3.5.4 LFXO crystal
For higher LFCLK accuracy (better than ±250 ppm) a low frequency crystal oscillator of
32.768 kHz (LFXO) shall be used. A crystal, 3.2 ×1.6 mm package, order code
with capacitors C9 (12 pF) and C10 (12 pF), 0402 package is used in the reference design.
Figure 11: LFXO
Depending on parasitic capacitance of PCB, the capacitance value of C9 and C10 shall be
calculated.
nRF52840 pin P0.00/XL1 and P0.01/XL12 are connected to module pad XL1 and XL2 re-
spectively.
The input capacitance of the pad XL1 and XL2 are 4 pF. The values of C9 and C10 can be
calculated as follows.
The load capacitance of LFXO is given by
C
l
=
C
9
l
∗
C
10
l
C
9
l
+
C
10
l
(1)
If
C
9
l
=
C
10
l
= C, then
C
l
=
C
2
(2)
whereas,
C
9 =
C
−
C
XL
1
−
C
P CB
(3)
C
10 =
C
−
C
XL
2
−
C
P CB
(4)
C
l
= Load capacitance of LFXO crystal.
C
XL
1
= Input capacitance of Pad XL1 (4pF)
C
XL
2
= Input capacitance of Pad XL2 (4pF)
C
P CB
= Parasitic capacitance of PCB
Parasitic capacitance of the PCB can vary depending on design and track length. Typical
values for parasitic capacitors range from 0.5 pF to 2 pF.
For the crystal
with load capacitance of 9 pF and parasitic capacitance of 2 pF
(for the Thetis-I Mini EV board), the value of C9 and C10 results in 12 pF, which was also
tested on the Thetis-I Mini EV board.
Thetis-I Mini Evaluation Board user manual version 1.0
© February 2021
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