CY28326
Document #: 38-07616 Rev. *A
Page 10 of 23
Crystal Recommendations
The CY28326 requires a Parallel Resonance Crystal.
Substituting a series resonance crystal will cause the
CY28326 to operate at the wrong frequency and violate the
ppm specification. For most applications there is a 300-ppm
frequency shift between series and parallel crystals due to
incorrect loading.
Crystal Loading
Crystal loading plays a critical role in achieving low ppm perfor-
mance. To realize low ppm performance, the total capacitance
the crystal will see must be considered to calculate the appro-
priate capacitive loading (CL).The following diagram shows a
typical crystal configuration using the two trim capacitors. An
important clarification for the following discussion is that the
trim capacitors are in series with the crystal not parallel. It’s a
common misconception that load capacitors are in parallel
with the crystal and should be approximately equal to the load
capacitance of the crystal. This is not true.
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 calcu-
lated 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 2 times 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 calcu-
lated to provide equal capacitative loading on both sides.
Table 7. Crystal Recommendations
Frequency
(Fund)
Cut
Loading Load Cap
Drive
(max.)
Shunt Cap
(max.)
Motional
(max.)
Tolerance
(max.)
Stability
(max.)
Aging
(max.)
14.31818 MHz
AT
Parallel
20 pF
0.1 mW
5 pF
0.016 pF
50 ppm
50 ppm
5 ppm
Figure 1. Crystal Capacitive Clarification
XTAL
Ce2
Ce1
Cs1
Cs2
X1
X2
Ci1
Ci2
Clock Chip
(CY28326)
Trace
2.8pF
Trim
33pF
Pin
3 to 6p
Figure 2. Crystal Loading Example
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