// LED-display Clock

#define F_CPU   8000000
#define F_TIMER 600
#define PERIOD ((F_CPU/F_TIMER)-1)

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

volatile uint8_t tf;            // timer flag for 1 second
volatile uint8_t digit[6];
const int8_t code7seg[10] = {20, 215, 76, 69, 135, 37, 36, 87, 4, 5};   // 7 segment code
uint8_t hr=20, min=0, sec=0;     //set start time

void inc_time(void) {
    sec++;
    if (sec==60) {
        sec=0; min++;
        if (min==60) {
            min=0; hr++;
            if (hr==24) hr=0;
        }
    }
}

// distribute number to digits

void number_output(int h, int m, int s) {
    digit[0]=  h/10;
    digit[1]=  h%10;
    digit[2] = m/10;
    digit[3] = m%10;
    digit[4] = s/10;
    digit[5] = s%10;
}

// timer interrupt

ISR(TIMER1_COMPA_vect) {    
    static uint8_t active_digit=0, select=1;
    static int cnt;

    // multilex display
    PORTC = 0b00111111;     // all digits off
    active_digit++; select <<= 1;
    if (active_digit == 6 ) {
        active_digit = 0;
        select = 1;
    }
    PORTD = code7seg[digit[active_digit]];
    PORTC = ~select;                // Low = current digit on

    cnt++;      
    if (cnt == F_TIMER) {
        cnt = 0;
        tf  = 1;                // 1 second flag
    }
}

int main(void) {

    // init ports
    DDRD = 0b11111111;
    DDRC = 0b00111111;
    
    // initialize timer1
    // mode 4, CTC
    // prescaler 1
    
    TCCR1A = 0;
    TCCR1B = (1<<WGM12) | (1<<CS10);
    OCR1A  = PERIOD;
    TIMSK |= (1<<OCIE1A);

    sei();  
    
    while(1) {
        set_sleep_mode(SLEEP_MODE_IDLE);
        sleep_mode();
        if (tf) {
            tf=0;
            inc_time();
            number_output(hr, min, sec);
        } 
    }   
    return 0;
}