DS3231 and LCD 1602 Clock: Accurate Time Without the Internet

About the project

The ESP32 can get the time from the internet over NTP, but without Wi-Fi it starts from zero after every reset. The DS3231 module is a real-time clock with a temperature-compensated crystal: it drifts about ±2 min a year, and a CR2032 cell keeps it running for years. We show the time, date and temperature on an LCD 1602.

What you need

• ESP32 DevKit
• DS3231 module (ZS-042 or the mini version) + a CR2032 cell
• LCD 1602 with an I2C backpack (PCF8574, address 0x27 or 0x3F)
• The RTClib (Adafruit) and LiquidCrystal_I2C libraries

Wiring

Both modules share one I2C bus, each at its own address (DS3231 is 0x68, the LCD 0x27):

The display works from 3.3 V but with lower contrast: turn the blue trimmer on the I2C board. If you power the LCD from 5 V, remember that its pull-up resistors pull SDA/SCL to 5 V. The ESP32 usually tolerates it, but it is out of spec.

ZS-042 modules have a "charging" circuit for the cell. With a plain CR2032 (not a rechargeable LIR2032) it is better disabled by removing the resistor or diode next to the holder.

How it works

• rtc.lostPower() returns true if the clock stopped, for example because the cell ran flat or was removed.
• DateTime(F(__DATE__), F(__TIME__)) is the date and time the sketch was compiled. Upload the sketch and the clock sets itself (a few seconds behind, because of the upload time).
• rtc.getTemperature() is the built-in temperature sensor, in 0.25 °C steps. It measures the chip itself, so inside a case it reads 1–2 °C above the air.
• snprintf builds a fixed-width line, so the digits do not jump around and leftovers of old text are overwritten.

The HD44780 (the LCD controller) has no Cyrillic in its built-in font, so the weekdays are written in Latin letters.

Code

// Clock: DS3231 (time) + LCD 1602 over I2C
#include <Wire.h>
#include <RTClib.h>
#include <LiquidCrystal_I2C.h>

RTC_DS3231 rtc;
LiquidCrystal_I2C lcd(0x27, 16, 2);
const char* DAYS[] = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};

void setup() {
  lcd.init();
  lcd.backlight();
  if (!rtc.begin()) {
    lcd.print("RTC not found");
    while (true) delay(1000);
  }
  if (rtc.lostPower()) {
    // the cell was removed: set the clock to the moment the sketch was compiled
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }
}

void loop() {
  DateTime now = rtc.now();
  char line[17];   // 16 characters + the terminating zero
  snprintf(line, sizeof line, "%02d:%02d:%02d   %s", now.hour(), now.minute(), now.second(), DAYS[now.dayOfTheWeek()]);
  lcd.setCursor(0, 0);
  lcd.print(line);
  snprintf(line, sizeof line, "%02d.%02d.%04d %3.0fC", now.day(), now.month(), now.year(), rtc.getTemperature());
  lcd.setCursor(0, 1);
  lcd.print(line);
  delay(200);
}

Common mistakes

• Blank screen or a row of blocks. Wrong address (try 0x3F) or the contrast is not set. An I2C scanner sketch shows the addresses of every device on the bus.
• The time resets after power-off. The cell is flat, or the module is "charging" a plain CR2032.
• The clock is a few seconds slow after an upload. That is expected: __TIME__ is the moment of compilation, not of the upload.

What next

Add buttons to set the time, or sync the DS3231 with NTP once a day: the ESP32 connects to Wi-Fi, gets the exact time and writes it into the RTC.