CY23FS04
Document #: 38-07304 Rev. *C
Page 8 of 12
Calculated value of the pullability range for the XTAL with
C0/C1 ratio of 200, 300 and 400 are shown in Table 3. For this
calculation Cl(min) = 8 pF and Cl(max)= 32 pF has been used.
Using a XTAL that has a nominal frequency specified at load
capacitance of 14 pF, almost symmetrical pullability range has
been obtained.
Next, it is important to calculate the pullability range including
error tolerances. This would be the capture range of the input
reference frequency that the FailSafe device and XTAL combi-
nation would reliably span.
Calculating the capture range involves subtracting error toler-
ances 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 will help the user in selecting the appropriate
XTAL for use in the FailSafe application.
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
Table 3. Pullability Range fro XTAL with Different C0/C1
Ratio
C0/C1 Ratio
Cload(min.) Cload(max.)
Pullability
Range
200
8.0
32.0
–385 333
300
8.0
32.0
–256 222
400
8.0
32.0
–192 166
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