Sample program to set the time on the Real Time Clock, and to read the time

#define RTC_READ_SECONDS  0x20
#define RTC_WRITE_SECONDS 0xa0

void put_spi(unsigned char c);
unsigned char get_spi(void);

main()
{
    int i;
    unsigned char time_data[] = 
        {0x00,    /* Seconds:        Address 0x20 0xA0 */
         0x10,    /* Minutes:        Address 0x21 0xA1 */
         0x14,    /* Hours:          Address 0x22 0xA2 */
         0x04,    /* Day of Week:    Address 0x23 0xA3 */
         0x03,    /* Date of Month:  Address 0x24 0xA4 */
         0x04,    /* Month:          Address 0x25 0xA5 */
         0x98,    /* Year:           Address 0x26 0xA6 */
         0x00,    /* Unused:         Address 0x27 0xA7 */
         0x00,    /* Seconds Alarm:  Address      0xA8 */
         0x00,    /* Minutes Alarm:  Address      0xA9 */
         0x07,    /* Hours Alarm:    Address      0xAA */
         0x00,    /* Unused:         Address 0x2B 0xAB */
         0x00,    /* Unused:         Address 0x2C 0xAC */
         0x00,    /* Unused:         Address 0x2D 0xAD */
         0x00,    /* Unused:         Address 0x2E 0xAE */
         0x00,    /* Unused:         Address 0x2F 0xAF */
         0x00,    /* Status:         Address 0x30      */
         0x??,    /* Interuupt Cntl: Address 0x31 0xB1 */
         0x??};   /* Clock Control:  Address 0x32 0xB2 */

    /* Put SPI setup in here */

    /* Write time to RTC */
    PORTD |= 0x20;                 /* Select RTC */
    put_spi(RTC_WRITE_SECONDS);    /* Send address of seconds */
    i = 0;
    while (i <= 0x12)
    {
        put_spi(time_data[i]);
        i = i + 1;
    }
    PORTD &= ~0x20;                /* Deselect RTC */

    /* Read time from RTC */
    PORTD |= 0x20;                 /* Select RTC */
    put_spi(RTC_READ_SECONDS);     /* Send address of seconds */
    i = 0;
    while (i <= 7)
    {
        time_data[i] = get_spi();
        i = i + 1;
    }
    PORTD &= ~0x20;                /* Deselect RTC */
}

/* Function to send a byte to the Serial Peripheral Interface */
void put_spi(unsigned char c)
{
    SPSR;                          /* First step in clearing SPIF */
    SPDR = c;                      /* Send char over SPI */
    while ((SPSR & 0x80) == 0) ;   /* Wait for transfer to complete */
}

/* Function to read a byte from the Serial Peripheral Interface */
unsigned char get_spi(void)
{
    SPSR;                          /* First step in clearing SPIF */
    SPDR = 0;                      /* Send junk byte over SPI */
    while ((SPSR & 0x80) == 0) ;   /* Wait for transfer to complete */
    return SPDR;                   /* Return byte read */
}