#include <avr/io.h>

uint8_t c = 0;

/* Make sure the WDTON Fuse is unprogrammed */

int main() {
  DDRB = (1 << PB0) | (1 << PB2) | (1 << PB3);
  SPCR = (1 << SPE) | (1 << MSTR);

  WDTCR = (1 << WDP2) | (1 << WDP1);
  GIMSK = (1 << PCIE);
  PCMSK = (1 << PCINT4);
  sei();
  
  while(1) {
    sleep_cpu();
  }
}

uint8_t sent = 0;
ISR(WDT_vect) {
  if(sent) {
    while(!(SPSR & (1 << SPIF)));
  }
  SPDR = c;
  c = 0;
  sent = 1;
}

ISR(PCINT0_vect) {
  if(c < -1 && (PORTB & (1 << PB4))) {
    c++;
  }
}