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9.6.3
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 (when used), and
package. The following equation may be used to choose the external capacitance values:
Where:
Cload = Crystal's load capacitance. This value may be obtained from crystal's specification.
Cin1, Cin2 = input capacitances at RTCX1, RTCX2 balls of the ICH3-S. These values may
be obtained in the Intel
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:
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 may be selected such that C1 = C2. Using the equation of Cload above, the
value of C1 and C2 may be calculated to give the best accuracy (closest to 32.768 kHz) of the RTC
circuit at room temperature. However, C2 may be chosen such that C2 > C1. Then C1 may be
trimmed to obtain 32.768 kHz.
In certain conditions, both C1 and C2 values may 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 may be higher.
The following example illustrates the use of the practical values C1 and C2 in the case that
theoretical values cannot ensure the accuracy of the RTC in a low temperature condition.
9.6.3.1
Example with Load Capacitance
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 zero ppm
at 25° C). This makes the RTC circuit oscillate at 32.767246 kHz instead of 32.768 kHz.
When the values of C1 and C2 are chosen to be 6.8 pF instead of 10 pF, this causes the RTC to
oscillate at a higher frequency at room temperature (+23 ppm). However, this configuration of C1
and 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.
Design Guide
®
®
Intel
Pentium
M Processor and Intel
Cload = [(C1 + Cin1 + Ctrace1)*(C2 + Cin2 + Ctrace2)]/[(C1 + Cin1 + Ctrace1
+ C2 + Cin2 +
Ctrace2)] + Cparasitic
®
82801CA I/O Controller Hub 3 (ICH3-S) Datasheet.
Ctrace = trace length * 2 pF / inch (dependent upon board characteristics)
®
E7501 Chipset Platform
I/O Controller Hub 3 (Intel
®
ICH3-S)
165
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