Cirrus Logic CS2000-CP - Manual (Page 20)

CS2000-CP
20 DS761F2
5.3.3 Ratio Modifier (R-Mod)
The Ratio Modifier is used to internally multiply/divide the currently addressed R
UD
(the Ratio
0-3
stored in
the register space remain unchanged). The available options for R
MOD
are summarized in Table 1 on
page 20 .
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 20 . 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
5.3.4 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 8 .
Selection of the user defined ratio from the four stored ratios is made by using the RSel[1:0] bits unless
auto clock switching is enabled in which case the LockClk[1:0] bits also select the ratio (see
“Manual Frac-
tional-N Source Selection for the Frequency Synthesizer” on page 21 ).
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
Ratio0-3................................. “Ratio 0 - 3 (Address 06h - 15h)” on page 31
RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 29
Referenced Control Register Location
RSel[1:0] ............................... “Ratio Selection (RSel[1:0])” on page 29
LockClk[1:0] .......................... “Lock Clock Ratio (LockClk[1:0])” section on page 30