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

5.1.3
CS2200-CP
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
5.2.2
Manual Ratio Modifier (R-Mod)
The manual Ratio Modifier is used to internally multiply/divide the R_UD (the Ratio stored in the register
space remains unchanged). The available options for R_MOD are summarized in Table 1 on page 12.
The R-Mod value selected by RModSel[2:0] is always used in the calculation for the Effective Ratio
(R_EFF), see “Effective Ratio (REFF)” on page 13. If R-Mod is not desired, RModSel[2:0] should be left at
its default value of ‘000’, which corresponds to an R-Mod value of 1, thereby effectively disabling the ratio
modifier.
RModSel[2:0]
000
001
010
011
100
101
110
111
Ratio Modifier
1
2
4
8
0.5
0.25
0.125
0.0625
Table 1. Ratio Modifier
Referenced Control
Register Location
Ratio......................................“Ratio (Address 06h - 09h)” on page 21
RModSel[2:0] ........................“R-Mod Selection (RModSel[2:0])” section on page 20
12
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