Cirrus Logic CS2100-CP - Manual (Page 18)

CS2100-CP
18 DS840F2
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 30 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
.
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.
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 18 .
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 19 . 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
Referenced Control Register Location
Ratio...................................... “Ratio (Address 06h - 09h)” on page 27
LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 29
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 27
RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 26