I2C RTC DS1307 and Arduino

In this article I will:

  • describe main differences between DS1307 and PCF8563 (see this detailed article about PCF8563 on my blog before reading on)

Then, I will show how to:

  • connect DS1307 to Arduino
  • set date & time
  • get date & time with, hopefully, an optimized code


DS1307 is an I2C real time clock that works at I2C standard mode (100 kbit/s).

From DS1307 datasheet:

The DS1307 serial real-time clock (RTC) is a low-power, full binary-coded decimal (BCD) clock/calendar plus 56 bytes of NV SRAM.
Address and data are transferred serially through an I2C, bidirectional bus.
The  lock/calendar provides seconds, minutes, hours, day, date, month, and year information.
The end of the month date is automatically adjusted for months with fewer than 31 days, including corrections for leap year.
The clock operates in either the 24-hour or 12-hour format with AM/PM indicator.
The DS1307 has a built-in power-sense circuit that detects power failures and automatically switches to the battery supply.

So the main differences between DS1307 and PCF8563 are:

  • Although not expressly stated, DS1307 works only at I2C standard mode (100 kbit/s) while PCF8563 can also work at I2C fast mode (400 kbit/s). This means that if you need to build a fast I2C circuit, as I need, you have to use PCF8563 (because every I2C component of the circuit has to be capable of running at fast mode), otherwise you can use DS1307 (that is much more common)
  • DS1307 accept a standard 3V alternate power supply (a lithium cell battery, for example) that in case of failure of the primary power supply can back up the RTC for a long time even (a lithium battery with 48mAhr or greater will back up
    the DS1307 for more than 10 years)
  • DS1307 does not have the possibility to manage alarm and timer (countdown), as PCF8563 can do
  • Other minor differences exists on how registers are organized

This is register organization of DS1307 (click to enlarge):

This is the PIN configuration of DS1307 (DIP8 package – from DS1307 datasheet):

DS1307 pin configurationWhere:

  • X1 = oscillator input
  • X2 = oscillator output
  • VBAT = backup supply input for any standard 3V lithium cell
  • GND = ground
  • SDA = serial data input and output
  • SCL = serial clock input
  • SQL/OUT = square wave/output driver
  • VCC = primary power supply

With respect to PCF8563, as far as pin configuration is concerned, the only difference is the presence of the alternate power supply pin (VBAT) in place of the interrupt pin of PCF8563 (INT). This means that if you are using only basic functions of the RTC (set date & time and get date & time) DS1307 and PCF8563 are pin-compatible.

The circuit is exactly the same as the PCF8563 (since we are not using neither alarms, timers, interrupts nor a backup battery):


The code is slightly different from the code of the PCF8563. The only differences are:

  • DS1307 7 bit address is 68 (hexadecimal), while PCF8563 address is 51 (hexadecimal)
  • when reading seconds, DS1307 does not raise a bit if the integrity of the clock is not guaranteed (as PCF8563 do)
  • week day goes from 1 to 7 in DS1307 while goes from 0 to 6 in PCF8563
  • as seen, registry organization is a little bit different:
    • DS1307 does not have the century register
    • DS1307 does not have alarm and timer registers
    • Day and weekday registers are in reverse order
    • DS1307 has one control register that is the last register (PCF8563 has two control registers in first position)

Once again, the code is optimized, in a way that Arduino ask to the RTC only pieces of information that changed: this way the code can be added to an existing complex application without breaking anything (hopefully).

#include <Wire.h>//I2C header file

// Defines
//#define DEBUG // Uncomment to turn on verbose mode
#define I2C_RTC 0x68 // 7 bit address (without last bit - look at the datasheet)
#define ERROR_LED 13

// Errors
#define ERROR_RTC_SET 1 // Unable to set RTC time and date
#define ERROR_RTC_GET 2 // Unable to get RTC time and date

// Global variables
byte result;
byte second;
byte second_old; // The code ask the RTC for data only when the previous value has changed
byte minute;
byte minute_old; // The code ask the RTC for data only when the previous value has changed
byte hour;
byte hour_old; // The code ask the RTC for data only when the previous value has changed
byte weekday;
byte day;
byte month;
byte year;
char* weekdayname[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

// Function prototypes
byte BcdToDec(byte);
byte DecToBcd(byte);
void SetError(int);

void setup()
  pinMode(ERROR_LED, OUTPUT); // Set error LED
  Wire.begin(); // Initiate the Wire library and join the I2C bus as a master
  Serial.begin(9600); // Initiate serial communication

