#include <avr/io.h> 
#include <avr/interrupt.h> 
#include <avr/pgmspace.h> 
#include <util/delay.h>  

volatile unsigned char PinChange = 0;
volatile unsigned char PinState = 0x00;

void pcint0_init(void);

int main(void) {

  portSetup();
  pcint0_init();

  while(1) {
    if(PinChange) {
      PORTA = PinState;
      _delay_ms(1000);
      PinState = 0x00;
      PORTA = PinState;
      PinChange = 0;
    }
  }
}

void portSetup(void) {
  DDRB = 0xF0;
  PORTB = 0x0F;
  DDRA = 0xFF;
}

void pcint0_init(void) {
  cli();
  PCICR |= (1 << PCIE0);
  PCMSK0 = PCMSK0 | (1 << PCINT0) | (1 << PCINT1) | (1 << PCINT2) | (1 << PCINT3);
  PCIFR |= (1 << PCIF0);
  sei();
}

ISR(PCINT0_vect) {
  PinChange = 1;
  PinState = (~PINB & 0x0F);
  debounceButton(PinState);
  PCIFR |= (1 << PCIF0);
}

void debounceButton(char state) {
  _delay_ms(5);

  if (state != (~PINB & 0x0F)) {
    PinChange = 0;
  }
}