Intel CHIPSET 820E - Manual (Page 99)

Intel ® 820E Chipset
R
Design Guide 99
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.
2.19.5. RTC External RTCRST Circuit
The ICH2 RTC requires 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 (V
BAT
)
were selected to create an RC time delay, such that RTCRST# will go high some time after the battery
voltage becomes valid. The RC time delay should be within the range 10 ms–20 ms. 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 cleared by software. As a result, when the system boots, the BIOS knows that the
RTC battery has been removed.
Figure 61. RTCRST External Circuit for ICH2 RTC
VCC3_3SBY
VccRTC
1.0 µF
1 k Ω
2.2 µF
8.2 k Ω
RTCRST#
RTCRST
Circuit
Diode/
Battery
Circuit
rtc_rtcrst_ich
This RTCRST# circuit is combined with the diode circuit (Figure 60. Diode Circuit Connecting RTC
External Battery), which allows the RTC well to be powered by the battery when system power is
unavailable. Figure 59 is an example of the circuit used in conjunction with the external diode circuit.