CS2200-CP_09 Datasheet, PDF (12/26 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

5.1.2
CS2200-CP
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 7. 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.
XTI
XTO
40 pF
40 pF
Figure 7. External Component Requirements for Crystal Circuit
5.1.3
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 pulled low through a 47 kΩ resistor to
GND.
5.2 Output to Input Frequency Ratio Configuration
5.2.1
User Defined Ratio (R_UD)
The User Defined Ratio, R_UD, is a 32-bit un-signed fixed-point number, stored in the Ratio register set,
which determines the basis for the desired input to output clock ratio. The 32-bit R_UD is represented in a
12.20 format where the 12 MSBs represent the integer binary portion while the remaining 20 LSBs repre-
sent the fractional binary portion. The maximum multiplication factor is approximately 4096 with a resolu-
tion of 0.954 PPM in this configuration. See “Calculating the User Defined Ratio” on page 23 for more
information.
The status of internal dividers, such as the internal timing reference clock divider, are automatically taken
into account. Therefore R_UD is simply the desired ratio of the output to input clock frequencies.
Referenced Control
Register Location
Ratio......................................“Ratio (Address 06h - 09h)” on page 21
12
DS759F1