XOSC_Q1
XOSC_Q2
XTAL
C51
C71
L
parasitic
51
71
1
C
C
1
1
C
C
=
+
+
SWRS037B – JANUARY 2006 – REVISED MARCH 2015
6.2.3
Crystal
A crystal in the frequency range of 26 to 27 MHz must be connected between the XOSC_Q1 and
XOSC_Q2 pins. The oscillator is designed for parallel mode operation of the crystal. In addition, loading
capacitors (C51 and C71) for the crystal are required. The loading capacitor values depend on the total
load capacitance, C
L
, specified for the crystal. The total load capacitance seen between the crystal
terminals should equal C
L
for the crystal to oscillate at the specified frequency.
(10)
The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. Total
parasitic capacitance is typically 2.5 pF.
The crystal oscillator circuit is shown in
. Typical component values for different values of C
L
are
given in
.
Figure 6-3. Crystal Oscillator Circuit
The crystal oscillator is amplitude regulated. This means that a high current is used to start up the
oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain
approximately 0.4 Vpp signal swing. This ensures a fast start up, and keeps the drive level to a minimum.
The ESR of the crystal should be within the specification in order to ensure a reliable start up (see
).
The initial tolerance, temperature drift, aging and load pulling should be carefully specified in order to meet
the required frequency accuracy in a certain application.
Table 6-3. Crystal Oscillator Component Values
COMPONENT
C
L
= 10 pF
C
L
= 13 pF
C
L
= 16 pF
C51
15 pF
22 pF
27 pF
C71
15 pF
22 pF
27 pF
54
Applications, Implementation, and Layout
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