Cirrus Logic CS2300-CP - Manual (Page 17)

CS2300-CP
DS843F2 17
in either a high resolution (12.20) or high multiplication (20.12) format selectable by the LFRatioCfg bit,
with 20.12 being the default.
The R
UD
for high resolution (12.20) format is encoded with 12 MSBs representing the integer binary por-
tion with the remaining 20 LSBs representing 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 29 for more information.
The R
UD
for high multiplication (20.12) format is encoded with 20 MSBs representing the integer binary
portion with the remaining 12 LSBs representing the fractional binary portion. In this configuration, the
maximum multiplication factor is approximately 1,048,575 with a resolution of 244 PPM. It is recommend-
ed that the 12.20 High-Resolution format be utilized whenever the desired ratio is less than 4096 since
the output frequency accuracy of the PLL is directly proportional to the accuracy of the timing reference
clock and the resolution of the R
UD
.
5.3.2 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 17 .
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 17 . 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.
Table 1. Ratio Modifier
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
Referenced Control Register Location
Ratio...................................... “Ratio (Address 06h - 09h)” on page 26
LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 28
RModSel[2:0] Ratio Modifier
000 1
001 2
010 4
011 8
100 0.5
101 0.25
110 0.125
111 0.0625
Referenced Control Register Location
Ratio...................................... “Ratio (Address 06h - 09h)” on page 26
RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 25