Si52131-A11A

Rev 1.0

7

2. Functional Description

2.1. Crystal Recommendations

The clock device requires a parallel resonance crystal.

2.1.1. Crystal Loading

Crystal loading is critical in achieving low ppm performance. To realize low ppm performance, use the total

Figure 1 shows a typical crystal configuration using the 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. Again, the capacitance on each side is in series with the crystal. The total capacitance

capacitive loading on both sides.

Figure 2. Crystal Loading Example

Table 4. Crystal Recommendations

Frequency

(Fund)

Cut

Loading

Load Cap

Shunt

Cap (max)

Motional

(max)

Tolerance

(max)

Stability

(max)

Aging

(max)

25 MHz

AT

Parallel

12–15 pF

5 pF

0.016 pF

35 ppm

30 ppm

5 ppm