CS2200-OTP Datasheet, PDF (10/22 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

5.2.2
CS2200-OTP
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 4. 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 4. External Component Requirements for Crystal Circuit
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 kΩ resistor to
GND.
5.3 Output to Input Frequency Ratio Configuration
5.3.1
User Defined Ratio (R_UD)
The User Defined Ratio, R_UD, is a 32-bit un-signed fixed-point number which determines the basis for the
desired input to output clock ratio. Up to four different ratios, Ratio_0-3, can be stored in the CS2200’s one
time programmable memory. Selection between the four ratios is achieved by the M[1:0] mode select
pins. 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 represent the fractional binary portion. The maximum multiplication
factor is approximately 4096 with a resolution of 0.954 PPM in this configuration. See “Calculating the
User Defined Ratio” on page 19 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
Parameter Definition
Ratio 0-3................................“Ratio 0 - 3” on page 17
M[1:0] ....................................“M1 and M0 Mode Pin Functionality” on page 13
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