CY23FS08
Document #: 38-07518 Rev. *A
Page 7 of 12
XTAL Selection Criteria and Application Example
Choosing the appropriate XTAL will ensure the FailSafe device
will be able to span an appropriate frequency of operation.
Also, the XTAL parameters will determine the holdover
frequency stability. Critical parameters are as follows. Our
recommendation is to choose:
• Low C0/C1 ratio (240 or less) so that the XTAL has enough
range of pullability.
• Low temperature frequency variation
• Low manufacturing frequency tolerance
• Low aging.
C0 is the XTAL shunt capacitance (3 pF – 7 pF typ.).
C1 is the XTAL motional capacitance (10 fF – 30 fF typ).
The capacitive load as “seen” by the XTAL is across its
terminals. It is named Clmin (for minimum value), and Clmax
(for maximum value).These are used for calculating the pull
range.
Please note that the Cl range “center” is approximately 20 pF,
but we may not want a XTAL calibrated to that load. This is
because the pullability is not linear, as represented in the
equation above. Plotting the pullability of the XTAL shows this
expected behavior as shown in Figure 8. In this example,
specifying a XTAL calibrated to 14 pF load provides a
balanced ppm pullability range around the nominal frequency.
Notes:
5.
The above example shows the maximum range the FailSafe internal capacitor array is capable of (0 to 48.6 pF).Cypress recommends the min./max capacitor
array values be programmed to a narrower range such as 6 pF–30 pF, or 7.5 pF–27 pF. This ensures the XTAL operates between series resonance and
anti-resonance. Please contact Cypress for choosing these range settings.
Clmin = (12 pF IC input cap + 0 pF pulling cap+ 6 pF trace cap on board)/2 = 9 pF
Clmax = (12 pF IC input cap + 48 pF pulling cap+ 6 pF trace cap on board)/2 = 33 pF
Pull Range =(fClmin–fClmax)/fClmin = ((C1)/2)[(1/(C0+Clmin))–(1/(C0+Clmax))]
Pull Range in ppm
= ((C1)/2)[(1/(C0+Clmin))–(1/(C0+Clmax))] × 106
Example:[5]
Figure 8. Frequency vs. Cload Behavior for Example XTAL
Pullability Range Vs. Cload
(Normalized to 14pF Cload)
-400.00
-300.00
-200.00
-100.00
0.00
100.00
200.00
300.00
400.00
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34
Cload (pF)
C0/C1 = 200
C0/C1 = 300
C0/C1 = 400
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