Техническая Спецификация для Atmel Xplained Pro Evaluation Kit ATSAMD20-XPRO ATSAMD20-XPRO
Модели
ATSAMD20-XPRO
614
Atmel | SMART SAM D20 [DATASHEET]
Atmel-42129K–SAM-D20_datasheet–06/2014
34.6.4 Calculating the Correct Crystal Decoupling Capacitor
In order to calculate correct load capacitor for a given crystal one can use the model shown in
internal capacitors C
Ln
, external parasitic capacitance C
ELn
and external load capacitance C
Pn
.
Figure 34-9. Crystal Circuit With Internal, External and Parasitic Capacitance
Using this model the total capacitive load for the crystal can be calculated as shown in the equation below:
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 in most applications be disregarded as these are usually very small. If accounted for
the value is dependent on the PCB material and PCB layout.
For some crystal the internal capacitive load provided by the device itself can be enough. To calculate the total load
capacitance in this case. C
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:
XOUT
In
ter
n
al
C
EL1
C
L1
C
L2
C
P1
C
P2
C
EL2
Ex
te
rn
a
l
XIN
C
tot
∑
C
L1
C
P1
C
EL1
+
+
) C
L2
C
P2
C
EL2
+
+
)
(
(
C
L1
C
P1
C
EL1
C
L2
C
P2
C
EL2
+
+
+
+
+
--------------------------------------------------------------------------------------------------------
=
C
tot
∑
C
L
2
-------
=