include <Wire.h>
#if (ARDUINO >= 100)
#define WireWrite Wire.write
#define WireRead Wire.read()
#else
#define WireWrite Wire.send
#define WireRead Wire.receive()
#endif

// DS1307 Real Time Clock
// SDA (pin 5) to Arduino A4
// SCL (pin 6) to Arduino A5
#define DS1307_ADDRESS 0x68
typedef struct {
  byte second;
  byte minute;
  byte hour;
  byte weekDay;
  byte monthDay;
  byte month;
  int year;
}
s_dateTime;
s_dateTime curDateTime;

byte bcdToDec(byte val) {
  return ((val/16*10)+(val%16));
}

byte decToBcd(byte val){
  return ((val/10*16)+(val%10));
}

void setDateTime() {
  Wire.beginTransmission(DS1307_ADDRESS);
  WireWrite(0);
  WireWrite(decToBcd(curDateTime.second));
  WireWrite(decToBcd(curDateTime.minute));
  WireWrite(decToBcd(curDateTime.hour));
  WireWrite(decToBcd(curDateTime.weekDay));
  WireWrite(decToBcd(curDateTime.monthDay));
  WireWrite(decToBcd(curDateTime.month));
  WireWrite(decToBcd(curDateTime.year-2000));
  WireWrite(0);
  Wire.endTransmission();
}

void updateCurDateTime() {
  static unsigned long last_update = -200;
  if(millis()-last_update>100) {

    Wire.beginTransmission(DS1307_ADDRESS);
    WireWrite(0);
    Wire.endTransmission();
    Wire.requestFrom(DS1307_ADDRESS, 7);
    curDateTime.second = bcdToDec(WireRead);
    curDateTime.minute = bcdToDec(WireRead);
    curDateTime.hour = bcdToDec(WireRead & 0b111111);

    curDateTime.weekDay = bcdToDec(WireRead);

    curDateTime.monthDay = bcdToDec(WireRead);
    curDateTime.month = bcdToDec(WireRead);
    curDateTime.year = bcdToDec(WireRead)+2000;

    if(curDateTime.second & 0b1000000) {
      curDateTime.second = 0; // disable Clock Halt bit on fresh ds1307 chip
      setDateTime();
    }
  }
}

void setup()
{
  Serial.begin(115200);
  Wire.begin();
  curDateTime.year = 2014;
  curDateTime.month = 11;
  curDateTime.monthDay = 24;
  curDateTime.weekDay = 1;
  curDateTime.hour = 23;
  curDateTime.minute = 0;
  curDateTime.second = 0;
  setDateTime();
  delay(200);
}

void loop()
{
  updateCurDateTime();
  Serial.print(curDateTime.hour);
  Serial.print(":");
  Serial.print(curDateTime.minute);
  Serial.print(":");
  Serial.println(curDateTime.second);
  delay(1000);
}