// Set initial date and time
  second_old = second = 53; // Second (0-59)
  minute_old = minute = 59; // Minute (0-59)
  hour_old = hour = 23; // Hour (0-23)
  weekday = 1; // Day of the week (1-7)
  day = 31; // Day (1-31)
  month = 12; // Month (1-12)
  year = 11; // Year (0-99)
  Wire.beginTransmission(I2C_RTC); // Select RTC
  Wire.send(0);        // Start address
  Wire.send(DecToBcd(second));     // Second
  Wire.send(DecToBcd(minute));    // Minute
  Wire.send(DecToBcd(hour));    // Hour
  Wire.send(DecToBcd(weekday));    // Weekday
  Wire.send(DecToBcd(day));    // Day
  Wire.send(DecToBcd(month));     // Month (with century bit = 0)
  Wire.send(DecToBcd(year));    // Year
  Wire.send(0b10000);           // Output clock frequency enabled (1 Hz)
  result = Wire.endTransmission();

#ifdef DEBUG
  Serial.print("Result of setting date and time: ");
  Serial.println(result, DEC);

  if (result) SetError(ERROR_RTC_SET);

void loop()
  Wire.send(0); // Start address
  result = Wire.endTransmission();
#ifdef DEBUG
  Serial.print("Result of asking for date and time: ");
  Serial.println(result, DEC);
  if (result) SetError(ERROR_RTC_GET);

  Wire.requestFrom(I2C_RTC, 1);
  second = BcdToDec(Wire.receive());
  if (second != second_old) // Cycle begins only when it has changed
    second_old = second;
    if (second == 0) // If second is zero I need to ask for the minute
      Wire.requestFrom(I2C_RTC, 1);
      minute = BcdToDec(Wire.receive());
      if (minute != minute_old) // Cycle begins only when it has changed
        minute_old = minute;
        if (minute == 0) // If minute is zero I need to ask for the hour
          Wire.requestFrom(I2C_RTC, 1);
          hour = BcdToDec(Wire.receive() & 0b111111);
          if (hour != hour_old) // Cycle begins only when it has changed
            hour_old = hour;
            if (hour == 0) // If hour is zero I need to ask for other elements
              Wire.requestFrom(I2C_RTC, 4);
              weekday = BcdToDec(Wire.receive());
              day = BcdToDec(Wire.receive());
              month = BcdToDec(Wire.receive());
              year = BcdToDec(Wire.receive());
    Serial.print(weekdayname[weekday - 1]);
    Serial.print(weekday, DEC);
    Serial.print(") 20");
    if (year < 10) Serial.print("0");
    Serial.print(year, DEC);
    if (month < 10) Serial.print("0");
    if (day < 10) Serial.print("0");
    Serial.print(day, DEC);
    if (hour < 10) Serial.print("0");
    if (minute < 10) Serial.print("0");
    Serial.print(minute, DEC);
    if (second < 10) Serial.print("0");
    Serial.println(second, DEC);

// Converts a BCD (binary coded decimal) to decimal
byte BcdToDec(byte value)
  return ((value / 16) * 10 + value % 16);

// Converts a decimal to BCD (binary coded decimal)
byte DecToBcd(byte value){
  return (value / 10 * 16 + value % 10);

void SetError(int error) // Blinks forever the error led a number of times corresponding to error number
  while(1) // Forever
    for (byte index = 0; index < error; index++)
      digitalWrite(ERROR_LED, HIGH);
      digitalWrite(ERROR_LED, LOW);

The final result

Finally, this is the video of the DS1307 connected to the arduino. Look at what happens when I detach one of the I2C lines…


6 thoughts on “I2C RTC DS1307 and Arduino

  1. hello, thank you for making a DS1307 lib! but i’m having a problem… i inserted your code into mine, and the part that you have running in void loop() i have running in a function that gets checked once ever second:
    if (millis() – lastOutput > 1000 || millis() < lastOutput)
    {// Output the current status once a second.
    lastOutput = millis();

    the only problem is, that when i serial print the output, the seconds is all screwed up, and i think it is screwing up the minutes too. it sets the time no problem, but when the clock starts ticking everything goes haywire. here is some of the output:

    11-12-31 23:59:104

    11-12-31 23:59:37

    11-12-31 23:59:158

    11-12-31 23:59:162

    11-12-31 23:59:40

    11-12-31 23:59:25

    that is my XML formatted serial.print, and the serial.print from your code minus the weekday stuff. what do you think is going on here?? any help would be greatly appreciated. fwiw, its for a smart thermostat for my office so i can save money. 🙂

  2. lol, the comments didn’t display the XML stuff… but its there, and it matches what your code is printing. also not show, but I have verified that you are setting the time ok, it shoes up as 11-12-31 23:59:53, but the next second after 53 is 104… lol wtf.

    • Thanks for your comment, I’ll suggest you to make these checks:
      – try to run my code without any modification and see if it works
      – otherwise tell me which version of arduino ide are you using
      – send your code to michele dot michielin at gmail dot com and I’ll check it out

    • At first sight I did not notice any strange thing (but I dit not try to compile and execute the complete code…)
      I suggest you to try to “minimize” the code starting from the RTC stuff and adding one by one other pieces of code to find where is the problem.
      But I have a question: why do you need an RTC if you need to do something every second?

  3. Pingback: TiZakit

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