text: // Code by JeeLabs http://news.jeelabs.org/code/
  // Released to the public domain! Enjoy!
  
  #include <Wire.h>
  #include "RTClib.h"
  #ifdef __AVR__
   #include <avr/pgmspace.h>
  #elif defined(ESP8266)
   #include <pgmspace.h>
  #elif defined(ARDUINO_ARCH_SAMD)
  // nothing special needed
  #elif defined(ARDUINO_SAM_DUE)
   #define PROGMEM
   #define pgm_read_byte(addr) (*(const unsigned char *)(addr))
   #define Wire Wire1
  #endif
  
  
  
  #if (ARDUINO >= 100)
   #include <Arduino.h> // capital A so it is error prone on case-sensitive filesystems
   // Macro to deal with the difference in I2C write functions from old and new Arduino versions.
   #define _I2C_WRITE write
   #define _I2C_READ  read
  #else
   #include <WProgram.h>
   #define _I2C_WRITE send
   #define _I2C_READ  receive
  #endif
  
  
  static uint8_t read_i2c_register(uint8_t addr, uint8_t reg) {
    Wire.beginTransmission(addr);
    Wire._I2C_WRITE((byte)reg);
    Wire.endTransmission();
  
    Wire.requestFrom(addr, (byte)1);
    return Wire._I2C_READ();
  }
  
  static void write_i2c_register(uint8_t addr, uint8_t reg, uint8_t val) {
    Wire.beginTransmission(addr);
    Wire._I2C_WRITE((byte)reg);
    Wire._I2C_WRITE((byte)val);
    Wire.endTransmission();
  }
  
  
  ////////////////////////////////////////////////////////////////////////////////
  // utility code, some of this could be exposed in the DateTime API if needed
  
  const uint8_t daysInMonth [] PROGMEM = { 31,28,31,30,31,30,31,31,30,31,30,31 };
  
  // number of days since 2000/01/01, valid for 2001..2099
  static uint16_t date2days(uint16_t y, uint8_t m, uint8_t d) {
      if (y >= 2000)
          y -= 2000;
      uint16_t days = d;
      for (uint8_t i = 1; i < m; ++i)
          days += pgm_read_byte(daysInMonth + i - 1);
      if (m > 2 && y % 4 == 0)
          ++days;
      return days + 365 * y + (y + 3) / 4 - 1;
  }
  
  static long time2long(uint16_t days, uint8_t h, uint8_t m, uint8_t s) {
      return ((days * 24L + h) * 60 + m) * 60 + s;
  }
  
  ////////////////////////////////////////////////////////////////////////////////
  // DateTime implementation - ignores time zones and DST changes
  // NOTE: also ignores leap seconds, see http://en.wikipedia.org/wiki/Leap_second
  
  DateTime::DateTime (uint32_t t) {
    t -= SECONDS_FROM_1970_TO_2000;    // bring to 2000 timestamp from 1970
  
      ss = t % 60;
      t /= 60;
      mm = t % 60;
      t /= 60;
      hh = t % 24;
      uint16_t days = t / 24;
      uint8_t leap;
      for (yOff = 0; ; ++yOff) {
          leap = yOff % 4 == 0;
          if (days < 365 + leap)
              break;
          days -= 365 + leap;
      }
      for (m = 1; ; ++m) {
          uint8_t daysPerMonth = pgm_read_byte(daysInMonth + m - 1);
          if (leap && m == 2)
              ++daysPerMonth;
          if (days < daysPerMonth)
              break;
          days -= daysPerMonth;
      }
      d = days + 1;
  }
  
  DateTime::DateTime (uint16_t year, uint8_t month, uint8_t day, uint8_t hour, uint8_t min, uint8_t sec) {
      if (year >= 2000)
          year -= 2000;
      yOff = year;
      m = month;
      d = day;
      hh = hour;
      mm = min;
      ss = sec;
  }
  
  DateTime::DateTime (const DateTime& copy):
    yOff(copy.yOff),
    m(copy.m),
    d(copy.d),
    hh(copy.hh),
    mm(copy.mm),
    ss(copy.ss)
  {}
  
  static uint8_t conv2d(const char* p) {
      uint8_t v = 0;
      if ('0' <= *p && *p <= '9')
          v = *p - '0';
      return 10 * v + *++p - '0';
  }
  
