External Capacitors; Rtc Layout Considerations; Rtc External Battery Connection - Intel Pentium 4 Design Manual

I/O Controller Hub 2
9.8.2

External Capacitors

To maintain the RTC accuracy, the external capacitor C1 needs to be 0.047 µF, and the external
capacitor values (C2 and C3) should be chosen to provide the manufacturer's specified load
capacitance (Cload) for the crystal when combined with the parasitic capacitance of the trace,
socket (if used), and package. When the external capacitor values are combined with the
capacitance of the trace, socket, and package, the closer the capacitor value can be matched to the
actual load capacitance of the crystal used, the more accurate the RTC will be.
Equation 9-1 can be used to choose the external capacitance values (C2 and C3):
Equation 9-1. External Capacitance Values
C = (C
LOAD
Note: C3 can be chosen such that C3 > C2. Then C2 can be trimmed to obtain the 32.768 kHz.
9.8.3

RTC Layout Considerations

• Keep the RTC lead lengths as short as possible; around ¼ inch is sufficient.
• Minimize the capacitance between X
• 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 VCC should be clean; use a filter, such as a RC lowpass, or a ferrite inductor.
9.8.4

RTC External Battery Connection

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: Duracell* 2032, 2025, or 2016 (or equivalent), which can give 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 uA, the battery life will be at
least:
170,000 uAh / 3 uA = 56,666 h = 6.4 years
The voltage of the battery 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
in the range of 3.0v to 3.3v.
158
* C )/(C +C ) + C
2 3 2 3 PARASITIC
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Intel Pentium 4 Processor / Intel
and X in the routing.
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850 Chipset Family Platform Design Guide
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