DS3231 — Extremely Accurate I2C Real-Time Clock with Integrated TCXO

Overview

The DS3231 is a low-cost, highly accurate I2C real-time clock (RTC) manufactured by Maxim Integrated (now Analog Devices). It incorporates an integrated temperature-compensated crystal oscillator (TCXO) and a 32.768 kHz quartz crystal, providing exceptional timekeeping accuracy ( $\pm 2\text{ ppm}$ , ~1 minute per year error).

The device maintains seconds, minutes, hours, day, date, month, and year with leap-year compensation valid up to the year 2100. It includes a battery input (VBAT) for main power loss backup, two programmable time-of-day alarms, a square-wave/32.768 kHz output pin, and an internal digital temperature sensor.

Quick reference


Operating voltage (`VCC`)	2.3 V to 5.5 V DC
Battery backup (`VBAT`)	2.3 V to 5.5 V (typically CR2032 3V coin cell)
Accuracy	$\pm 2\text{ ppm}$ ( $0^\circ\text{C}$ to $+40^\circ\text{C}$ ), $\pm 3.5\text{ ppm}$ ( $-40^\circ\text{C}$ to $+85^\circ\text{C}$ )
Battery backup current	$< 0.8\text{ }\mu\text{A}$ typical
Communication interface	I2C (address `0x68`, up to 400 kHz)
Output signals	`SQW` / `INT` (Programmable interrupt/square wave), `32kHz`
Temperature sensor accuracy	$\pm 3\text{ }^\circ\text{C}$

Pinout

Standard DS3231 Breakout Header

Pin	Name	Type	Description
1	`32K`	Digital Output	32.768 kHz open-drain square wave output
2	`SQW` / `INT`	Digital Output	Interrupt or programmable square wave output (Active-LOW, Open-drain)
3	`SCL`	Digital Input	I2C Serial Clock input
4	`SDA`	Digital I/O	I2C Serial Data line
5	`VCC`	Power	Main supply voltage (+2.3 V to +5.5 V)
6	`GND`	Power	Ground (0 V)

Memory / AT24C32 EEPROM

Most DS3231 breakout modules (such as ZS-042) include an onboard 32 Kbit (4 KB) AT24C32 I2C EEPROM chip at base address 0x57 (selectable 0x50–0x57 via solder pads A0, A1, A2).

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Main Supply Voltage	$V_{CC}$	2.3	3.3	5.5	V	DC
Battery Supply Voltage	$V_{BAT}$	2.3	3.0	5.5	V	Coin cell backup
Active I2C Supply Current	$I_{CCA}$	—	100	200	µA	$f_{SCL} = 400\text{ kHz}$
Standby Current	$I_{CCS}$	—	70	150	µA	$V_{CC} = 5.0\text{ V}$ , I2C bus idle
Timekeeping Battery Current	$I_{BATATT}$	—	0.85	3.0	µA	$V_{CC} = 0\text{ V}$ , $V_{BAT} = 3.0\text{ V}$
Frequency Accuracy	$\Delta f/f$	-2	—	+2	ppm	$0^\circ\text{C} \text{ to } +40^\circ\text{C}$
Temp Sensor Resolution	$T_{res}$	—	0.25	—	°C	10-bit output

Register map

Address	Register	Access	Description
`0x00`	`SECONDS`	R/W	BCD format (00–59)
`0x01`	`MINUTES`	R/W	BCD format (00–59)
`0x02`	`HOURS`	R/W	BCD format (12-hr with AM/PM or 24-hr 00–23)
`0x03`	`DAY`	R/W	Day of week (1–7)
`0x04`	`DATE`	R/W	Day of month BCD (01–31)
`0x05`	`MONTH`	R/W	Month BCD (01–12) + Century bit
`0x06`	`YEAR`	R/W	Year BCD (00–99)
`0x0E`	`CONTROL`	R/W	Enable oscillator (`EOSC`), Enable alarms (`INTCN`), Rate select (`RS1/RS2`)
`0x0F`	`STATUS`	R/W	Oscillator stop flag (`OSF`), Alarm flags (`A1F`, `A2F`)
`0x11`	`TEMP_MSB`	R	Temperature Integer part (°C, 2's complement)
`0x12`	`TEMP_LSB`	R	Temperature Fractional part (bits [7:6] represent .00, .25, .50, .75 °C)

Note

Registers 0x00 through 0x06 store values in Binary Coded Decimal (BCD) format. To convert BCD to decimal in software: decimal = (bcd >> 4) * 10 + (bcd & 0x0F).

Wiring

DS3231 Breakout Pin	→	Microcontroller (Arduino Uno / ESP32)
`VCC`		`5V` (or `3.3V`)
`GND`		`GND`
`SDA`		`SDA` (A4 on Uno, GPIO21 on ESP32)
`SCL`		`SCL` (A5 on Uno, GPIO22 on ESP32)
`SQW` / `INT`		GPIO Pin with internal pull-up (for alarm interrupts)

Warning

Charging Circuit Danger on ZS-042 Modules: Many cheap "ZS-042" DS3231 modules include a simple diode-resistor trickle charge circuit designed for rechargeable LIR2032 cells. If using a non-rechargeable CR2032 coin cell, this circuit can force current into the primary cell, risking leakage or rupture. Desolder the 200 Ω resistor or diode on ZS-042 boards when using standard CR2032 batteries.

Common mistakes

Not converting BCD to Binary/Decimal: Reading 0x15 from register 0x01 returns $0\times15$ (hex), which equals $21$ in decimal BCD. Treating it directly as an integer yields 21 instead of 15.
Forgetting to clear OSF (Oscillator Stop Flag): On first power up or after battery replacement, the OSF bit in 0x0F is set to 1 indicating the oscillator was stopped. Software must clear OSF to confirm time valid status.
Incompatible trickle charging with CR2032: Using non-rechargeable CR2032 batteries on ZS-042 boards without disabling the onboard charging circuit.

Notes

The TCXO updates crystal compensation registers automatically every 64 seconds based on internal temperature readings.

Interface	I2C
Supply	2.3–5.5 V
Package	16-pin SO (300 mil) / Breakout PCB