  // A convenient constructor for using "the compiler's time":
  //   DateTime now (__DATE__, __TIME__);
  // NOTE: using F() would further reduce the RAM footprint, see below.
  DateTime::DateTime (const char* date, const char* time) {
      // sample input: date = "Dec 26 2009", time = "12:34:56"
      yOff = conv2d(date + 9);
      // Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 
      switch (date[0]) {
          case 'J': m = date[1] == 'a' ? 1 : m = date[2] == 'n' ? 6 : 7; break;
          case 'F': m = 2; break;
          case 'A': m = date[2] == 'r' ? 4 : 8; break;
          case 'M': m = date[2] == 'r' ? 3 : 5; break;
          case 'S': m = 9; break;
          case 'O': m = 10; break;
          case 'N': m = 11; break;
          case 'D': m = 12; break;
      }
      d = conv2d(date + 4);
      hh = conv2d(time);
      mm = conv2d(time + 3);
      ss = conv2d(time + 6);
  }
  
  // A convenient constructor for using "the compiler's time":
  // This version will save RAM by using PROGMEM to store it by using the F macro.
  //   DateTime now (F(__DATE__), F(__TIME__));
  DateTime::DateTime (const __FlashStringHelper* date, const __FlashStringHelper* time) {
      // sample input: date = "Dec 26 2009", time = "12:34:56"
      char buff[11];
      memcpy_P(buff, date, 11);
      yOff = conv2d(buff + 9);
      // Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
      switch (buff[0]) {
          case 'J': m = buff[1] == 'a' ? 1 : m = buff[2] == 'n' ? 6 : 7; break;
          case 'F': m = 2; break;
          case 'A': m = buff[2] == 'r' ? 4 : 8; break;
          case 'M': m = buff[2] == 'r' ? 3 : 5; break;
          case 'S': m = 9; break;
          case 'O': m = 10; break;
          case 'N': m = 11; break;
          case 'D': m = 12; break;
      }
      d = conv2d(buff + 4);
      memcpy_P(buff, time, 8);
      hh = conv2d(buff);
      mm = conv2d(buff + 3);
      ss = conv2d(buff + 6);
  }
  
  uint8_t DateTime::dayOfTheWeek() const {    
      uint16_t day = date2days(yOff, m, d);
      return (day + 6) % 7; // Jan 1, 2000 is a Saturday, i.e. returns 6
  }
  
  uint32_t DateTime::unixtime(void) const {
    uint32_t t;
    uint16_t days = date2days(yOff, m, d);
    t = time2long(days, hh, mm, ss);
    t += SECONDS_FROM_1970_TO_2000;  // seconds from 1970 to 2000
  
    return t;
  }
  
  long DateTime::secondstime(void) const {
    long t;
    uint16_t days = date2days(yOff, m, d);
    t = time2long(days, hh, mm, ss);
    return t;
  }
  
  DateTime DateTime::operator+(const TimeSpan& span) {
    return DateTime(unixtime()+span.totalseconds());
  }
  
  DateTime DateTime::operator-(const TimeSpan& span) {
    return DateTime(unixtime()-span.totalseconds());
  }
  
  TimeSpan DateTime::operator-(const DateTime& right) {
    return TimeSpan(unixtime()-right.unixtime());
  }
  
  ////////////////////////////////////////////////////////////////////////////////
  // TimeSpan implementation
  
  TimeSpan::TimeSpan (int32_t seconds):
    _seconds(seconds)
  {}
  
  TimeSpan::TimeSpan (int16_t days, int8_t hours, int8_t minutes, int8_t seconds):
    _seconds((int32_t)days*86400L + (int32_t)hours*3600 + (int32_t)minutes*60 + seconds)
  {}
  
  TimeSpan::TimeSpan (const TimeSpan& copy):
    _seconds(copy._seconds)
  {}
  
  TimeSpan TimeSpan::operator+(const TimeSpan& right) {
    return TimeSpan(_seconds+right._seconds);
  }
  
  TimeSpan TimeSpan::operator-(const TimeSpan& right) {
    return TimeSpan(_seconds-right._seconds);
  }
  
  ////////////////////////////////////////////////////////////////////////////////
  // RTC_DS1307 implementation
  
  static uint8_t bcd2bin (uint8_t val) { return val - 6 * (val >> 4); }
  static uint8_t bin2bcd (uint8_t val) { return val + 6 * (val / 10); }
  
  boolean RTC_DS1307::begin(void) {
    Wire.begin();
    return true;
  }
  
  uint8_t RTC_DS1307::isrunning(void) {
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE((byte)0);
    Wire.endTransmission();
  
    Wire.requestFrom(DS1307_ADDRESS, 1);
    uint8_t ss = Wire._I2C_READ();
    return !(ss>>7);
  }
  
  void RTC_DS1307::adjust(const DateTime& dt) {
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE((byte)0); // start at location 0
    Wire._I2C_WRITE(bin2bcd(dt.seconds()));
    Wire._I2C_WRITE(bin2bcd(dt.minute()));
    Wire._I2C_WRITE(bin2bcd(dt.hour()));
    Wire._I2C_WRITE(bin2bcd(0));
    Wire._I2C_WRITE(bin2bcd(dt.day()));
    Wire._I2C_WRITE(bin2bcd(dt.month()));
    Wire._I2C_WRITE(bin2bcd(dt.year() - 2000));
    Wire.endTransmission();
  }
  
