I/O Subsystem
R
208
Intel
®
855PM Chipset Platform Design Guide
9.8.2. 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 can 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 can be obtained from Crystal’s specification.
C
in1
, C
in2
= input capacitances at RTCX1, RTCX2 balls of the ICH4-M. These values can be
obtained in the
Intel
®
82801DBM I/O Controller Hub 4 Mobile (ICH4-M) Datasheet
.
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 existence of 2
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
can be chosen such that C
1
= C
2
. Using the equation of C
load
above, the value of C
1
, C
2
can be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC circuit at room
temperature. However, C
2
can be chosen such that C
2
> C
1
. Then C
1
can be trimmed to obtain the
32.768 kHz.
In certain conditions, both C
1
, C
2
values can be shifted away from the
theoretical values
(calculated
values from the above equation) to obtain the closest oscillation frequency to 32.768 kHz. When C
1
, C
2
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
1
, C
2
in the case that theoretical
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
1
= C
2
is 10 pF at room temperature (25
°
C) to yield a 32.768 kHz oscillation.
At 0
°
C the frequency stability of crystal gives – 23 ppm (assumed that the circuit has 0 ppm at 25
°
C).
This makes the RTC circuit oscillate at 32.767246 kHz instead of 32.768 kHz.
If the values of C
1
, C
2
are chosen to be 6.8 pF instead of 10 pF, the RTC will oscillate at a higher
frequency at room temperature (+23 ppm) but this configuration of C
1
/ C
2
makes the circuit oscillate
closer to 32.768 kHz at 0
°
C. The 6.8-pF value of C1 and 2 is the
practical value
.
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
°
C (25
°
C below room
temperature) is the same when operating at 50
°
C (25
°
C above room temperature).
