Intel 815 - Manual (Page 127)

I/O Subsystem
R
Intel
®
815 Chipset Platform Design Guide 127
10.9.3 RTC Layout Considerations
•
Keep the RTC lead lengths as short as possible. 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 any 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 low-pass or a ferrite inductor.
10.9.4 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 the Duracell* 2032, 2025 or 2016 (or equivalent), which 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
µ
A, the battery life will be at
least:
170,000 µ Ah / 3 µ A = 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
within 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 66 is an example of a diode circuitry that can be used.
Figure 66. Diode Circuit to Connect RTC External Battery
VCC3_3SBY
VccRTC
1.0 µF
1 k Ω
RTC_ext_batt_diode_circ
-
+
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.