I/O Controller Hub
130
Design Guide
9.6.2
External Capacitors
To maintain the RTC accuracy, the external capacitor C3 must be 0.047 µF, and capacitor values
C1 and C2 should be chosen to provide the manufacturer's specified load capacitance (Cload) 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:
Cload = [(C1 + Cin1 + Ctrace1)*(C2 + Cin2 + Ctrace2)]/[(C1 + Cin1 + C C2 + Cin2 +
Ctrace2)] + Cparasitic
Where:
Cload = Crystal's load capacitance. This value can be obtained from crystal's specification.
Cin1, Cin2 = input capacitances at RTCX1, RTCX2 balls of the ICH3-S. These values can be
obtained in the
Intel
®
82801CA I/O Controller Hub 3 (ICH3-S) Datasheet
.
Ctrace1, Ctrace2 = Trace length capacitances measured from crystal terminals to the RTCX1
and RTCX2 balls. These values depend on the characteristics of board material, the width of
signal traces, and the length of the traces. Typical value is approximately:
Ctrace = trace length * 2 pF / inch (dependent upon board characteristics)
Cparasitic = Crystal's parasitic capacitance. This capacitance is created by the existence 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, C1 and C2 can be chosen such that C1 = C2. Using the equation of Cload above, the value
of C1 and C2 can be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC circuit
at room temperature. However, C2 can be chosen such that C2 > C1. Then C1 can be trimmed to
obtain 32.768 kHz.
In certain conditions, both C1 and C2 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 C1 and C2 values are smaller then the theoretical values, the RTC oscillation
frequency will be higher.
The following example illustrates the use of the practical values C1, C2 in the case that theoretical
values can not guarantee the accuracy of the RTC in a low temperature condition:
Example
According to a required 12 pF load capacitance of a typical crystal that is used with the ICH3-S, the
calculated values of C1 = C2 are 10 pF at room temperature (25 °C) to yield a 32.768 kHz
oscillation.
At 0 °C, the frequency stability of the 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 C1 and C2 are chosen to be 6.8 pF instead of 10 pF. This will make the RTC
oscillate at a higher frequency at room temperature (+23 ppm), but this configuration of C1 / C2
makes the circuit oscillate closer to 32.768 kHz at 0 °C. The 6.8 pF value of C1 and C2 is the
practical value.
Note that the temperature dependency of crystal frequency is a parabolic relationship (ppm / degree
squared). The effect of the changing 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).
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