CS2000-CP Datasheet, PDF (13/36 Pages) Cirrus Logic – Fractional-N Clock Synthesizer & Clock Multiplier

5. APPLICATIONS
CS2000-CP
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 CS2000 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 Figure 7.
XTI/REF_CLK
Timing Reference Clock
8
MHz
<
RefClk
<
50
75
MHz (XTI)
MHz (REF_CLK)
Timing Reference
Clock Divider
÷1
÷2
÷4
Internal Timing
Reference Clock
8 MHz < SysClk < 18.75 MHz
Fractional-N
Frequency
Synthesizer
PLL Output
RefClkDiv[1:0]
N
Figure 7. Internal Timing Reference Clock Divider
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 8 for more details.
Referenced Control
Register Location
RefClkDiv[1:0] .......................“Reference Clock Input Divider (RefClkDiv[1:0])” on page 31
5.1.2
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 8. 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 8 for the allowed crystal frequency range.
XTI
XTO
40 pF
40 pF
Figure 8. External Component Requirements for Crystal Circuit
DS761PP1
13