Unit D - Sensors & Buses

19. DS1307 RTC & Formatted Text

Keep calendar time through power loss and format it for displays.

Estimated time 4 hours

Learning outcomes

Distinguish millis from an RTC
Wire a DS1307 on I2C
Set time once and preserve it
Format fixed-width date/time text

Parts and preparation

Uno, DS1307 RTC module, suitable backup cell, and RTClib by Adafruit (see Libraries box).

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Libraries for this lesson

In Arduino IDE 2 open Tools → Manage Libraries…. Search by the Library Manager name and install the package by the exact author below. Similar names from other authors can use different APIs and will break the example.

Include	Library Manager name	Author	Install note
`Wire.h`	Wire	Arduino	Built into the Arduino core. No Library Manager install required.
`RTClib.h`	RTClib	Adafruit	Install via Library Manager. Choose Adafruit, not similarly named RTC packages.

DS1307 RTC & Formatted Text instructional connection diagram — Open the diagram for a larger view. Trace every connection before building.

RTC purpose

millis measures uptime and resets with power. An RTC tracks calendar date/time and uses a backup cell while main power is absent.

DS1307

The DS1307 communicates over I2C and runs from a 32.768 kHz crystal. It is less accurate than temperature-compensated devices such as DS3231.

Setting time

Set the RTC once from compile time, then comment out that adjustment and upload again. Repeated adjustment makes time jump back on every reset.

Formatting

snprintf writes formatted text into a bounded character buffer. %02d pads a number to two digits.

Wiring and safe build sequence

Breadboard wiring for lesson 19: DS1307 RTC & Formatted Text — Breadboard layout for this lesson. Match colours and pins before powering the circuit. Click the image for a larger view.

RTC VCC -> 5 V
RTC GND -> GND
RTC SDA -> A4
RTC SCL -> A5
Install the correct backup cell with correct polarity

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

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

RTC_DS1307 rtc;
char line[24];

void setup() {
  Serial.begin(9600);
  if (!rtc.begin()) {
    Serial.println("RTC not found");
    while (true) { delay(100); }
  }
  // Run once if required, then comment and upload again:
  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
}

void loop() {
  DateTime now = rtc.now();
  snprintf(line, sizeof(line), "%02d/%02d/%04d %02d:%02d:%02d",
           now.day(), now.month(), now.year(),
           now.hour(), now.minute(), now.second());
  Serial.println(line);
  delay(1000);
}

How the code works

sizeof(line) prevents snprintf from writing beyond the buffer.
The failure message distinguishes wiring/address faults from display faults.

Test and record evidence

Expected result: A correctly formatted date and time updates once each second and survives Uno power removal.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Use an I2C scanner to confirm the RTC address.
Replace/check the backup cell if time resets.
Do not leave rtc.adjust active.

Extension challenge: Display time on row 1 and date on row 2 of an LCD.

Check your understanding

Q1. Why not use millis as a clock?

Show answer

It measures uptime only and resets after power loss/reset.

Q2. Why comment rtc.adjust after setting?

Show answer

Otherwise each reset overwrites the RTC time.