Intel 855GME User Manual

January 2007 235
Intel
®
855GME Chipset and Intel
®
6300ESB ICH Embedded Platform Design Guide
Intel
®
6300ESB Design Guidelines
9.11.2 External Capacitors
To maintain the RTC accuracy, the external capacitor C
3
needs to be 0.047 µF and capacitor values
C
1
and C
2
should be chosen to provide the manufacturer’s specified load capacitance (C
load
) for the
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:
C
load
= [(C
1
+ C
in1
+ C
trace1
)*(C
2
+ C
in2
+ C
trace2
)]/[(C
1
+ C
in1
+ C
trace1
+ C
2
+
C
in2
+ C
trace2
)] + C
parasitic
Where:
C
load
= Crystal’s load capacitance. This value may be obtained from Crystal’s
specification.
C
in1
, C
in2
= input capacitances at RTCX1, RTCX2 balls of the 6300ESB.
These values may be obtained in the ICHn’s data sheet.
C
trace1
, C
trace2
= Trace length capacitances measured from Crystal terminals
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
= trace length * 2 pF/inch
C
parasitic
= Crystal’s parasitic capacitance. This capacitance is created by the
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.
Ideally, C
1
, C
2
may be chosen such that C
1
= C
2
. Using the equation of C
load
above, the value of
C
1
, C
2
may be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC circuit at
room temperature. However, C
2
may be chosen so that C
2
> C
1
. Then C
1
may be trimmed to obtain
the 32.768 kHz.
Table 101. RTC Routing Summary
Trace
Impedance
RTC Routing
Requirements
Maximum Trace
Length To Crystal
Signal
Length
Matching
R1, R2, C1, and C2
tolerances
Signal
Referencing
55
Ω ± 10%
5 mil trace width
(results in ~2pF
per inch)
1 inch NA
R1 = 10M
Ω ± 5%
C1 = C2 = (NPO class)
See Section 9.11.2 for
calculating a specific
capacitance value for
C1 and C2.
Ground

Intel 855GME Specifications

General

Brand	Intel
Model	855GME
Category	Computer Hardware
Language	English