DS3231 on OLED: Real-Time Clock + Temperature
Overview & Wiring
What it is: The DS3231 is a very accurate real-time clock (±2 ppm, about a minute a year) with a coin-cell backup so it keeps time even when your project is unplugged. It also exposes its internal temperature, which it uses to stay accurate.
How it works: I2C device at 0x68. On first upload the sketch sets the clock to your computer's compile time; after that the coin cell keeps it running. The library returns the date, time and chip temperature.
Wiring (shared I2C bus with the OLED):
DS3231 SDA -> GPIO4
DS3231 SCL -> GPIO5
DS3231 VCC -> 3V3
DS3231 GND -> GND
OLED VCC=3V3, GND=GND, SDA=GPIO4, SCL=GPIO5
Libraries: "RTClib" by Adafruit (installs "Adafruit BusIO"), plus "Adafruit SSD1306" and "Adafruit GFX Library".
On screen / Serial: the current time (HH:MM:SS), the date, and the chip temperature in °C.
Gotchas: Insert a CR2032 backup battery or it forgets the time on power loss. The sketch sets the time to the compile moment only when it detects a power loss — to force a reset, uncomment the rtc.adjust line. Cheap modules sometimes try to recharge a non-rechargeable CR2032; that's a known quirk but harmless for hobby use.
Circuit Diagram
Parts You Need
Source Code (Arduino / ESP32)
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <RTClib.h>
Adafruit_SSD1306 display(128, 64, &Wire, -1);
RTC_DS3231 rtc;
void setup() {
Serial.begin(115200);
Wire.begin(4, 5);
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
if (!rtc.begin()) {
Serial.println("DS3231 not found");
display.setTextColor(SSD1306_WHITE);
display.setCursor(0, 28); display.println("RTC not found");
display.display();
while (1) delay(10);
}
if (rtc.lostPower()) { // first run or dead battery
rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); // set to compile time
}
// rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); // uncomment to force-reset
}
void loop() {
DateTime now = rtc.now();
float t = rtc.getTemperature();
char timeStr[9], dateStr[11];
sprintf(timeStr, "%02d:%02d:%02d", now.hour(), now.minute(), now.second());
sprintf(dateStr, "%04d-%02d-%02d", now.year(), now.month(), now.day());
display.clearDisplay();
display.setTextColor(SSD1306_WHITE);
display.setTextSize(2);
display.setCursor(0, 4); display.println(timeStr);
display.setTextSize(1);
display.setCursor(0, 34); display.println(dateStr);
display.setCursor(0, 50); display.printf("chip %.1f C", t);
display.display();
Serial.printf("%s %s (%.1f C)\n", dateStr, timeStr, t);
delay(1000);
}