DS1307 64 x 8, Serial, I

2C Real-Time Clock

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PIN DESCRIPTION

PIN

NAME

FUNCTION

1

X1

2

X2

Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is

designed for operation with a crystal having a specified load capacitance (CL) of 12.5pF.

X1 is the input to the oscillator and can optionally be connected to an external 32.768kHz

oscillator. The output of the internal oscillator, X2, is floated if an external oscillator is

connected to X1.

Note: For more information on crystal selection and crystal layout considerations, refer to

Application Note 58: Crystal Considerations with Dallas Real-Time Clocks.

3

VBAT

Backup Supply Input for Any Standard 3V Lithium Cell or Other Energy Source. Battery

voltage must be held between the minimum and maximum limits for proper operation.

Diodes in series between the battery and the VBAT pin may prevent proper operation. If a

backup supply is not required, VBAT must be grounded. The nominal power-fail trip point

(VPF) voltage at which access to the RTC and user RAM is denied is set by the internal

circuitry as 1.25 x VBAT nominal. A lithium battery with 48mAh or greater will back up the

DS1307 for more than 10 years in the absence of power at +25°C.

UL recognized to ensure against reverse charging current when used with a lithium

battery. Go to:

4

GND

Ground

5

SDA

Serial Data Input/Output. SDA is the data input/output for the I

2C serial interface. The

SDA pin is open drain and requires an external pullup resistor. The pullup voltage can be

up to 5.5V regardless of the voltage on VCC.

6

SCL

Serial Clock Input. SCL is the clock input for the I

2C interface and is used to synchronize

data movement on the serial interface. The pullup voltage can be up to 5.5V regardless of

the voltage on VCC.

7

SQW/OUT

Square Wave/Output Driver. When enabled, the SQWE bit set to 1, the SQW/OUT pin

outputs one of four square-wave frequencies (1Hz, 4kHz, 8kHz, 32kHz). The SQW/OUT

pin is open drain and requires an external pullup resistor. SQW/OUT operates with either

VCC or VBAT applied. The pullup voltage can be up to 5.5V regardless of the voltage on

VCC. If not used, this pin can be left floating.

8

VCC

Primary Power Supply. When voltage is applied within normal limits, the device is fully

accessible and data can be written and read. When a backup supply is connected to the

device and VCC is below VTP, read and writes are inhibited. However, the timekeeping

function continues unaffected by the lower input voltage.

DETAILED DESCRIPTION

The DS1307 is a low-power clock/calendar with 56 bytes of battery-backed SRAM. The clock/calendar provides

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 DS1307 operates as a slave

device on the I

2C bus. Access is obtained by implementing a START condition and providing a device identification

code followed by a register address. Subsequent registers can be accessed sequentially until a STOP condition is

executed. When VCC falls below 1.25 x VBAT, the device terminates an access in progress and resets the device

address counter. Inputs to the device will not be recognized at this time to prevent erroneous data from being

written to the device from an out-of-tolerance system. When VCC falls below VBAT, the device switches into a low-

current battery-backup mode. Upon power-up, the device switches from battery to VCC when VCC is greater than

VBAT +0.2V and recognizes inputs when VCC is greater than 1.25 x VBAT. The block diagram in Figure 1 shows the

main elements of the serial RTC.