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I/O Subsystem
9.8.2.
External Capacitors
To maintain the RTC accuracy, the external capacitor C
and C
should be chosen to provide the manufacturer's specified load capacitance (C
2
when combined with the parasitic capacitance of the trace, socket (if used), and package. The following
equation can be used to choose the external capacitance values:
C
= [(C
load
Where:
C
load
C
, C
in1
obtained in the Intel
C
trace1
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
parasitic
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
, C
1
can be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC circuit at room
temperature. However, C
32.768 kHz.
In certain conditions, both C
values from the above equation) to obtain the closest oscillation frequency to 32.768 kHz. When C
values are smaller then the theoretical values, the RTC oscillation frequency will be higher.
The following example will illustrates the use of the practical values C
values cannot guarantee the accuracy of the RTC in low temperature condition:
Example 1:
According to a required 12-pF load capacitance of a typical crystal that is used with the ICH4-M, the
calculated values of C
°
At 0
C the frequency stability of crystal gives – 23 ppm (assumed that the circuit has 0 ppm at 25
This makes the RTC circuit oscillate at 32.767246 kHz instead of 32.768 kHz.
If the values of C
frequency at room temperature (+23 ppm) but this configuration of C
closer to 32.768 kHz at 0
Note that the temperature dependency of crystal frequency is a parabolic relationship (ppm / degree
square). The effect of changing the crystal's frequency when operating at 0
temperature) is the same when operating at 50
208
+ C
+ C
)*(C
+ C
1
in1
trace1
2
in2
= Crystal's load capacitance. This value can be obtained from Crystal's specification.
= input capacitances at RTCX1, RTCX2 balls of the ICH4-M. These values can be
in2
®
82801DBM I/O Controller Hub 4 Mobile (ICH4-M) Datasheet.
, C
= Trace length capacitances measured from Crystal terminals to RTCX1, RTCX2
trace2
C
= trace length * 2 pF/inch
trace
= Crystal's parasitic capacitance. This capacitance is created by the existence of 2
can be chosen such that C
2
can be chosen such that C
2
, C
values can be shifted away from the theoretical values (calculated
1
2
= C
is 10 pF at room temperature (25
1
2
, C
are chosen to be 6.8 pF instead of 10 pF, the RTC will oscillate at a higher
1
2
°
C. The 6.8-pF value of C1 and 2 is the practical value .
needs to be 0.047 µF and capacitor values C
3
+ C
)]/[(C
+ C
+ C
trace2
1
in1
trace1
= C
. Using the equation of C
1
2
> C
. Then C
2
1
°
C) to yield a 32.768 kHz oscillation.
°
°
C (25
C above room temperature).
®
Intel
855PM Chipset Platform Design Guide
) for the crystal
load
+ C
+ C
+ C
)] + C
2
in2
trace2
above, the value of C
load
can be trimmed to obtain the
1
, C
in the case that theoretical
1
2
/ C
makes the circuit oscillate
1
2
°
°
C (25
C below room
R
1
parasitic
, C
1
2
, C
1
2
°
C).
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