DS1302 Trickle-Charge Timekeeping Chip

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OSCILLATOR CIRCUIT

The DS1302 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or

capacitors to operate. Table 1 specifies several crystal parameters for the external crystal. Figure 2 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 accuracy of the clock is dependent upon 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 will be added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the

oscillator circuit may result in the clock running fast. Figure 3 shows a typical PC board layout for isolating the

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

for detailed information.

Table 1. Crystal Specifications*

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

Nominal Frequency

32.768

kHz

Series Resistance

ESR

45

k

Ω

Load Capacitance

6

pF

*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.

Figure 2. Oscillator Circuit Showing Internal Bias Network

Figure 3. Typical PC Board Layout for Crystal

COUNTDOWN

CHAIN

RTC

X1

X2

CRYSTAL

C

C

RTC

REGISTERS

LOCAL GROUND PLANE (LAYER 2)

CRYSTAL

X1

X2

GND

NOTE: AVOID ROUTING SIGNALS IN THE

CROSSHATCHED AREA (UPPER LEFT-

HAND QUADRANT) OF THE PACKAGE

UNLESS THERE IS A GROUND PLANE

BETWEEN THE SIGNAL LINE AND THE

PACKAGE.