Arduino time with Real Time Clock (RTC)

You often need a time for a project. Maybe you want to build an alarm clock or time-controlled electronics. Then you look for the command with which you can read the time and find out that the Arduino board does not provide any time. WT*??? Yes, it is true but help is on the way – with a Real Time Clock (RTC).

A real time clock is a hardware module with battery and memory. Many of these modules are based on the DS1307 chip. It is addressed via the I2C interface. I use the Tiny RTC module* in this example, but a lot of RTCs work with the same code.

Circuit for Arduino time with Real Time Clock (RTC)

The RTC module is connected to 5V + and GND. To do this, the I2C pins are connected: SDA to SDA, SCL to SCL.

Arduino Code

To operate the RTC module, you need a program library. To install, in the Arduino software click Sketch > Embed Library > Manage Libraries … search for RTC by Makuna in the search field. Install the latest version. The sample code can now be found under File > Examples > RTC by Makuna > DS1307_Simple. If you use a different module, you must of course open the corresponding example.

// CONNECTIONS:
// DS1307 SDA --> SDA
// DS1307 SCL --> SCL
// DS1307 VCC --> 5v
// DS1307 GND --> GND

/* for software wire use below
#include <SoftwareWire.h>  // must be included here so that Arduino library object file references work
#include <RtcDS1307.h>

SoftwareWire myWire(SDA, SCL);
RtcDS1307<SoftwareWire> Rtc(myWire);
 for software wire use above */

/* for normal hardware wire use below */
#include <Wire.h> // must be included here so that Arduino library object file references work
#include <RtcDS1307.h>
RtcDS1307<TwoWire> Rtc(Wire);
/* for normal hardware wire use above */

void setup () 
{
    Serial.begin(57600);

    Serial.print("compiled: ");
    Serial.print(__DATE__);
    Serial.println(__TIME__);

    //--------RTC SETUP ------------
    // if you are using ESP-01 then uncomment the line below to reset the pins to
    // the available pins for SDA, SCL
    // Wire.begin(0, 2); // due to limited pins, use pin 0 and 2 for SDA, SCL
    
    Rtc.Begin();

    RtcDateTime compiled = RtcDateTime(__DATE__, __TIME__);
    printDateTime(compiled);
    Serial.println();

    if (!Rtc.IsDateTimeValid()) 
    {
        if (Rtc.LastError() != 0)
        {
            // we have a communications error
            // see https://www.arduino.cc/en/Reference/WireEndTransmission for 
            // what the number means
            Serial.print("RTC communications error = ");
            Serial.println(Rtc.LastError());
        }
        else
        {
            // Common Causes:
            //    1) first time you ran and the device wasn't running yet
            //    2) the battery on the device is low or even missing

            Serial.println("RTC lost confidence in the DateTime!");
            // following line sets the RTC to the date & time this sketch was compiled
            // it will also reset the valid flag internally unless the Rtc device is
            // having an issue

            Rtc.SetDateTime(compiled);
        }
    }

    if (!Rtc.GetIsRunning())
    {
        Serial.println("RTC was not actively running, starting now");
        Rtc.SetIsRunning(true);
    }

    RtcDateTime now = Rtc.GetDateTime();
    if (now < compiled) 
    {
        Serial.println("RTC is older than compile time!  (Updating DateTime)");
        Rtc.SetDateTime(compiled);
    }
    else if (now > compiled) 
    {
        Serial.println("RTC is newer than compile time. (this is expected)");
    }
    else if (now == compiled) 
    {
        Serial.println("RTC is the same as compile time! (not expected but all is fine)");
    }

    // never assume the Rtc was last configured by you, so
    // just clear them to your needed state
    Rtc.SetSquareWavePin(DS1307SquareWaveOut_Low); 
}

void loop () 
{
    if (!Rtc.IsDateTimeValid()) 
    {
        if (Rtc.LastError() != 0)
        {
            // we have a communications error
            // see https://www.arduino.cc/en/Reference/WireEndTransmission for 
            // what the number means
            Serial.print("RTC communications error = ");
            Serial.println(Rtc.LastError());
        }
        else
        {
            // Common Causes:
            //    1) the battery on the device is low or even missing and the power line was disconnected
            Serial.println("RTC lost confidence in the DateTime!");
        }
    }

    RtcDateTime now = Rtc.GetDateTime();

    printDateTime(now);
    Serial.println();

    delay(10000); // ten seconds
}

#define countof(a) (sizeof(a) / sizeof(a[0]))

void printDateTime(const RtcDateTime& dt)
{
    char datestring[20];

    snprintf_P(datestring, 
            countof(datestring),
            PSTR("%02u/%02u/%04u %02u:%02u:%02u"),
            dt.Month(),
            dt.Day(),
            dt.Year(),
            dt.Hour(),
            dt.Minute(),
            dt.Second() );
    Serial.print(datestring);
}

If you now open the serial monitor, you can see the output of the real-time clock. Attention, the baud rate of the serial monitor must be set to 57600.

