Intel 815 - Manual (Page 129)

I/O Subsystem
R
Intel
®
815 Chipset Platform Design Guide 129
10.9.7 RTC Routing Guidelines
•
All RTC OSC signals (RTCX1, RTCX2, VBIAS) should be routed with trace lengths shorter
than 1 inch. The shorter, the better.
•
Minimize the capacitance between RTCX1 and RTCX2 in the routing (optimally, there 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).
10.9.8 Guidelines to Minimize ESD Events
Guidelines to minimize ESD events that may cause loss of CMOS contents:
•
Provide a 1
µ
F 805 X5R dielectric, monolithic, ceramic capacitor on the VCCRTC pin. This
capacitor connection should not be stubbed off the trace run and should be as close as possible
to the ICH. If a stub is required, its maximum length should be a few mm. The ground
connection should be made through a via to the plane, with no trace between the capacitor pad
and the via.
•
Place the battery, the 1 k
Ω
series current limit resistor, and the common-cathode isolation
diode very close to the ICH. If this is not possible, place the common-cathode diode and the
1 k
Ω
resistor as close as possible to the 1
µ
F capacitor. Do not place these components
between the capacitor and the ICH. The battery can be placed remotely from the ICH.
•
On boards that have chassis intrusion utilizing inverters powered by the VCCRTC pin, place
the inverters as close as possible to the common-cathode diode. If this is not possible, keep
the trace run near the center of the board.
•
Keep the ICH VCCRTC trace away from the board edge. If this trace must run from opposite
ends of the board, keep the trace run towards the board center, away from the board edge
where contact could be made by those handling the board.
10.9.9 VBIAS and DC Voltage and Noise Measurements
•
Steady-state VBIAS will be a DC voltage of about 0.38V ± 0.06V.
•
VBIAS will be “kicked” when the battery is inserted, to about 0.7–1.0 V, but it will return to
its DC value within a few msec.
•
Noise on VBIAS must be kept to a minimum (200 mV or less).
•
VBIAS is very sensitive and cannot be probed directly. It can be probed through a 0.01
µ
F
capacitor.
•
Excessive noise on VBIAS can cause the ICH internal oscillator to misbehave or even stop
completely.
•
To minimize the VBIAS noise, it is necessary to implement the routing guidelines described
previously as well as the required external RTC circuitry.