  DateTime RTC_DS1307::now() {
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE((byte)0);	
    Wire.endTransmission();
  
    Wire.requestFrom(DS1307_ADDRESS, 7);
    uint8_t ss = bcd2bin(Wire._I2C_READ() & 0x7F);
    uint8_t mm = bcd2bin(Wire._I2C_READ());
    uint8_t hh = bcd2bin(Wire._I2C_READ());
    Wire._I2C_READ();
    uint8_t d = bcd2bin(Wire._I2C_READ());
    uint8_t m = bcd2bin(Wire._I2C_READ());
    uint16_t y = bcd2bin(Wire._I2C_READ()) + 2000;
    
    return DateTime (y, m, d, hh, mm, ss);
  }
  
  Ds1307SqwPinMode RTC_DS1307::readSqwPinMode() {
    int mode;
  
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE(DS1307_CONTROL);
    Wire.endTransmission();
    
    Wire.requestFrom((uint8_t)DS1307_ADDRESS, (uint8_t)1);
    mode = Wire._I2C_READ();
  
    mode &= 0x93;
    return static_cast<Ds1307SqwPinMode>(mode);
  }
  
  void RTC_DS1307::writeSqwPinMode(Ds1307SqwPinMode mode) {
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE(DS1307_CONTROL);
    Wire._I2C_WRITE(mode);
    Wire.endTransmission();
  }
  
  void RTC_DS1307::readnvram(uint8_t* buf, uint8_t size, uint8_t address) {
    int addrByte = DS1307_NVRAM + address;
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE(addrByte);
    Wire.endTransmission();
    
    Wire.requestFrom((uint8_t) DS1307_ADDRESS, size);
    for (uint8_t pos = 0; pos < size; ++pos) {
      buf[pos] = Wire._I2C_READ();
    }
  }
  
  void RTC_DS1307::writenvram(uint8_t address, uint8_t* buf, uint8_t size) {
    int addrByte = DS1307_NVRAM + address;
    Wire.beginTransmission(DS1307_ADDRESS);
    Wire._I2C_WRITE(addrByte);
    for (uint8_t pos = 0; pos < size; ++pos) {
      Wire._I2C_WRITE(buf[pos]);
    }
    Wire.endTransmission();
  }
  
  uint8_t RTC_DS1307::readnvram(uint8_t address) {
    uint8_t data;
    readnvram(&data, 1, address);
    return data;
  }
  
  void RTC_DS1307::writenvram(uint8_t address, uint8_t data) {
    writenvram(address, &data, 1);
  }
  
  ////////////////////////////////////////////////////////////////////////////////
  // RTC_Millis implementation
  
  long RTC_Millis::offset = 0;
  
  void RTC_Millis::adjust(const DateTime& dt) {
      offset = dt.unixtime() - millis() / 1000;
  }
  
  DateTime RTC_Millis::now() {
    return (uint32_t)(offset + millis() / 1000);
  }
  
  ////////////////////////////////////////////////////////////////////////////////
  
  ////////////////////////////////////////////////////////////////////////////////
  // RTC_PCF8563 implementation
  
  boolean RTC_PCF8523::begin(void) {
    Wire.begin();
    return true;
  }
  
  boolean RTC_PCF8523::initialized(void) {
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE((byte)PCF8523_CONTROL_3);
    Wire.endTransmission();
  
    Wire.requestFrom(PCF8523_ADDRESS, 1);
    uint8_t ss = Wire._I2C_READ();
    return ((ss & 0xE0) != 0xE0);
  }
  
  void RTC_PCF8523::adjust(const DateTime& dt) {
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE((byte)3); // start at location 3
    Wire._I2C_WRITE(bin2bcd(dt.seconds()));
    Wire._I2C_WRITE(bin2bcd(dt.minute()));
    Wire._I2C_WRITE(bin2bcd(dt.hour()));
    Wire._I2C_WRITE(bin2bcd(dt.day()));
    Wire._I2C_WRITE(bin2bcd(0)); // skip weekdays
    Wire._I2C_WRITE(bin2bcd(dt.month()));
    Wire._I2C_WRITE(bin2bcd(dt.year() - 2000));
    Wire.endTransmission();
  
    // set to battery switchover mode
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE((byte)PCF8523_CONTROL_3);
    Wire._I2C_WRITE((byte)0x00);
    Wire.endTransmission();
  }
  
  DateTime RTC_PCF8523::now() {
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE((byte)3);	
    Wire.endTransmission();
  