Arduino time with the Real Time Clock (RTC) and LCD

Now I would like to show you how to display the time on an LCD display. To do this, build the following circuit.

Code: Arduino time with the Real Time Clock (RTC) and LCD

#include <LiquidCrystal.h>

// CONNECTIONS:
// DS1307 SDA --> SDA
// DS1307 SCL --> SCL
// DS1307 VCC --> 5v
// DS1307 GND --> GND

/* for software wire use below
  #include <SoftwareWire.h>  // must be included here so that Arduino library object file references work
  #include <RtcDS1307.h>

  SoftwareWire myWire(SDA, SCL);
  RtcDS1307<SoftwareWire> Rtc(myWire);
  for software wire use above */

/* for normal hardware wire use below */

#include <Wire.h> // must be included here so that Arduino library object file references work
#include <RtcDS1307.h>
RtcDS1307<TwoWire> Rtc(Wire);
/* for normal hardware wire use above */

const int rs = 12, en = 11, d4 = 5, d5 = 4, d6 = 3, d7 = 2;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

void setup (){
  lcd.begin(16, 2);

  Serial.begin(57600);

  Serial.print("compiled: ");
  Serial.print(__DATE__);
  Serial.println(__TIME__);

  //--------RTC SETUP ------------
  // if you are using ESP-01 then uncomment the line below to reset the pins to
  // the available pins for SDA, SCL
  // Wire.begin(0, 2); // due to limited pins, use pin 0 and 2 for SDA, SCL

  Rtc.Begin();

  RtcDateTime compiled = RtcDateTime(__DATE__, __TIME__);
  printDateTime(compiled);

  if (!Rtc.IsDateTimeValid())
  {
    if (Rtc.LastError() != 0)
    {
      // we have a communications error
      // see https://www.arduino.cc/en/Reference/WireEndTransmission for
      // what the number means
      Serial.print("RTC communications error = ");
      Serial.println(Rtc.LastError());
    }
    else
    {
      // Common Causes:
      //    1) first time you ran and the device wasn't running yet
      //    2) the battery on the device is low or even missing

      Serial.println("RTC lost confidence in the DateTime!");
      // following line sets the RTC to the date & time this sketch was compiled
      // it will also reset the valid flag internally unless the Rtc device is
      // having an issue

      Rtc.SetDateTime(compiled);
    }
  }

  if (!Rtc.GetIsRunning())
  {
    Serial.println("RTC was not actively running, starting now");
    Rtc.SetIsRunning(true);
  }

  RtcDateTime now = Rtc.GetDateTime();
  if (now < compiled)
  {
    Serial.println("RTC is older than compile time!  (Updating DateTime)");
    Rtc.SetDateTime(compiled);
  }
  else if (now > compiled)
  {
    Serial.println("RTC is newer than compile time. (this is expected)");
  }
  else if (now == compiled)
  {
    Serial.println("RTC is the same as compile time! (not expected but all is fine)");
  }



  // never assume the Rtc was last configured by you, so
  // just clear them to your needed state
  Rtc.SetSquareWavePin(DS1307SquareWaveOut_Low);
}

void loop ()
{
  Serial.print(".");
  if (!Rtc.IsDateTimeValid())
  {
    if (Rtc.LastError() != 0)
    {
      // we have a communications error
      // see https://www.arduino.cc/en/Reference/WireEndTransmission for
      // what the number means
      Serial.print("RTC communications error = ");
      Serial.println(Rtc.LastError());
    }
    else
    {
      // Common Causes:
      //    1) the battery on the device is low or even missing and the power line was disconnected
      Serial.println("RTC lost confidence in the DateTime!");
    }
  }

  RtcDateTime now = Rtc.GetDateTime();

  printDateTime(now);
  lcdShowTime(now);


  delay(1000); // ten seconds
}

#define countof(a) (sizeof(a) / sizeof(a[0]))

void lcdShowTime(const RtcDateTime& dt)
{
  char datestring[20];
  char timestring[20];

  snprintf_P(datestring,
             countof(datestring),
             PSTR("%02u.%02u.%04u"),
             dt.Day(),
             dt.Month(),
             dt.Year() );

  snprintf_P(timestring,
             countof(timestring),
             PSTR("%02u:%02u:%02u"),
             dt.Hour(),
             dt.Minute(),
             dt.Second() );             

  lcd.setCursor(0, 0);
  lcd.print(timestring);

  lcd.setCursor(0, 1);
  lcd.print(datestring);
}

void printDateTime(const RtcDateTime& dt)
{
  char datestring[20];

  snprintf_P(datestring,
             countof(datestring),
             PSTR("%02u/%02u/%04u %02u:%02u:%02u"),
             dt.Month(),
             dt.Day(),
             dt.Year(),
             dt.Hour(),
             dt.Minute(),
             dt.Second() );
  Serial.print(datestring);
}