http://actinium.org/devel/avr/7seg-led-clock

7seg_led_clock.c

#define F_CPU 1000000UL

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

#define DIGITS 6
#define REFRESH_RATE 60
#define IS_ANODE_COMMON
#define LONG_PRESS_THRESHOLD_SEC 0.8
#define DAYSECOND 86400


volatile unsigned long current_time = 0;
volatile char buf[DIGITS + 1];

void current_time_format(void)
{
    snprintf(
        buf,
        sizeof(buf),
        "%02u%02u%02u",
        (unsigned int)(current_time / 3600),
        (unsigned int)((current_time % 3600) / 60),
        (unsigned int)(current_time % 60)
    );
}

void current_time_increment(unsigned long increments)
{
    unsigned long a = current_time + increments;
    if (a >= DAYSECOND)
    {
        current_time = a - DAYSECOND;
    }
    else
    {
        current_time = a;
    }
    current_time_format();
}

void current_time_decrement(unsigned long decrements)
{
    if (current_time < decrements)
    {
        current_time = DAYSECOND - decrements;
    }
    else
    {
        current_time -= decrements;
    }
    current_time_format();
}

volatile int is_setting_mode = 0;

ISR(TIMER1_COMPA_vect)
{
    if (!is_setting_mode)
    {
        current_time_increment(1);
    }
}

ISR(PCINT1_vect)
{
    _delay_ms(1);
    int pc0s = PINC & _BV(PC0);
    int pc1s = PINC & _BV(PC1);
    int pc2s = PINC & _BV(PC2);
    
    if (pc0s == 0)
    {
        is_setting_mode ^= 1;
        if (!is_setting_mode)
        {
            TCNT1 = 0;
        }
    }
    else if (is_setting_mode && pc1s == 0)
    {
        current_time_increment(1);
    }
    else if (is_setting_mode && pc2s == 0)
    {
        current_time_decrement(1);
    }
}

int char_to_segment(char input)
{
    switch (input)
    {
        case '0':
            return 0b11111100;
        case '1':
            return 0b01100000;
        case '2':
            return 0b11011010;
        case '3':
            return 0b11110010;
        case '4':
            return 0b01100110;
        case '5':
            return 0b10110110;
        case '6':
            return 0b10111110;
        case '7':
            return 0b11100100;
        case '8':
            return 0b11111110;
        case '9':
            return 0b11110110;
        default:
            return 0b00000000;
    }
}

void Initialize(void)
{    
    // 16bit timer(timer1) interrupt compare A
    TCCR1B = _BV(WGM12) | _BV(CS11) | _BV(CS10);// clock / 64
    OCR1A = 15625;// 1000000 / 64
    TIMSK1 = _BV(OCIE1A);
    
    // I/O
    DDRB = 0b11111111;
    DDRD = 0b11111111;
    
    // Pin Change INTerrupt
    DDRC = 0b00000000;
    PORTC |= _BV(PC0) | _BV(PC1) | _BV(PC2);
    PCMSK1 |= _BV(PCINT8) | _BV(PCINT9) | _BV(PCINT10);
    PCICR |= _BV(PCIE1);
    
    // Enable Interrupt
    sei();
}

int main (void)
{
    Initialize();
    
    unsigned int i;
    double wait_ms = 1000 / DIGITS / REFRESH_RATE;
    unsigned long long_press_count_threshold = REFRESH_RATE * LONG_PRESS_THRESHOLD_SEC;
    unsigned long long_press_count_increase = 0;
    unsigned long long_press_count_decrease = 0;
    
    while(1)
    {
        // long press
        if (is_setting_mode)
        {
            if (PINC & _BV(PC1))
            {
                long_press_count_increase = 0;
            }
            else
            {
                long_press_count_increase++;
            }
            
            if (PINC & _BV(PC2))
            {
                long_press_count_decrease = 0;
            }
            else
            {
                long_press_count_decrease++;
            }
            
            unsigned int lpc_inc = long_press_count_increase / long_press_count_threshold;
            unsigned int lpc_dec = long_press_count_decrease / long_press_count_threshold;
            if (lpc_inc > 4)
            {
                current_time_increment(30);
            }
            else if (lpc_inc > 2)
            {
                current_time_increment(15);
            }
            else if (lpc_inc > 0)
            {
                current_time_increment(1);
            }
            else if (lpc_dec > 4)
            {
                current_time_decrement(30);
            }
            else if (lpc_dec > 2)
            {
                current_time_decrement(15);
            }
            else if (lpc_dec > 0)
            {
                current_time_decrement(1);
            }
        }
        
        // output
        for (i = 0; i < DIGITS; i++)
        {
            PORTD = 1 << (DIGITS - i - 1);
#ifdef IS_ANODE_COMMON
            PORTB = ~(char_to_segment(buf[i]) | (is_setting_mode ? 1 : 0));
#else
            PORTB = char_to_segment(buf[i]) | (is_setting_mode ? 1 : 0);
#endif
            _delay_ms(wait_ms);
        }
    }
}