#include <avr/io.h>
#include <avr/interrupt.h>

char count = 0x20;

int main (void)
{ DDRC  = 0x00;         // all input
  PORTC = 0xFF;         // enable pull-up for all pins
  DDRB  = 0xFF;         // all output, use LED at B5

  PCMSK1 = 0x10;        // interrupt line C4 (PCINT12)
  PCIFR  = 0x02;        // clear any pending interrupt
  PCICR  = 0x02;        // enable interrupt for pin-change-1

  sei();                // global interrupt enable
  while(1)
    PORTB = count;      // endless loop – display counter
}       

ISR(PCINT1_vect)
{ count = ~count;       // toggle all pins: pin5 = LED
  PCIFR = 0x02;         // Delete pin change interrupt flags 
}