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®
Intel
855GME Chipset and Intel
Table 101.
RTC Routing Summary
Trace
Impedance
55 Ω ± 10%
9.11.2
External Capacitors
To maintain the RTC accuracy, the external capacitor C
C
and C
1
2
crystal when combined with the parasitic capacitance of the trace, socket (if used), and package.
The following equation may be used to choose the external capacitance values:
Where:
Ideally, C
C
, C
may be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC circuit at
1
2
room temperature. However, C
the 32.768 kHz.
RTC Routing
Maximum Trace
Requirements
Length To Crystal
5 mil trace width
(results in ~2pF
per inch)
should be chosen to provide the manufacturer's specified load capacitance (C
C
= [(C
+ C
load
1
in1
C
+ C
)] + C
in2
trace2
C
= Crystal's load capacitance. This value may be obtained from Crystal's
load
specification.
C
, C
= input capacitances at RTCX1, RTCX2 balls of the 6300ESB.
in1
in2
These values may be obtained in the ICHn's data sheet.
C
, C
= Trace length capacitances measured from Crystal terminals
trace1
trace2
to RTCX1, RTCX2 balls. These values depend on the characteristics of board
material, the width of signal traces and the length of the traces. A typical
value, based on a 5 mil wide trace and a ½ ounce copper pour, is
approximately equal to:
C
= trace length * 2 pF/inch
trace
C
= Crystal's parasitic capacitance. This capacitance is created by the
parasitic
exist of two electrode plates and the dielectric constant of the crystal blank
inside the Crystal part. Refer to the crystal's specification to obtain this value.
, C
may be chosen such that C
1
2
may be chosen so that C
2
January 2007
®
6300ESB ICH Embedded Platform Design Guide
Intel
Signal
Length
Matching
1 inch
NA
needs to be 0.047 µF and capacitor values
3
+ C
)*(C
+ C
+ C
trace1
2
in2
parasitic
= C
. Using the equation of C
1
2
> C
. Then C
2
1
®
6300ESB Design Guidelines
R1, R2, C1, and C2
tolerances
Referencing
R1 = 10M Ω ± 5%
C1 = C2 = (NPO class)
See
Section 9.11.2
for
calculating a specific
capacitance value for
C1 and C2.
load
)]/[(C
+ C
+ C
trace2
1
in1
trace1
above, the value of
load
may be trimmed to obtain
1
Signal
Ground
) for the
+ C
+
2
235
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