#define F_CPU 1000000UL

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

// font for a 7-segment display - numbers 0-9
uint8_t font[10] = {0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f};

volatile uint8_t framebuffer[4];     // use to buffer contents of display

volatile uint8_t current_digit = 0;  // which digit is active now

ISR(TIMER0_OVF_vect)
{
  // the code for the interrupt service routine
  PORTD = 0xff;                  // all segments off
  current_digit += 1;            // step one digit
  current_digit %= 4;            //    and keep between 0-3
  PORTB = (1 << current_digit);
  PORTD = ~font[current_digit];  // -- to be replaced! --
}

void init(void)
{
  // we can collect all hardware initialization here
  DDRB = 0b00001111;    // lower B-pins as output
  DDRD = 0b11111111;    // all D-pins as output


  // setting up TIMER0
  TCCR0A = (0 << COM0A1) | (0 << COM0A0)   // we'll use PWM later
         | (0 << COM0B1) | (0 << COM0B0)   // we'll use PWM later
         | (_ << WGM01)  | (_ << WGM00);   // WGMxx sets the mode of the timer

  TCCR0B = (_ << WGM02)                    // WGMxx sets the mode of the timer
         | (_ << CS02) | (_ << CS01) | (_ << CS00);  // clock prescaler

  // enable the TIMER0 OVERFLOW INTERRUPT
  TIMSK0 = (0 << OCIE0B) | (0 << OCIE0A) | (1 << TOIE0);

  // don't forget to enable interrupts
  sei();
}

int main(void)
{
  init();
  while (1)
  {
    // your specific code goes here
  }
}