Si52147

Preliminary Rev. 0.1

7

2. Functional Description

2.1. Crystal Recommendations

The clock device requires a parallel resonance crystal. Substituting a series resonance crystal causes the clock

device to operate at the wrong frequency and violates the ppm specification. For most applications there is a

300 ppm frequency shift between series and parallel crystals due to incorrect loading.

2.1.1. Crystal Loading

Crystal loading plays a critical role in achieving low ppm performance. To realize low ppm performance, use the

total capacitance the crystal sees to calculate the appropriate capacitive loading (CL).

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. It is not true that load capacitors are in parallel with the crystal and are approximately

equal to the load capacitance of 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

on both side 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

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