Cirrus Logic CS2100-CP - Manual (Page 19)

CS2100-CP
DS840F2 19
5.3.3 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 .
5.3.4 Ratio Configuration Summary
The R
UD
is the user defined ratio stored in the register space. The resolution for the R
UD
is selectable by
setting LFRatioCfg. R-Mod is applied if selected. The user defined 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 17 summarizes the fea-
tures involved in the calculation of the ratio values used to generate the fractional-N value which controls
the Frequency Synthesizer.
Figure 17. Ratio Feature Summary
Referenced Control Register Location
Ratio...................................... “Ratio (Address 06h - 09h)” on page 27
LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 29
RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 26
RefClkDiv[1:0] ....................... “Reference Clock Input Divider (RefClkDiv[1:0])” on page 28
Effective Ratio R
EFF
Ratio Format
Frequency Reference Clock
(CLK_IN)
SysClk
PLL Output
Frequency
Synthesizer
Digital PLL &
Fractional N Logic
N
Ratio
12.20
20.12
LFRatioCfg
RModSel[2:0]
Ratio
Modifier
R Correction
RefClkDiv[1:0]
Timing Reference Clock
(XTI/REF_CLK)
Divide
RefClkDiv[1:0]
User Defined Ratio R
UD