DS3231 RTC with Arduino Nano — accurate time and the battery-charging trap

Clocks, school bells, timed pumps and data loggers all need to know the real time after a power cut — something the Nano cannot do on its own. The DS3231 keeps time on a coin cell and talks I2C on A4/A5.

On first run (or after a battery change) the sketch sets the clock from your PC’s time at compile, then prints the date, time and the chip’s own temperature sensor once a second.

Wiring

Board quirks

• The popular ZS-042 board has a charging circuit (a diode and 200 Ω resistor) that pushes current into the coin cell. With 5 V on VCC and a normal non-rechargeable CR2032, that is unsafe. Either fit a rechargeable LIR2032, or remove the resistor/diode.
• The module also carries an AT24C32 EEPROM at 0x57 — an I2C scanner shows two addresses, and that is normal.
• The time is set from the moment the sketch was compiled, so it is a few seconds behind. Upload straight after compiling.
• Leave 32K and SQW unconnected. SQW can wake a sleeping Nano once a second or on an alarm, via D2 (INT0).
• The DS3231 drifts about a minute a year — far better than millis() on a Nano, whose resonator can drift minutes a day.

Code

// DS3231 RTC on an Arduino Nano: sets itself from the compile time after a battery change,
// then prints the date, time and the DS3231's own temperature every second.
// VCC → 5V, GND → GND, SDA → A4, SCL → A5.
#include <Wire.h>
#include <RTClib.h>

RTC_DS3231 rtc;

void setup() {
  Serial.begin(115200);
  if (!rtc.begin()) {
    Serial.println(F("DS3231 not found - check A4/A5"));
    for (;;) {}
  }
  if (rtc.lostPower()) {
    Serial.println(F("RTC lost power - setting it to the sketch's compile time"));
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }
  rtc.disable32K();  // the 32K output is not needed
}

void loop() {
  DateTime now = rtc.now();
  char buf[] = "DD-MM-YYYY hh:mm:ss";
  Serial.print(now.toString(buf));
  Serial.print(F("  RTC temp: "));
  Serial.print(rtc.getTemperature(), 2);
  Serial.println(F(" C"));
  delay(1000);
}

// DS3231 RTC on an Arduino Nano: sets itself from the compile time after a battery change,
// then prints the date, time and the DS3231's own temperature every second.
// VCC → 5V, GND → GND, SDA → A4, SCL → A5.
#include <Wire.h>
#include <RTClib.h>

RTC_DS3231 rtc;

void setup() {
  Serial.begin(115200);
  if (!rtc.begin()) {
    Serial.println(F("DS3231 not found - check A4/A5"));
    for (;;) {}
  }
  if (rtc.lostPower()) {
    Serial.println(F("RTC lost power - setting it to the sketch's compile time"));
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }
  rtc.disable32K();  // the 32K output is not needed
}

void loop() {
  DateTime now = rtc.now();
  char buf[] = "DD-MM-YYYY hh:mm:ss";
  Serial.print(now.toString(buf));
  Serial.print(F("  RTC temp: "));
  Serial.print(rtc.getTemperature(), 2);
  Serial.println(F(" C"));
  delay(1000);
}

Library: RTClib (Adafruit) — compiled with RTClib 2.1.4.

Expected serial output

RTC lost power - setting it to the sketch's compile time
03-11-2026 14:22:07  RTC temp: 28.25 C
03-11-2026 14:22:08  RTC temp: 28.25 C

RTC lost power - setting it to the sketch's compile time
03-11-2026 14:22:07  RTC temp: 28.25 C
03-11-2026 14:22:08  RTC temp: 28.25 C

Common errors