#include <avr/interrupt.h>

typedef struct { 
	unsigned char second;
	unsigned char minute;
	unsigned char hour;                                     
	unsigned char date;       
	unsigned char month;
	unsigned int year;     
	unsigned char changed;
} time_t;

time_t time={0,0,12,1,1,2007};

char not_leap(void)      //check for leap year
{
    if (!(time.year%100)) return (char)(time.year%400);
    			     else return (char)(time.year%4);
}         

SIGNAL(SIG_OVERFLOW2) //overflow interrupt vector
{ 
    if (++time.second==60)        //keep track of time, date, month, and year
    {
        time.second=0;
        if (++time.minute==60) 
        {
            time.minute=0;
            if (++time.hour==24)
            {
                time.hour=0;
                if (++time.date==32)
                {
                    time.month++;
                    time.date=1;
                }
                else if (time.date==31) 
                {                    
                    if ((time.month==4) || (time.month==6) || (time.month==9) || (time.month==11)) 
                    {
                        time.month++;
                        time.date=1;
                    }
                }
                else if (time.date==30)
                {
                    if(time.month==2)
                    {
                       time.month++;
                       time.date=1;
                    }
                }              
                else if (time.date==29) 
                {
                    if((time.month==2) && (not_leap()))
                    {
                        time.month++;
                        time.date=1;
                    }                
                }                          
                if (time.month==13)
                {
                    time.month=1;
                    time.year++;
                }
            }
        }
    }  

	time.changed=1;
}  
 

void RTC_init(void)
{
	
	TIMSK2 = 0;             // disable OCIE2A and TOIE2
	ASSR = (1<<AS2);        // select asynchronous operation of Timer2

	TCNT2 = 0;              // clear TCNT2A
	TCCR2B = (1<<CS22) | (1<<CS20);             // select precaler: 32.768 kHz / 128 = 1 sec between each overflow

	while(ASSR & (0x01 | 0x04));       // wait for TCN2UB and TCR2UB to be cleared

	TIFR2 = 0xFF;           // clear interrupt-flags
	TIMSK2 = (1<<TOIE2);    // enable Timer2 overflow interrupt
}

int main(void)
{
RTC_init();
sei();
while(1) { 

}