/*
ATmega8, 48, 88, 168, 328

/Reset PC6|1   28|PC5
 seg_A PD0|2   27|PC4
 seg_B PD1|3   26|PC3 
 seg_C PD2|4   25|PC2 
 seg_D PD3|5   24|PC1 
 seg_E PD4|6   23|PC0 
       Vcc|7   22|Gnd
       Gnd|8   21|Aref
       PB6|9   20|AVcc
       PB7|10  19|PB5 SCK   Anode_1 
 seg_F PD5|11  18|PB4 MISO  Anode_2
 seg_G PD6|12  17|PB3 MOSI  Anode_3
 dot   PD7|13  16|PB2       Anode_4
       PB0|14  15|PB1
*/

#define F_CPU 1000000UL
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

/*     _   _
      |  A  |
      F     B
      |- G -|
      E     C
      |_ D _| .H   
*/

uint8_t font[] =
  {// HGFEDCBA
    0b11000000,     // 0
    0b11111001,     // 1
    0b10100100,     // 2
    0b10110000,     // 3
    0b10011001,     // 4
    0b10010010,     // 5
    0b10000010,     // 6
    0b11111000,     // 7
    0b10000000,     // 8
    0b10010000,     // 9
  };

volatile uint8_t framebuffer[4];

ISR(TIMER0_OVF_vect)
{
  static uint8_t digit;
  PORTB &= 0b11000011;             // clear all digits
  PORTB |= (0b00000100 << digit);  // set current digit
  PORTD = framebuffer[digit];      // update segments
  digit++;
  digit = digit % 4;               // keep digit in [0:3]
}

void init(void)
{
  DDRD = 0b?;   // segments
  DDRB = 0b?;   // digits
  
  // we start with 
  //   normal operation
  //   clk/1024 prescaler (about 1kHz)  
  //   interrupt on Timer 0 overflow
  
  TCCR0A = (0 << COM0A1) | (0 << COM0A0) | (0 << COM0B1) | (0 << COM0B0)
         | (? << WGM01)  | (? << WGM00);   
  TCCR0B = (0 << WGM02) 
         | (? << CS02)   | (? << CS01)   | (? << CS00);
  TIMSK0 = (0 << OCIE0B) | (0 << OCIE0A) | (? << TOIE0); 
  
  // allow interrupts globally
  sei();     
}

int main(void)
{
  uint16_t count=0; 
  uint16_t dummy;
  uint8_t  i;
  init();
  while(1)
  {
    dummy = count;
    for (i=0; i<4; i++)                  
    {
      framebuffer[i] = font[dummy % 10];
      dummy = dummy / 10;
    }
    count++;
    if (count >= 10000)
    {
      count = 0;
    }
    _delay_ms(70);
  }
}