User's Manual

Layout/Routing Guidelines
2-70 Intel
®
820 Chipset Design Guide
2.20.5 RTC External RTCRST Circuit
The ICH 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 10-20 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 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 2-57) which allows the RTC well
to be powered by the battery when the system power is not available. Figure 2-56 is an example of
this circuitry that is used in conjuction with the external diode circuit.
2.20.6 RTC Routing Guidelines
•
All RTC OSC signals (RTCX1, RTCX2, VBIAS) should all be routed with trace lengths of
less than 1”, the shorter the better
•
Minimize the capacitance between RTCX1 and RTCX2 in the routing (optimal would be a
ground line between them)
•
Put a ground plane under all of the external RTC circuitry
•
Do not route any switching signals under the external components (unless on the other side of
the ground plane)
Figure 2-58. RTCRST External Circuit for the ICH RTC
VCC3_3SBY
VccRTC
1.0 uF
1K
2.2 uF
8.2K
RTCRST#
RTCRST
Circuit
Diode/
Battery
Circuit