CS2100-CP_09 datasheet(18/32 Pages) CIRRUS

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CS2100-CP

DS840F1

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 RUD 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 RUD 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 RUD.

The status of internal dividers, such as the internal timing reference clock divider, are automatically taken

into account. Therefore RUD 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 RUD (the Ratio stored in the register space re-

mains unchanged). The available options for RMOD 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

(REFF), 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

Ratio...................................... “Ratio (Address 06h - 09h)” on page 27

LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 29

RModSel[2:0]

Ratio Modifier

000

001

010

011

100

0.5

101

0.25

110

0.125

111

0.0625

Referenced Control

Ratio...................................... “Ratio (Address 06h - 09h)” on page 27

RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 26

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