GIMSK |= (1 << INT0); // Enable pin change interrupt

  // MCUCR – MCU Control Register
  MCUCR |= (1 << ISC01); // The falling edge of INT0 generates an interrupt request
  MCUCR &= ~(1 << ISC00); // (ISC01 = 1, ISC00 = 0)
  sei(); // Enable global interrupts
}

bool is_button_pressed(void) {
  // Check if button is pressed (logic low)
  if (!(PINB & (1 << BUTTON_PIN))) {
    _delay_ms(DEBOUNCE_TIME); // Debounce delay

    // Check again to confirm stable press
    if (!(PINB & (1 << BUTTON_PIN))) {
      // Wait for button release to avoid multiple toggles
      while (!(PINB & (1 << BUTTON_PIN))) {
        _delay_ms(1); // Simple hold loop
      }
      return true;
    }
  }
  return false;
}
// ==== END FUNCTIONS =====

ISR(INT0_vect) {
  if (is_button_pressed()) { // debounce function
    PORTB ^= (1 << LED_PIN); // Toggle LED on interrupt
  }
}

int main(void) {
  DDRB |= (1 << LED_PIN); // Set LED as output
  PORTB |= (1 << BUTTON_PIN); // Pull-up resistor on Pb2

  setup_external_interrupt();

  while (1) {
    // Main loop can sleep, or do other work
  }
  return 0;
}