CY28411

Rev 1.0, November 22, 2006

Page 8 of 18

Calculating Load Capacitors

In addition to the standard external trim capacitors, trace

capacitance and pin capacitance must also be considered to

correctly calculate crystal loading. As mentioned previously,

the capacitance on each side of the crystal is in series with the

crystal. This means the total capacitance on each side of the

crystal must be twice the specified crystal load capacitance

(CL). While the capacitance on each side of the crystal is in

series with the crystal, trim capacitors (Ce1,Ce2) should be

calculated to provide equal capacitive loading on both sides.

As mentioned previously, the capacitance on each side of the

crystal is in series with the crystal. This mean the total capac-

itance on each side of the crystal must be twice the specified

load capacitance (CL). While the capacitance on each side of

the crystal is in series with the crystal, trim capacitors

(Ce1,Ce2) should be calculated to provide equal capacitance

loading on both sides.

Use the following formulas to calculate the trim capacitor

values for Ce1 and Ce2.

CL....................................................Crystal load capacitance

CLe......................................... Actual loading seen by crystal

using standard value trim capacitors

Ce..................................................... External trim capacitors

Cs .............................................. Stray capacitance (terraced)

Ci ...........................................................Internal capacitance

(lead frame, bond wires etc.)

CL....................................................Crystal load capacitance

CLe......................................... Actual loading seen by crystal

using standard value trim capacitors

Ce..................................................... External trim capacitors

Cs .............................................. Stray capacitance (terraced)

Ci ...........................................................Internal capacitance

(lead frame, bond wires etc.)

Figure 1. Crystal Capacitive Clarification

XTAL

Ce2

Ce1

Cs1

Cs2

X1

X2

Ci1

Ci2

Clock Chip

Trace

2.8pF

Trim

33pF

Pin

3 to 6p

Figure 2. Crystal Loading Example

Load Capacitance (each side)

Total Capacitance (as seen by the crystal)

Ce = 2 * CL – (Cs + Ci)

Ce1 + Cs1 + Ci1

1

+

Ce2 + Cs2 + Ci2

1

()

1

=

CLe