CS2000-CP datasheet(19/36 Pages) CIRRUS

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CS2000-CP

DS761PP1

set to 10), the Frequency Range Indicator would then reflect the frequency range of the audio sample rate.

An RUD of 512 would then generate the audio oversampling clocks as shown in Table 4.

5.3.5

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:

Frequency Synthesizer (Static Ratio) Mode: REFF = RUD

• R

MOD

Hybrid PLL (Dynamic Ratio) Mode: REFF = RUD

• R

MOD

• Auto 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 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 20).

FsDetect[1:0]

Inferred Audio Sample Rate

when SysClk=12.288 MHz

Speed Mode (used for

audio converters)

Audio Oversampling

Clock

< 54.8 kHz

Single Speed

512 x

54.8 kHz to 128 kHz

Double Speed

256 x

> 128 kHz

Quad Speed

128 x

Table 4. Example Audio Oversampling Clock Generation from CLK_IN

Referenced Control

Ratio0-3.................................“Ratio 0 - 3 (Address 06h - 15h)” on page 30

RModSel[2:0] ........................ “R-Mod Selection (RModSel[2:0])” section on page 28

AutoRMod .............................“Auto R-Modifier Enable (AutoRMod)” on page 29

Referenced Control

RSel[1:0] ...............................“Ratio Selection (RSel[1:0])” on page 28

LockClk[1:0] .......................... “Lock Clock Ratio (LockClk[1:0])” section on page 29

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