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CS2300-CP

18 DS843F2

Ratio modifiers which would produce an overflow or truncation of R

EFF

should not be used; For example

if R

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

is the user defined ratio stored in the register space. The resolution for the R

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 conceptual

diagram in Figure 15 summarizes the features involved in the calculation of the ratio values used to gen-

erate the fractional-N value which controls the Frequency Synthesizer.

Figure 15. Ratio Feature Summary

Referenced Control Register Location

Ratio......................................“Ratio (Address 06h - 09h)” on page 26

LFRatioCfg............................“Low-Frequency Ratio Configuration (LFRatioCfg)” on page 28

RModSel[2:0] ........................“R-Mod Selection (RModSel[2:0])” section on page 25

Effective Ratio R

EFF

Ratio Format

Frequency Reference Clock

(CLK_IN)

PLL Output

Frequency

Synthesizer

Digital PLL &

Fractional N Logic

Ratio

12.20

20.12

LFRatioCfg

RModSel[2:0]

Ratio

Modifier

LCO

User Defined Ratio R