RTC设计和layout guideline

RTC design and layoutApplication Note 58 Crystal Considerations for Dallas Real-Time Clocks
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OVERVIEW
This application note describes crystal selection and layout techniques for connecting a 32,768Hz crystal to a real-time clock (RTC). It also provides information about oscillator circuit-design criteria, system design, and manufacturing issues.

OSCILLATOR BASICS
The oscillator used in Dallas Semiconductor RTCs is a CMOS inverter variation of a Pierce-type oscillator. Figure 1 shows a general configuration. These RTCs include integrated load capacitors (CL1 and CL2) and bias resistors. The Pierce oscillator uses a crystal operating in parallel mode. Crystals used in parallel mode are specified for a certain frequency with a specific load capacitance. The crystal is trimmed at the factory to o