Intel 820E Benutzerhandbuch

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Design 

Guide 

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Minimize the RTC lead lengths. Approximately 0.25 inch is sufficient. 

• 

Minimize the capacitance between Xin and Xout in the routing. 

• 

Put a ground plane under the XTAL components. 

• 

Do not route switching signals under the external components (unless on the other side of the 
board).  

• 

The oscillator V

CC

 should be clean. Use a filter (e.g., an RC low-pass) or a ferrite inductor. 

The RTC requires an external battery connection to maintain its functionality and its RAM while the 
ICH2 is not powered by the system. 

Example batteries are the Duracell* 2032, 2025 or 2016 (or equivalent), which provide many years of 
operation. Batteries are rated by storage capacity. The battery life can be calculated by dividing the 
capacity by the average current required. For example, if the battery storage capacity is 170 mAh 
(assumed usable) and the average current required is 3 µA, the battery life will be at least:  

170,000 µAh / 3 µA = 56,666 h = 6.4 years 

The battery voltage can affect the RTC accuracy. In general, when the battery voltage decays, the RTC 
accuracy also decreases. High accuracy can be obtained when the RTC voltage is within the range 3.0 V 
to 3.3 V. 

The battery must be connected to the ICH2 via an isolation Schottky diode circuit. The Schottky diode 
circuit allows the ICH2 RTC well to be powered by the battery when system power is unavailable, but by 
system power when it is available. For this purpose, the diodes are set to be reverse-biased when system 
power is unavailable. The following figure is an example diode circuit. 

VCC3_3SBY

VccRTC

1.0 µF

1 k

Ω

RTC_ext_batt_diode_circ

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