Figure 9. ref_clk frequency vs. a fixed clk_out, 2 crystal connections (xti and xto), 3 external reference clock (ref_clk) – Cirrus Logic CS2000-OTP User Manual
Page 12: 3 frequency reference clock input, clk_in, Figure 9, Cs2000-otp
CS2000-OTP
12
DS758F2
example of how to determine the range of RefClk frequencies around 12 MHz to be used in order to
achieve the lowest jitter PLL output at a frequency of 12.288 MHz is as follows:
where:
and
5.2.2
Crystal Connections (XTI and XTO)
An external crystal may be used to generate RefClk. To accomplish this, a 20 pF fundamental mode par-
allel resonant crystal must be connected between the XTI and XTO pins as shown in
. As shown,
nothing other than the crystal and its load capacitors should be connected to XTI and XTO. Please refer
to the
“AC Electrical Characteristics” on page 7
for the allowed crystal frequency range.
5.2.3
External Reference Clock (REF_CLK)
For operation with an externally generated REF_CLK signal, XTI/REF_CLK should be connected to the
reference clock source and XTO should be left unconnected or terminated through a 47 k
Ω resistor to
GND.
5.3
Frequency Reference Clock Input, CLK_IN
The frequency reference clock input (CLK_IN) is used in Hybrid PLL Mode by the Digital PLL and Fractional-
N Logic block to dynamically generate a fractional-N value for the Frequency Synthesizer (see
). The Digital PLL first compares the CLK_IN frequency to the PLL output. The
Fractional-N logic block then translates the desired ratio based off of CLK_IN to one based off of the internal
timing reference clock (SysClk). This allows the low-jitter timing reference clock to be used as the clock
Referenced Control
Parameter Definition
RefClkDiv[1:0] .......................
“Reference Clock Input Divider (RefClkDiv[1:0])” on page 24
-80
-60
-40
-20
0
20
40
60
80
20
40
60
80
100
120
140
160
180
Normalized REF__CLK Frequency (kHz)
T
y
p
ica
l B
a
s
e
B
a
n
d
Ji
tt
er
(
p
s
e
c
)
CLK__OUT Jitter
-15 kHz
+15 kHz
CLK__OUT
f
*32/N
Figure 9. REF_CLK Frequency vs. a Fixed CLK_OUT
f
L
f
RefClk
f
H
≤
≤
f
L
f
CLK_OUT
31
32
------
15kHz
+
×
=
12.288MHz
0.96875
15kHz
+
×
=
11.919MHz
=
f
H
f
CLK_OUT
32
32
------
15kHz
–
×
=
12.288MHz
1
15kHz
+
×
=
12.273MHz
=
XTI
XTO
40 pF
40 pF
Figure 10. External Component Requirements for Crystal Circuit