led.c

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

volatile unsigned char count = 0; // should overflow to 0

void write_leds(unsigned char value) {
  int i;
  for (i=0;i<8;++i) {
    // set data line
    if ((value & (1 << i)) != 0) {
      PORTB |= _BV(PORTB2);
    } else {
      PORTB &= ~(_BV(PORTB2));
    }

    // pulse clock (maybe a delay would be good? works without it anyway)
    PORTB |= _BV(PORTB3);
    PORTB &= ~(_BV(PORTB3));
  }
}

void read_buttons() {
  // top button
  if (bit_is_set(PIND, PIND2) == 0) {
    PORTB ^= _BV(PORTB0);
  // middle button
  } else if (bit_is_set(PIND, PIND3) == 0) {
    PORTB ^= _BV(PORTB1);
  // bottom button
  } else if (bit_is_set(PIND, PIND4) == 0) {
    PORTB &= ~(_BV(PORTB0) | _BV(PORTB1));
  // button release, early exit
  } else {
    return;
  }

  write_leds(++count);
}

ISR(PCINT2_vect) {
  read_buttons();
}

int main (void) {
  // configure pins 0, 1 and 5 of upper port for output
  // 0 and 1 are led's
  // 2 and 3 are input and clock of a shift register
  // 5 is the status led onboard
  DDRB |= _BV(DDB0);
  DDRB |= _BV(DDB1);
  DDRB |= _BV(DDB2);
  DDRB |= _BV(DDB3);
  DDRB |= _BV(DDB5);

  // activate pull-up circuits for buttons
  PORTD |= _BV(PORTD2);
  PORTD |= _BV(PORTD3);
  PORTD |= _BV(PORTD4);

  // activate pin change interrupt 2
  PCICR |= _BV(PCIE2);
  // activate interrupt for pins 18, 19 and 20 (buttons)
  PCMSK2 |= _BV(PCINT18);
  PCMSK2 |= _BV(PCINT19);
  PCMSK2 |= _BV(PCINT20);

  sei();

  write_leds(count);

  do {
    set_sleep_mode(SLEEP_MODE_IDLE);
    sleep_enable();
    PORTB &= ~(_BV(PORTB5));
    sleep_mode();
    sleep_disable();
    PORTB |= _BV(PORTB5);
  } while (1);

  cli();

  return 0;
}