SI52143 Datasheet, PDF (7/21 Pages) Silicon Laboratories – PCI-EXPRESS GEN 1, GEN 2, & GEN 3 QUAD OUTPUT CLOCK GENERATOR WITH 25 MHZ REFERENCE CLOCK

Si52143
2. Functional Description
2.1. Crystal Recommendations
If using crystal input, the device requires a parallel resonance 25 MHz crystal.
Table 4. Crystal Recommendations
Frequency
(Fund)
25 MHz
Cut Loading Load Cap Shunt Motional Tolerance
Cap (max) (max)
(max)
AT Parallel 12–15 pF 5 pF
0.016 pF 35 ppm
Stability
(max)
30 ppm
Aging
(max)
5 ppm
2.1.1. Crystal Loading
Crystal loading is critical for ppm accuracy. In order to achieve low/zero ppm error, use the calculations below in
section 2.1.2 to estimate the appropriate capacitive loading (CL).
Figure 1 shows a typical crystal configuration using two trim capacitors. It is important that the trim capacitors are in
series with the crystal.
Figure 1. Crystal Capacitive Clarification
2.1.2. Calculating Load Capacitors
In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate
the crystal loading correctly. The capacitance on each side is in series with the crystal. The total capacitance on
both sides is twice the specified crystal load capacitance (CL). Trim capacitors are calculated to provide equal
capacitive loading on both sides.
Figure 2. Crystal Loading Example
Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2.
Rev 1.3
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