Board and part

Arduino Uno DS3231 RTC Simulator: Wiring, Code and a Live Circuit

This is a DS3231 real-time clock wired to an Arduino Uno over I2C, read live with Adafruit's RTClib: the date and time every second, and the chip's own temperature.

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5 V logic
Library: RTClib.h
I2C address 0x68
SDA A4, SCL A5

Arduino Uno · DS3231 RTClive0.000 s 0.00x

Click to open it in the editor

The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Connections

Wiring the DS3231 RTC to the Arduino Uno

Four wires: 5V, GND, SDA to pin A4 and SCL to pin A5. The sketch sets the clock to a fixed time, so the first line is always the same.

Every connection in the circuit above, with the board’s own pin names.

DS3231 RTC pin	Arduino Uno pin	Why
VCC	5V	Most DS3231 modules run from 3.3 to 5.5 V.
GND	GND	Common ground.
SCL	pin A5	I2C clock. The module has its own pull-up resistors.
SDA	pin A4	I2C data.

Power

On an Uno the I2C bus is A4 (SDA) and A5 (SCL). The SDA and SCL pins beside AREF are the same two wires, so either pair works. The module carries its own I2C pull-ups, and the chip answers at the fixed address 0x68. The 32K and SQW pins are outputs this page leaves unwired.

Pins to leave alone on the Arduino Uno

Pins 0 and 1 are the USB serial port on a real Uno, so anything wired there fights the upload and the serial monitor. Pin 13 carries the onboard LED.

On a real bench

On a real bench the Uno stands beside the breadboard: the DS3231 RTC goes into the breadboard and male-to-male jumpers run from the Uno’s female headers to its rows. Power the Uno from USB while you build, and unplug it before you move a wire.

The sketch

The code, and what it does.

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

DS3231 clock on Arduino Uno I2C

#include <Wire.h>
#include <RTClib.h>

RTC_DS3231 rtc;   // on I2C: SDA pin A4, SCL pin A5, address 0x68

void setup() {
  Serial.begin(9600);
  if (!rtc.begin()) {
    Serial.println("No DS3231 found");
    for (;;) {}
  }
  if (rtc.lostPower()) {
    Serial.println("Clock lost power, setting it");
    rtc.adjust(DateTime(2026, 11, 7, 9, 30, 0));
  }
}

void loop() {
  DateTime now = rtc.now();
  char text[24];
  snprintf(text, sizeof(text), "%04d-%02d-%02d %02d:%02d:%02d",
           now.year(), now.month(), now.day(), now.hour(), now.minute(), now.second());
  Serial.print(text);
  Serial.print("  Chip: ");
  Serial.print(rtc.getTemperature(), 2);
  Serial.println(" C");
  delay(1000);
}

rtc.begin() looks for the chip at 0x68. lostPower() reads the oscillator-stop flag, which a DS3231 sets whenever it has lost its time. On a real module with a coin cell it is clear after the first set, so a sketch sets the clock once. The simulated chip has no coin cell and starts every run with the flag set, so this sketch sets it each run.

rtc.now() reads all seven time registers in one transfer and converts them from BCD, so the sketch never sees a BCD byte. getTemperature() is the thermometer inside the chip, which it uses to correct its crystal; it measures the chip, not the room.

Serial.begin(9600) opens the serial monitor. Serial goes over the USB cable, and the serial monitor in the editor reads it at whatever baud rate the sketch sets.

Pitfalls

Common mistakes with the DS3231 RTC on the Arduino Uno

Setting the time on every boot

A sketch that calls rtc.adjust() unconditionally resets the clock each time the board restarts. Guard it with lostPower(), as above, so a real module with a battery keeps its time.

Another chip at 0x68

The MPU6050 also answers at 0x68. Two such parts on one bus clash; the DS3231 has no address pin to move it.

Using compile time on an old build

DateTime(F(__DATE__), F(__TIME__)) sets the time the sketch was compiled, not now. If you upload a day later, the clock is a day behind.

FAQ

Questions and answers

Can I simulate a DS3231 with an Arduino Uno?

Yes. The circuit at the top of this page is an Arduino Uno with a DS3231 real-time clock, running in your browser on a simulated ATmega328P. The simulated chip counts real seconds and answers the library over I2C, registers and all. It has no coin cell, so it forgets the time between runs. Press Run it in the editor to change the wiring or the code; it is free and needs no account.

Where does the DS3231 connect on the Arduino Uno?

On an Uno the I2C bus is A4 (SDA) and A5 (SCL). The SDA and SCL pins beside AREF are the same two wires, so either pair works.

How accurate is a DS3231?

It corrects its crystal for temperature and keeps time to within about a minute a year, where a DS1307 can drift that much in a month.

Does it keep time when the board is off?

A real module does, from a coin cell on the back. The simulated one has no battery, so it starts each run at the time the sketch sets.

Build your own DS3231 RTC project

Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.

Start building free Open the editor