// RTC is connected, address is Hex68 (Decimal 104)
#define DS3231_I2C_ADDRESS 0x68

// Convert normal decimal numbers to binary coded decimal
byte decToBcd(byte val) {
    return ((val / 10 * 16) + (val % 10));
}

// Convert binary coded decimal to normal decimal numbers
byte bcdToDec(byte val) {
    return ((val / 16 * 10) + (val % 16));
}

// sets time and date data to DS3231
void setDS3231time(byte second, byte minute, byte hour, byte dayOfWeek,
                   byte dayOfMonth, byte month, byte year) {
    
    Wire.beginTransmission(DS3231_I2C_ADDRESS);
    Wire.write(0); // set next input to start at the seconds register
    Wire.write(decToBcd(second)); // set seconds
    Wire.write(decToBcd(minute)); // set minutes
    Wire.write(decToBcd(hour)); // set hours
    Wire.write(decToBcd(dayOfWeek)); // set day of week (1=Sunday, 7=Saturday)
    Wire.write(decToBcd(dayOfMonth)); // set date (1 to 31)
    Wire.write(decToBcd(month)); // set month
    Wire.write(decToBcd(year)); // set year (0 to 99)
    Wire.endTransmission();
}

// reads time and date from DS3231
void readDS3231time(byte *second, byte *minute, byte *hour, byte *dayOfWeek,
                    byte *dayOfMonth, byte *month, byte *year) {
    Wire.beginTransmission(DS3231_I2C_ADDRESS);
    Wire.write(0); // set DS3231 register pointer to 00h
    Wire.endTransmission();
    Wire.requestFrom(DS3231_I2C_ADDRESS, 7);
    // request seven bytes of data from DS3231 starting from register 00h
    *second = bcdToDec(Wire.read() & 0x7f);
    *minute = bcdToDec(Wire.read());
    *hour = bcdToDec(Wire.read() & 0x3f);
    *dayOfWeek = bcdToDec(Wire.read());
    *dayOfMonth = bcdToDec(Wire.read());
    *month = bcdToDec(Wire.read());
    *year = bcdToDec(Wire.read());
}