CY28326 Datasheet, PDF (10/22 Pages) Cypress Semiconductor – FTG for VIA PT880 Serial Chipset

CY28326
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.
Table 7. Crystal Recommendations
Frequency
(Fund)
14.31818 MHz
Cut Loading Load Cap
AT Parallel 20 pF
Drive
(max.)
0.1 mW
Shunt Cap
(max.)
5 pF
Motional
(max.)
0.016 pF
Tolerance
(max.)
50 ppm
Stability
(max.)
50 ppm
Aging
(max.)
5 ppm
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.
Figure 1. Crystal Capacitive Clarification
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.
Clock Chip
(CY28326)
Ci1
Ci2
Pin
3 to 6p
X1
Cs1
X2
Cs2
XTAL
Trace
2.8pF
Ce1
Ce2
Trim
33pF
Figure 2. Crystal Loading Example
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.
Rev 1.0, November 20, 2006
Page 10 of 22