Cirrus Logic CS2000-OTP, General Description Manual

Page: 12 / 30

CS2000-OTP
12 DS758F3
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
Figure 10 . 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

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
“Hybrid An-
alog-Digital PLL” on page 10 ). 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
yp
ica
l B
a
se
B
a
n
d
Ji
tter
(
p
se
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
------ 15 kHz +

=
12.288 MHz 0.96875 15 kHz +

=
11.919 MHz =
f
H
f
CLK_OUT
32
32
------ 15 kHz –

=
12.288 MHz 1 15 kHz +

=
12.273 MHz =
XTI XTO
40 pF 40 pF
Figure 10. External Component Requirements for Crystal Circuit