//display numbers from 0000-9999 
//display numbers from 000.0 - 999.9

#include <stdint.h>

#define DDRA  (*(volatile uint8_t*)0x21)
#define PORTA (*(volatile uint8_t*)0x22)

#define DDRB  (*(volatile uint8_t*)0x24)
#define PORTB (*(volatile uint8_t*)0x25)

uint8_t seg[10] = {
    0x3F,0x06,0x5B,0x4F,0x66,
    0x6D,0x7D,0x07,0x7F,0x6F
};

void delay_small(void){
    for(volatile uint16_t i=0;i<2000;i++);
}

int main(void){
    DDRA = 0xFF;           // segments
    DDRB = 0x0F;           // digit enable PB0–PB3

    uint16_t count = 0;

    while(1){
        for(uint16_t refresh=0; refresh<300; refresh++){
            uint8_t d1 = count % 10;
            uint8_t d2 = (count/10) % 10;
            uint8_t d3 = (count/100) % 10;
            uint8_t d4 = (count/1000);

            PORTB = 0x00; PORTA = seg[d1]; delay_small();
            PORTB = 0x00; PORTA = seg[d2]; delay_small();
            PORTB = 0x00; PORTA = seg[d3]; delay_small();
            PORTB = 0x00; PORTA = seg[d4]; delay_small();
        }

        count++;
        if(count == 10000) count = 0;
    }
}