    Wire.requestFrom(PCF8523_ADDRESS, 7);
    uint8_t ss = bcd2bin(Wire._I2C_READ() & 0x7F);
    uint8_t mm = bcd2bin(Wire._I2C_READ());
    uint8_t hh = bcd2bin(Wire._I2C_READ());
    uint8_t d = bcd2bin(Wire._I2C_READ());
    Wire._I2C_READ();  // skip 'weekdays'
    uint8_t m = bcd2bin(Wire._I2C_READ());
    uint16_t y = bcd2bin(Wire._I2C_READ()) + 2000;
    
    return DateTime (y, m, d, hh, mm, ss);
  }
  
  Pcf8523SqwPinMode RTC_PCF8523::readSqwPinMode() {
    int mode;
  
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE(PCF8523_CLKOUTCONTROL);
    Wire.endTransmission();
    
    Wire.requestFrom((uint8_t)PCF8523_ADDRESS, (uint8_t)1);
    mode = Wire._I2C_READ();
  
    mode >>= 3;
    mode &= 0x7;
    return static_cast<Pcf8523SqwPinMode>(mode);
  }
  
  void RTC_PCF8523::writeSqwPinMode(Pcf8523SqwPinMode mode) {
    Wire.beginTransmission(PCF8523_ADDRESS);
    Wire._I2C_WRITE(PCF8523_CLKOUTCONTROL);
    Wire._I2C_WRITE(mode << 3);
    Wire.endTransmission();
  }
  
  
  
  
  ////////////////////////////////////////////////////////////////////////////////
  // RTC_DS3231 implementation
  
  boolean RTC_DS3231::begin(void) {
    Wire.begin();
    return true;
  }
  
  bool RTC_DS3231::lostPower(void) {
    return (read_i2c_register(DS3231_ADDRESS, DS3231_STATUSREG) >> 7);
  }
  
  void RTC_DS3231::adjust(const DateTime& dt) {
    Wire.beginTransmission(DS3231_ADDRESS);
    Wire._I2C_WRITE((byte)0); // start at location 0
    Wire._I2C_WRITE(bin2bcd(dt.seconds()));
    Wire._I2C_WRITE(bin2bcd(dt.minute()));
    Wire._I2C_WRITE(bin2bcd(dt.hour()));
    Wire._I2C_WRITE(bin2bcd(0));
    Wire._I2C_WRITE(bin2bcd(dt.day()));
    Wire._I2C_WRITE(bin2bcd(dt.month()));
    Wire._I2C_WRITE(bin2bcd(dt.year() - 2000));
    Wire.endTransmission();
  
    uint8_t statreg = read_i2c_register(DS3231_ADDRESS, DS3231_STATUSREG);
    statreg &= ~0x80; // flip OSF bit
    write_i2c_register(DS3231_ADDRESS, DS3231_STATUSREG, statreg);
  }
  
  DateTime RTC_DS3231::now() {
    Wire.beginTransmission(DS3231_ADDRESS);
    Wire._I2C_WRITE((byte)0);	
    Wire.endTransmission();
  
    Wire.requestFrom(DS3231_ADDRESS, 7);
    uint8_t ss = bcd2bin(Wire._I2C_READ() & 0x7F);
    uint8_t mm = bcd2bin(Wire._I2C_READ());
    uint8_t hh = bcd2bin(Wire._I2C_READ());
    Wire._I2C_READ();
    uint8_t d = bcd2bin(Wire._I2C_READ());
    uint8_t m = bcd2bin(Wire._I2C_READ());
    uint16_t y = bcd2bin(Wire._I2C_READ()) + 2000;
    
    return DateTime (y, m, d, hh, mm, ss);
  }
  
  Ds3231SqwPinMode RTC_DS3231::readSqwPinMode() {
    int mode;
  
    Wire.beginTransmission(DS3231_ADDRESS);
    Wire._I2C_WRITE(DS3231_CONTROL);
    Wire.endTransmission();
    
    Wire.requestFrom((uint8_t)DS3231_ADDRESS, (uint8_t)1);
    mode = Wire._I2C_READ();
  
    mode &= 0x93;
    return static_cast<Ds3231SqwPinMode>(mode);
  }
  
  void RTC_DS3231::writeSqwPinMode(Ds3231SqwPinMode mode) {
    uint8_t ctrl;
    ctrl = read_i2c_register(DS3231_ADDRESS, DS3231_CONTROL);
  
    ctrl &= ~0x04; // turn off INTCON
    ctrl &= ~0x18; // set freq bits to 0
  
    if (mode == DS3231_OFF) {
      ctrl |= 0x04; // turn on INTCN
    } else {
      ctrl |= mode;
    } 
    write_i2c_register(DS3231_ADDRESS, DS3231_CONTROL, ctrl);
  
    //Serial.println( read_i2c_register(DS3231_ADDRESS, DS3231_CONTROL), HEX);
  }
  