Oscillator Circuit

The DS1340 uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 2 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

error is added by crystal frequency drift caused by

temperature shifts. External circuit noise coupled into

the oscillator circuit can result in the clock running fast.

Figure 4 shows a typical PC board layout for isolating

the crystal and oscillator from noise. Refer to

Application Note 58: Crystal Considerations with Dallas

Real-Time Clocks (www.maxim-ic.com/RTCapps) for

detailed information.

DS1340C Only

The DS1340C integrates a standard 32,768Hz crystal

and +25°C is approximately +15ppm. Refer to

Application Note 58 for information about crystal accu-

racy vs. temperature.

Operation

The DS1340 operates as a slave device on the serial

bus. Access is obtained by implementing a START

condition and providing a device identification code fol-

lowed by data. Subsequent registers can be accessed

sequentially until a STOP condition is executed. The

device is fully accessible and data can be written and

is returned to nominal levels. The functional diagram

(Figure 5) shows the main elements of the serial RTC.

_____________________________________________________________________

7

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

Nominal

Frequency

32.768

kHz

Series Resistance

ESR

45,60**

kΩ

Load Capacitance

12.5

pF

Table 2. 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

CRYSTAL

X1

X2

GND

LOCAL GROUND PLANE (LAYER 2)

Figure 4. Layout Example

SERIAL BUS

INTERFACE

AND ADDRESS

REGISTER

OSCILLATOR

CONTROL

LOGIC

X2

"C" VERSION ONLY

SCL

SDA

512Hz

MUX/BUFFER

FT/OUT

USER BUFFER

(7 BYTES)

CLOCK AND

CALENDAR

REGISTERS

32,768Hz

1Hz

X1

POWER

CONTROL

DIVIDER AND

CALIBRATION

CIRCUIT

DS1340

Figure 5. Functional Diagram