DS1307 — Serial Real-Time Clock (RTC) with 56-Byte NV RAM

Overview

The DS1307 is a low-power, full binary-coded decimal (BCD) real-time clock (RTC) IC manufactured by Maxim Integrated (Analog Devices). It counts seconds, minutes, hours, day of the week, date of the month, month, and year with leap-year compensation valid up to the year 2100.

The chip incorporates a $32.768\text{ kHz}$ quartz crystal oscillator circuit, a power-sense circuit that automatically switches to a 3V CR2032 lithium battery backup during power outages, and 56 bytes of nonvolatile battery-backed SRAM for storing custom user settings.

Quick reference


Operating Voltage (`VCC`)	4.5 V to 5.5 V DC (5.0 V required)
Backup Battery Voltage (`VBAT`)	2.0 V to 3.5 V DC (3 V CR2032 coin cell)
Oscillator Frequency	32.768 kHz (requires external crystal with $12.5\text{ pF}$ load capacitance)
Nonvolatile SRAM	56 bytes (address `0x08` to `0x3F`)
Communication Interface	I2C (fixed slave address `0x68`, up to 100 kHz)
Square Wave Output (`SQW/OUT`)	Programmable output frequency: 1 Hz, 4.096 kHz, 8.192 kHz, 32.768 kHz
Backup Battery Current	$< 500\text{ nA}$ (battery lasts 10+ years)

Pinout

8-Pin DIP / SOIC Package & Tiny RTC Module Header

text

           ┌──────────┐
        X1 ──│ 1      8 │── VCC (5V)
        X2 ──│ 2      7 │── SQW/OUT
      VBAT ──│ 3      6 │── SCL
       GND ──│ 4      5 │── SDA
           └──────────┘

Pin	Name	Type	Description
1	`X1`	Oscillator Input	Connect to 32.768 kHz crystal terminal 1
2	`X2`	Oscillator Output	Connect to 32.768 kHz crystal terminal 2
3	`VBAT`	Battery Input	Backup +3V CR2032 lithium battery input
4	`GND`	Power	Ground (0 V)
5	`SDA`	Digital I/O	I2C Serial Data line
6	`SCL`	Digital Input	I2C Serial Clock line
7	`SQW` / `OUT`	Digital Output	Square-Wave / Interrupt output line (Open-drain)
8	`VCC`	Power Input	Primary supply voltage (+4.5 V to +5.5 V DC)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Primary Supply Voltage	$V_{CC}$	4.5	5.0	5.5	V	DC
Battery Voltage	$V_{BAT}$	2.0	3.0	3.5	V	CR2032 Lithium coin cell
Active Supply Current	$I_{CCA}$	—	0.8	1.5	mA	$V_{CC} = 5.0\text{ V}$ , I2C active
Battery Backup Current	$I_{BAT}$	—	300	500	nA	$V_{CC} = 0\text{ V}$ , $V_{BAT} = 3.0\text{ V}$
Power-Fail Threshold	$V_{PF}$	1.21 $V_{BAT}$	1.25 $V_{BAT}$	1.29 $V_{BAT}$	V	Switch to battery mode threshold
Crystal Load Capacitance	$C_L$	—	12.5	—	pF	$32.768\text{ kHz}$ quartz crystal

Register Map (BCD Format)

Time and calendar data are stored in Binary-Coded Decimal (BCD) format across registers 0x00 to 0x06:

Address	Register	Bit 7 (MSB)	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0 (LSB)	Range
`0x00`	`SECONDS`	`CH`	10 Seconds	Seconds	00–59
`0x01`	`MINUTES`	0	10 Minutes	Minutes	00–59
`0x02`	`HOURS`	0	12/24	10Hr / AM/PM	Hours	01–12 / 00–23
`0x03`	`DAY`	0	0	0	0	0	Day of Week	01–07
`0x04`	`DATE`	0	0	10 Date	Date	01–31
`0x05`	`MONTH`	0	0	0	10 Month	Month	01–12
`0x06`	`YEAR`	10 Year	Year	00–99
`0x07`	`CONTROL`	`OUT`	0	0	`SQWE`	0	0	`RS1`	`RS0`	Control byte
`0x08`–`0x3F`	`RAM`	56 Bytes General Purpose Non-Volatile User SRAM	`0x00`–`0xFF`

Note

Clock Halt (CH) Bit: Bit 7 of register 0x00 is the Clock Halt (CH) bit. When CH = 1, the internal crystal oscillator is STOPPED and the clock does not run. Software MUST write CH = 0 during initialization to start counting time!

Wiring

DS1307 Module Pin	Microcontroller (Arduino Uno / ESP32)	Notes
`VCC`	`5V`	Must be 5.0 V DC (Does NOT work on 3.3V)
`GND`	`GND`	Ground
`SDA`	`SDA` (A4 on Uno, GPIO21 on ESP32)	$4.7\text{ k}\Omega$ pull-up resistor
`SCL`	`SCL` (A5 on Uno, GPIO22 on ESP32)	$4.7\text{ k}\Omega$ pull-up resistor
`SQW`	Digital Pin (optional 1 Hz pulse interrupt)	Open-drain output

Common mistakes

Powering with 3.3V: The DS1307 requires $V_{CC} \ge 4.5\text{ V}$ . Operating on 3.3V causes the power-fail comparator to assume main power was lost, locking out I2C access. For native 3.3V systems, use the DS3231.
Forgetting to clear Clock Halt (CH) bit: New DS1307 ICs default to CH = 1. Time values stay frozen until software writes 0x00 to register 0x00.
Crystal temperature drift: Unlike the DS3231 (which has an integrated TCXO temperature-compensated crystal), the DS1307 uses an external uncompensated crystal. Ambient temperature changes can cause time drift up to 1–2 minutes per month.

Notes

Tiny RTC modules often contain an 8-pin 24C32 EEPROM chip on the same I2C bus at address 0x50.