CDK2000 datasheet(13/26 Pages) CIRRUS

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CS2300-OTP

DS844F1

5.4

Output to Input Frequency Ratio Configuration

5.4.1

User Defined Ratio (RUD)

The User Defined Ratio, RUD, is a 32-bit un-signed fixed-point number which determines the basis for the

desired input to output clock ratio. Up to four different ratios, Ratio0-3, can be stored in the CS2300’s one

time programmable memory. Selection between the four ratios is achieved by the M[1:0] mode select

pins. The 32-bit RUD can be expressed in either a high resolution (12.20) or high multiplication (20.12)

format selectable by the LFRatioCfg global parameter.

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 22 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.

5.4.2

Ratio Modifier (R-Mod)

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

page 13. R-Mod is enabled via the M2 pin in conjunction with the appropriate setting of the M2Config[2:0]

global parameter (see Section 5.7.2 on page 17).

Table 2. Ratio Modifier

5.4.3

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

• R-Mod

Referenced Control

Parameter Definition

Ratio 0-3................................“Ratio 0 - 3” on page 20

LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 20

M[1:0] ....................................“M1 and M0 Mode Pin Functionality” on page 16

RModSel[1:0]

R Modifier

0.5

0.25

0.125

0.0625

Referenced Control

Parameter Definition

Ratio 0-3................................“Ratio 0 - 3” on page 20

RModSel[1:0] ........................ “R-Mod Selection (RModSel[1:0])” section on page 19

M2Config[2:0]........................“M2 Pin Configuration (M2Config[2:0])” on page 21

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