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8 page 
DS1685/DS1687 3V/5V Real-Time Clocks 8 of 39 Table 1. Crystal Specifications* PARAMETER SYMBOL MIN TYP MAX UNITS Nominal Frequency fO 32.768 kHz Series Resistance ESR 50 kΩ Load Capacitance CL 6, 12.5 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. 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 X1 X2 Crystal C L1C L2 RTC Registers Local ground plane (Layer 2) crystal X1 X2 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. |
