CS2200-CP_09 Datasheet, PDF (13/26 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

5.2.2
CS2200-CP
Ratio Modifier (R-Mod)
The Ratio Modifier is used to internally multiply/divide the R_UD (the Ratio stored in the register space re-
mains unchanged). The available options for R_MOD are summarized in Table 1 on page 13.
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
5.2.3
Effective Ratio (R_EFF)
The Effective Ratio (R_EFF) is an internal calculation comprised of R_UD and the appropriate modifiers, as
previously described. R_EFF is calculated as follows:
R_EFF = R_UD • R_MOD
To simplify operation the device handles some of the ratio calculation functions automatically (such as
when the internal timing reference clock divider is set). For this reason, the Effective Ratio does not need
to be altered to account for internal dividers.
Ratio modifiers which would produce an overflow or truncation of R_EFF should not be used; For example
if R_UD is 1024 an R_MOD of 8 would produce an R_EFF value of 8192 which exceeds the 4096 limit of the
12.20 format. In all cases, the maximum and minimum allowable values for R_EFF are dictated by the fre-
quency limits for both the input and output clocks as shown in the “AC Electrical Characteristics” on
page 7.
DS759F1
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