Rainbow Electronics DS1687 - Manual (Page 8)

DS1685/DS1687
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OSCILLATOR STARTUP TIME
Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and
excessive capacitive loads are the major contributors to long startup times. A circuit using a crystal with
the recommended characteristics and following the recommended layout usually starts within one second.
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.
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.
RECOMMENDED LAYOUT FOR CRYSTAL