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

5.2.3
CS2200-CP
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.
5.2.4
Ratio Configuration Summary
The R_UD is the user defined ratio stored in the register space. R-Mod is applied if selected. The user de-
fined ratio and ratio modifier make up the effective ratio R_EFF, the final calculation used to determine the
output to input clock ratio. The effective ratio is then corrected for the internal dividers. The conceptual
diagram in Figure 7 summarizes the features involved in the calculation of the ratio values used to gen-
erate the fractional-N value which controls the Frequency Synthesizer.
Timing Reference Clock
(XTI/REF_CLK)
Effective Ratio REFF
User Defined Ratio RUD
Ratio
Ratio Format
12.20
RModSel[2:0]
Ratio
Modifier
Divide
RefClkDiv[1:0]
RefClkDiv[1:0]
R Correction
SysClk
N
Frequency
Synthesizer
PLL Outpu
Figure 7. Ratio Feature Summary
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
RefClkDiv[1:0] .......................“Reference Clock Input Divider (RefClkDiv[1:0])” on page 22
DS759PP1
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