CS2000-CP_15 datasheet(20/37 Pages) CIRRUS

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CS2000-CP

DS761F3

5.3.3

Ratio Modifier (R-Mod)

The Ratio Modifier is used to internally multiply/divide the currently addressed RUD (the Ratio0-3 stored in

the register space remain unchanged). The available options for RMOD 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

(REFF), 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 (REFF)

The Effective Ratio (REFF) is an internal calculation comprised of RUD and the appropriate modifiers, as

previously described. REFF is calculated as follows:

REFF = RUD  RMOD

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 REFF should not be used; For example

if RUD is 1024 an RMOD of 8 would produce an REFF value of 8192 which exceeds the 4096 limit of the

12.20 format. In all cases, the maximum and minimum allowable values for REFF 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

001

010

011

100

0.5

101

0.25

110

0.125

111

0.0625

Referenced Control

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

RSel[1:0] ...............................“Ratio Selection (RSel[1:0])” on page 29

LockClk[1:0] ..........................“Lock Clock Ratio (LockClk[1:0])” section on page 30

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