Detailed Description

The DS1340 is a low-power clock/calendar with a trickle

charger. Address and data are transferred serially

vides 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 DS1340

has a built-in power-sense circuit that detects power fail-

ures and automatically switches to the backup supply.

Oscillator Circuit

The DS1340 uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 1 specifies several crys-

tal parameters for the external crystal. Figure 3 shows a

functional schematic of the oscillator circuit. If using a

crystal with the specified characteristics, the startup

time is usually less than one second.

Clock Accuracy

The initial clock accuracy depends on the accuracy of

the crystal and the accuracy of the match between the

capacitive load of the oscillator circuit and the capaci-

tive load for which the crystal was trimmed. Additional

6

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Pin Description

PIN

8

16

NAME

FUNCTION

1

—

X1

2

—

X2

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

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.

3

14

This pin can be connected to a primary cell such as a lithium coin cell. Additionally, this pin can be

connected to a rechargeable cell or a super cap when used with the trickle-charge feature. UL

recognized to ensure against reverse charging when used with a lithium battery.

4

15

GND

Ground

5

16

SDA

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

drain and requires an external pullup resistor.

6

1

SCL

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

on the serial interface.

7

2

FT/OUT

Frequency Test/Output. This pin is used to output either a 512Hz signal or the value of the OUT bit. When

the FT bit is logic 1, the FT/OUT pin toggles at a 512Hz rate. When the FT bit is logic 0, the FT/OUT pin

reflects the value of the OUT bit. This open-drain pin requires an external pullup resistor, and operates

8

3

DC Power for Primary Power Supply

—

4–13

N.C.

No Connection. Must be connected to ground.

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

Nominal

Frequency

32.768

kHz

Series Resistance

ESR

45,60**

kΩ

Load Capacitance

12.5

pF

Table 1. Crystal Specifications*

*The crystal, traces, and crystal input pins should be isolated

from RF generating signals. Refer to Application Note 58:

Crystal Considerations for Dallas Real-Time Clocks for addi-

tional specifications.

**A crystal with up to 60kΩ ESR can be used if the minimum

COUNTDOWN

CHAIN

RTC

X1

X2

CRYSTAL

RTC

REGISTERS

Figure 3. Oscillator Circuit Showing Internal Bias Network