User's Manual

Intel
®
820 Chipset Design Guide 2-69
Layout/Routing Guidelines
2.20.3 RTC Layout Considerations
•
Keep the RTC lead lengths as short as possible; around ¼ 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 VCC should be clean; use a filter, such as an RC lowpass, or a ferrite inductor.
2.20.4 RTC External Battery Connection
The RTC requires an external battery connection to maintain its functionality and its RAM while
the ICH 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.
The battery must be connected to the ICH via an isolation schottky diode circuit. The schottky
diode circuit allows the ICH RTC-well to be powered by the battery when the system power is not
available, but by the system power when it is available. To do this, the diodes are set to be reverse
biased when the system power is not available. Figure 2-57 is an example of a diode circuitry that
is used.
A standby power supply should be used in a desktop system to provide continuous power to the
RTC when available, which will significantly increase the RTC battery life and thereby increase the
RTC accuracy.
Figure 2-57. Diode Circuit Connecting RTC External Battery
VCC3_3SBY
VccRTC
1.0 uF
1 K
Ω
rtc e t bat sd