Cirrus Logic CS2200-CP - Manual (Page 11)

CS2200-CP
DS759F2 11
5. APPLICATIONS
5.1 Timing Reference Clock Input
The low jitter timing reference clock (RefClk) can be provided by either an external reference clock or an
external crystal in conjunction with the internal oscillator. In order to maintain a stable and low-jitter PLL out-
put the timing reference clock must also be stable and low-jitter; the quality of the timing reference clock
directly affects the performance of the PLL and hence the quality of the PLL output.
5.1.1 Internal Timing Reference Clock Divider
The Internal Timing Reference Clock (SysClk) has a smaller maximum frequency than what is allowed on
the XTI/REF_CLK pin. The CS2200 supports the wider external frequency range by offering an internal
divider for RefClk. The RefClkDiv[1:0] bits should be set such that SysClk, the divided RefClk, then falls
within the valid range as indicated in
“AC Electrical Characteristics” on page 7 .
It should be noted that the maximum allowable input frequency of the XTI/REF_CLK pin is dependent
upon its configuration as either a crystal connection or external clock input. See the
“AC Electrical Char-
acteristics” on page 7 for more details.
For the lowest possible output jitter, attention should be paid to the absolute frequency of the Timing Ref-
erence Clock relative to the PLL Output frequency (CLK_OUT). To minimize output jitter, the Timing Ref-
erence Clock frequency should be chosen such that f
RefClk
is at least +/-15 kHz from f
CLK_OUT
*N/32
where N is an integer. Figure 6 shows the effect of varying the RefClk frequency around f
CLK_OUT
*N/32.
It should be noted that there will be a jitter null at the zero point when N = 32 (not shown in Figure 6 ). An
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
Referenced Control Register Location
RefClkDiv[1:0] ....................... “Reference Clock Input Divider (RefClkDiv[1:0])” on page 22
Figure 5. Internal Timing Reference Clock Divider
N
Internal Timing
Reference Clock
PLL Output
Fractional-N
Frequency
Synthesizer
Timing Reference
Clock Divider
ч
1
ч
2
ч
4
XTI/REF_CLK
RefClkDiv[1:0]
8 MHz < SysClk < 14 MHz 8 MHz < RefClk <
Timing Reference Clock
50 MHz (XTI)
58 MHz (REF_CLK)
-80 -60 -40 -20 0 20 40 60 80
20
40
60
80
100
120
140
160
180
Normalized REF__CLK Frequency (kHz)
Ty
pi
c
a
l B
a
se
B
a
nd
J
it
te
r
(p
se
c)
CLK__OUT Jitter
-15 kHz +15 kHz
CLK__OUT
f *32/N
Figure 6. 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 =