CY23FS08

Document Number: 38-07518 Rev. *F

Page 9 of 15

Calculated value of the pullability range for the XTAL with C0/C1

ratio of 200, 300, and 400 are shown in Table 4. For this

Using a XTAL that has a nominal frequency specified at load

capacitance of 14 pF, almost symmetrical pullability range is

obtained.

Next, it is important to calculate the pullability range including

error tolerances. This is the capture range of the input reference

frequency that the FailSafe device and XTAL combination can

reliably span.

Calculating the capture range involves subtracting error

tolerances as follows:

Parameter........................................................ f error (ppm)

Manufacturing frequency tolerance ...................................15

Temperature stability ..........................................................30

Aging ................................................................................... 3

Board/trace variation ........................................................... 5

Total ...................................................................................53

Example: Capture Range for XTAL with C0/C1 Ratio of 200

Negative Capture Range= –385 ppm + 53 ppm = –332 ppm

Positive Capture Range = 333 ppm – 53 ppm = +280 ppm

It is important to note that the XTAL with lower C0/C1 ratio has

wider pullability/capture range as compared to the higher

C0/C1 ratio. This helps to select the appropriate XTAL for use in

the FailSafe application.

-400

-300

-200

-100

0

100

200

300

400

6

8

10 12 14 16 18 20 22 24 26 28 30 32 34

C0/C1 = 200

C0/C1 = 300

C0/C1 = 400

Table 4. Pullability Range from XTAL with Different C0/C1

Ratio

C0/C1 Ratio

Pullability

Range

200

8

32

–385

333

300

8

32

–256

222

400

8

32

–192

166

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