Datasheet
42.5.4 Calculating the Correct Crystal Decoupling Capacitor
In order to calculate correct load capacitor for a given crystal, use the model shown in the next figure,
which includes internal capacitors C
Ln
, external parasitic capacitance C
ELn
and external load capacitance
C
Pn
.
Figure 42-9. Crystal Circuit With Internal, External and Parasitic Capacitance
XOUT
Internal
C
EL1
C
L1
C
L2
C
P1
C
P2
C
EL2
External
XIN
Using this model the total capacitive load for the crystal can be calculated as shown in the equation
below:
tot
=
1
+
1
+
EL1
2
+
2
+
EL2
1
+
1
+
EL1
+
2
+
2
+
EL2
where C
tot
is the total load capacitance seen by the crystal, this value should be equal to the load
capacitance value found in the crystal manufacturer datasheet.
The parasitic capacitance C
ELn
can be disregarded in most applications as these are usually very small. If
accounted for, the value is dependent on the PCB material and PCB layout.
For some crystals, the internal capacitive load provided by the device itself can be enough to calculate
the total load capacitance in this case. C
ELn
and C
Pn
are both zero, C
L1
= C
L2
= C
L
, and the equation
reduces to the following:
tot
=
2
The next table shows the device equivalent internal pin capacitance.
Table 42-8. Equivalent Internal Pin Capacitance
Symbol Value Description
C
XIN32
3.05pF Equivalent internal pin capacitance
SAM D21 Family
Schematic Checklist
© 2018 Microchip Technology Inc.
Datasheet Complete
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