Intel 810A3, Design Manual

Page: 78 / 166

Layout and Routing Guidelines
4-26 Intel
®
810A3 Chipset Design Guide
4.13.3 RTC Layout Considerations
•
Keep the XTAL lead lengths as short as possible; around 1 inch is sufficient.
•
Minimize the capacitance between RTCX1and RTCX2 in the routing.
•
Put a ground plane under the XTAL components.
•
Do not route any switching signals under the external components (unless on the other side of
the board).
•
The oscillator VCC should be clean; use a filter (e.g., an RC lowpass) or a ferrite inductor.
•
Keep high speed switching signals (e.g., PCI signals) away from VCCRTC, RTCX1, RTCX2
and VBIAS.
4.13.4 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 mA
per hour (assumed usable) and the average current required is 3 uA, the battery life will be at least:
170,000 uAhr / 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 diode circuit. The 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 4-27 is an example of a diode circuitry that can be used.
A standby power supply should be used to provide continuous power to the RTC when available,
which will significantly increase the RTC battery life and thereby the RTC accuracy.
Figure 4-27. A Diode Circuit to Connect RTC External Battery
V C C 3 _ 3 S B Y
V c c R T C
1.0uF
1 K