DS1685EN-5/T&R【PDF 32/34 页】IC RTC 5V 64BIT Y2K IND 24TSSOP

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DS1685/DS1687 3V/5V Real-Time Clocks

7 of 34

Table 1. Crystal Specifications*

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

Nominal Frequency

32.768

kHz

Series Resistance

ESR

Load Capacitance

6, 12.5

*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 additional specifications.

CLOCK ACCURACY

The accuracy of the clock is dependent on the accuracy of the crystal and the accuracy of the match between the

capacitive load of the oscillator circuit and the capacitive 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 3 shows a typical PC board layout for isolation of the crystal and

oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks for

detailed information.

The DS1685 can also be driven by an external 32.768 kHz oscillator. In this configuration, the X1 pin is connected

to the external oscillator signal and the X2 pin is floated. Refer to Application Note 58: Crystal Considerations with

Dallas Real-Time Clocks for detailed information about crystal selection and crystal layout.

Figure 2. Oscillator Circuit Showing Internal Bias Network

Figure 3. Typical Crystal Layout

Countdown

Chain

RTC

Crystal

CL1

CL2

RTC

Registers

Local ground plane (Layer 2)

crystal

GND

NOTE: AVOID ROUTING SIGNAL LINES IN

THE CROSSHATCHED AREA (UPPER

LEFT QUADRANT) OF THE PACKAGE

UNLESS THERE IS A GROUND PLANE

BETWEEN THE SIGNAL LINE AND THE

DEVICE PACKAGE.

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