//PCINT1 Triggered from external source at pin PB1

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

void initInterrupt() {
  GIMSK |= 1 << PCIE;       // enable PCINT[0:5] pin change interrupt
  PCMSK |= 1 << PCINT1;     // configure interrupt at PB1
  sei();                    // globaly enable interrupts
}

void setup() {
  DDRB |= 1 << DDB4;     // define PB4 as output
  DDRB &= ~ (1 << DDB1); // define PB1 as input
  PORTB |= 1 << PB1;    // enable pull up resistor at PB1
  initInterrupt();
}

ISR(PCINT0_vect) {
  PORTB ^= (1 << PB4);   // inverse logical level at PB4 / blink
}

void loop() {
  // nop
}