I2C RTC Real-Time Clock Module | DS3231 / DS1302 / DS1307 / PCF8563 / PCF8523T | Arduino ESP32 Raspberry Pi

I2C RTC Real-Time Clock Module — DS3231 / DS1302 / DS1307 / PCF8563 / PCF8523T | Arduino / ESP32 / Raspberry Pi

A real-time clock (RTC) module provides accurate timekeeping for microcontroller projects that need to track time, date, and day-of-week independently of the main processor — even when the system is powered off (using a backup coin cell battery). Without an RTC, microcontrollers lose track of time every time they are reset or powered off. This listing covers the most popular RTC chips used in Arduino and embedded projects: DS3231 (highest accuracy, temperature-compensated), DS1302 (3-wire SPI, low power), DS1307 (I2C, most common), PCF8563 (I2C, ultra-low power), PCF8523T (I2C, integrated capacitor backup), and DS3231 Mini (compact DS3231). Select the chip that matches your accuracy, power, and interface requirements.

Module Comparison

Module	Chip	Interface	Accuracy	Backup	Best For
DS3231	DS3231	I2C	±2ppm (TCXO)	Coin cell	Highest accuracy, temperature-compensated
DS3231 Mini	DS3231	I2C	±2ppm (TCXO)	Coin cell	DS3231 accuracy in compact form factor
DS1307	DS1307	I2C	±~2 min/month	Coin cell	Most common, widely supported
DS1302	DS1302	3-wire SPI	±~2 min/month	Coin cell + trickle charge	Low power, trickle-charge backup
PCF8563	PCF8563	I2C	±~1 min/month	Coin cell	Ultra-low power, IoT and wearables
PCF8523T	PCF8523	I2C	±~1 min/month	Integrated capacitor	No coin cell needed, compact backup
PI RTC DS1307	DS1307	I2C	±~2 min/month	Coin cell	Raspberry Pi HAT form factor
DS2121SN	DS2121	I2C	Standard	Coin cell	General purpose RTC

Specifications (All Modules)

Supply Voltage	3.3V – 5.5V DC
Operating Temperature	-40°C to +85°C
Package	SMD (surface mount chip on through-hole module)
Application	Arduino, ESP32, STM32, Raspberry Pi timekeeping
Origin	Mainland China

Why Your Project Needs an RTC

Microcontrollers like Arduino and ESP32 have no internal clock that persists through power cycles. Without an RTC:

• Time resets to zero on every power cycle or reset
• Data logging is inaccurate — timestamps are meaningless without real time
• Scheduled tasks fail — alarms, timers, and scheduled events cannot be set to real-world times
• NTP sync is unavailable offline — WiFi-connected devices can sync via NTP, but offline devices need an RTC

An RTC module with a coin cell backup maintains accurate time for years without main power, providing reliable timestamps for data loggers, alarm clocks, irrigation controllers, and any time-aware embedded system.

Key Features

• ✅ Multiple chip options — DS3231 (highest accuracy), DS1307 (most common), DS1302 (SPI), PCF8563 (ultra-low power)
• ✅ 3.3V–5.5V supply — compatible with Arduino (5V), ESP32 (3.3V), and Raspberry Pi (3.3V)
• ✅ Coin cell backup — maintains time through power cycles and system resets
• ✅ I2C interface — only 2 wires (SDA + SCL) for most modules
• ✅ -40°C to +85°C — suitable for industrial and outdoor applications
• ✅ Arduino library support — all chips have well-maintained Arduino libraries

Common Applications

• Data logger with accurate timestamps
• Arduino alarm clock and timer
• Automatic irrigation and lighting scheduler
• Industrial process time recording
• Energy meter with time-of-use tracking
• Attendance and access control system
• Weather station data logging
• Raspberry Pi real-time clock (offline timekeeping)

Arduino Quick Start (DS3231)

1. Connect VCC to Arduino 5V (or 3.3V), GND to GND, SDA to A4, SCL to A5.
2. Install the RTClib library by Adafruit via Arduino Library Manager.
3. Use RTC.adjust(DateTime(F(__DATE__), F(__TIME__))) to set the time to compile time.
4. Use DateTime now = RTC.now(); to read the current time.

Frequently Asked Questions

Q: Which RTC module is most accurate?
A: The DS3231 is the most accurate, with a temperature-compensated crystal oscillator (TCXO) rated at ±2ppm (±63 seconds per year). The DS1307 and DS1302 are less accurate (±2 minutes per month typical) but adequate for most applications.

Q: How long does the coin cell backup last?
A: A CR2032 coin cell provides backup power for 5–10 years in most RTC modules, depending on the chip’s backup current draw. The PCF8563 has the lowest backup current and longest coin cell life.

Package Contents

• 1× RTC Real-Time Clock Module (chip variant as selected)
• Coin cell battery may or may not be included depending on variant — check variant details

Enjoy free standard shipping on all orders over $199 USD. Orders are processed within 1–2 business days.

Not satisfied? Return any item within 90 days of purchase for a full refund or exchange. Items must be in original condition.

We are committed to quality, reliability, and customer satisfaction. Every product is carefully selected and tested to meet the highest standards.