I2C Real-Time Clock module with DS3231M

This project uses the DS3231M I2C real-time clock (RTC) module for extremely accurate timekeeping. It features a MEMS resonator for long-term accuracy, a 3V backup battery, and tracks full calendar data (including leap years). It also includes two alarms, a 1Hz output, and a power-monitoring circuit that provides a reset output.

Modules October 18, 2025 by mixos

This is low cost and extremely accurate I2C real-time clock (RTC) module that consists of DS3231M low-cost RTC chip, on board 3V battery to maintain accurate timekeeping when main power to the device is interrupted. The integration of the microelectromechanical systems (MEMS) resonator enhances the long-term accuracy of the device.

The RTC maintains seconds, minutes, hours, day, date, month, and year information. The date at the end of the month is automatically adjusted for months with fewer than 31 days, including corrections for leap year. The clock operates in either the 24-hour or 12-hour format with an active-low AM/PM indicator. Two programmable time-of-day alarms and a 1Hz output are provided. Address and data are transferred serially through an I2C bidirectional bus. A precision temperature-compensated voltage reference and comparator circuit monitors the status of VCC to detect power failures, to provide a reset output, and to automatically switch to the backup supply when necessary. Additionally, the active-low RST pin is monitored as a pushbutton input for generating a microprocessor reset.

Features

Supply Input 2.3V To 5.5V DC
On Board 3V Battery Holder
On Board Power LED
Header Connector for I2C Interface and Power
Simple Serial Interface Connects to Most Microcontrollers Fast (400kHz) I2C Interface
Battery-Backup Input for Continuous Timekeeping Low Power Operation Extends Battery-Backup Run Time
Complete Clock Calendar Functionality Including Seconds, Minutes, Hours, Day, Date, Month, and Year, with Leap-Year Compensation Up to Year 2100
Timekeeping Accuracy ±5ppm (±0.432 Second/Day) from -45°C to +85°C
Reset Output and Push button Input with De-bounce
Digital Temp Sensor with ±3°C Accuracy
1Hz and 32.768kHz Outputs
On Board Power LED
5mm Mounting Holes
PCB Dimensions 31.75 x 23.50 mm

Connections

CN1: Pin 1 = VCC 2.3V to 5.5V, Pin 2 = Reset, Pin 3 = SDA, Pin 4 = SCL, Pin 5 = SQW/INT, Pin 6 = 32Khz/1Hz, Pin 7 = GND

Schematic

Parts List

NO.	QNTY.	REF.	DESC	MANUFACTURER	SUPPLIER	SUPPLIER PART NO
1	1	BT1	3.3V BATTERY HOLDER CR2032	KEYSTONE	DIGIKEY	36-1066-ND
2	1	CN1	7 PIN MALE HEADER PITCH 2.54MM	WURTH	DIGIKEY	732-5320-ND
3	1	C1	10uF/6.3V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
4	1	C2	100nF/50V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
5	1	D1	LED RED SMD SIZE 0805	OSRAM	DIGIKEY	475-1278-1-ND
6	1	R1	1K 10% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
7	4	R2,R3,R4,R5	10K 5% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
8	1	U1	DS3231M SOIC8	ANALOG DEVICE	DIGIKEY	175-DS3231MZ+TRLCT-ND
9	1	BAT	3.3V BATTERY CR2032	PANASONIC	DIGIKEY