I/O Controller Hub 2
R
Intel
®
Pentium
®
4 Processor / Intel
®
850 Chipset Family Platform Design Guide 159
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 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 114 is an example of a diode
circuitry that is used.
Figure 114. 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 in a desktop system to provide continuous power to the
RTC when available, which will significantly increase the RTC battery life and thereby the RTC
accuracy.
9.8.5 RTC External RTCRST Circuit
The ICH2 RTC requires some additional external circuitry. The RTCRST# signal is used to reset
the RTC well. The external capacitor and the external resistor between RTCRST# and the RTC
battery (Vbat) were selected to create an RC time delay, such that RTCRST# will go high some
time after the battery voltage is valid. The RC time delay should be in the range of 10-20ms. When
RTCRST# is asserted, bit 2 (RTC_PWR_STS) in the GEN_PMCON_3 (General PM
Configuration 3) register is set to 1, and remains set until software clears it. As a result of this,
when the system boots, the BIOS knows that the RTC battery has been removed.
This RTCRST# circuit is combined with the diode circuit (Figure 114) which allows the RTC well
to be powered by the battery when the system power is not available. Figure 115 is an example of
this circuitry that is used in conjunction with the external diode circuit.
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