CY23FS08

Document Number: 38-07518 Rev. *F

Page 8 of 15

Figure 12. Phase Offset

XTAL Selection Criteria and Application Example

Selecting the appropriate XTAL ensures the FailSafe device is

able to span an appropriate frequency of operation. Also, the

XTAL parameters determine the holdover frequency stability.

Critical parameters are given here. Cypress recommends that

you choose:

range of pullability.

C0 is the XTAL shunt capacitance (3 pF to 7 pF typ).

C1 is the XTAL motional capacitance (10 fF to 30 fF typ).

The capacitive load as “seen” by the XTAL is across its terminals.

maximum value).These are used for calculating the pull range.

we may not want a XTAL calibrated to that load. This is because

the pullability is not linear, as represented in the equation below.

Plotting the pullability of the XTAL shows this expected behavior

as shown in Figure 13. In this example, specifying a XTAL

calibrated to 14 pF load provides a balanced ppm pullability

range around the nominal frequency.

Example:

REF

V

t

(

)

V

FBK

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