CL
XA
=
CL
XB
= (2 ×
CL
VAR
) = 2 × 4 . 85
pF
= 9 . 7
pF
Equation 2
Substituting Equation 1 for CL
VAR
into Equation 2 will solve for CL
XA
/CL
XB
in single equation form:
CL
XA
=
CL
XB
= 2 × (
Crystal
CL
-
CL
int
-
CL
ext
)
Equation 3
Equation 3 can now be used to determine the CL
XA
and CL
XB
values needed to set internal CL
XA
and CL
XB
register values. The range
of CL
XA
and CL
XB
values the Si5332 can support is 0 to 38.395 pF. CL
XA
and CL
XB
values less than 0 cannot be implemented and
any values greater than 38.395 pF cannot be implemented using internal capacitors alone. Remember, CL
XA
and CL
XB
are not derived
simply from the crystal’s CL because internal and external stray capacitance must be factored into the calculations.
Once the required CL
XA
and CL
XB
have been determined using Equation 3, use the following set of formulas to calculate the required
register values to implement the desired CL
XA
and CL
XB
.
If (CL
XA/XB
< 30.555 pF, then:
• Register xosc_cint_ena = 0
• Register xosc_ctrim_xin = Round to nearest integer (CL
XA
/ 0.485)
• Register xosc_ctrim_xout = Round to nearest integer (CL
XB
/ 0.485)
If (30.555 pF < CL
XA/XB
< 38.395 pF, then:
• • Register xosc_cint_ena = 1
• Register xosc_ctrim_xin = Round to nearest integer ((CL
XA
- 7.84) / 0.485)
• Register xosc_ctrim_xout = Round to nearest integer (CL
XB
- 7.84) / 0.485)
To summarize, use Equation 3 to calculate CL
XA
/CL
XB
, then use the above set of formulas to calculate register values to implement
CL
XA
/CL
XB
in the Si5332. Note that external loading capacitors can be used to supplement the internal loading capacitors if necessary.
Note:
Your unique PCB assembly’s stray capacitance value plays a role in determining correct internal capacitor settings and,
consequently, the crystal’s frequency of oscillation. Small differences in actual board stray capacitance value from the value you use in
the above equations will result in the crystal oscillating slightly off-frequency. Significant capacitance differences can result in significant
frequency error.
Si5332-AM1/2/3 Automotive Grade Device Reference Manual • Input Clocks
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.3 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • July 26, 2021
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