WM8944BECSSLASHR datasheet(78/165 Pages) CIRRUS

78 / 165 page

WM8944B

Rev 4.3

For best FLL performance, a non-integer value of N.K is required. In this case, the fractional mode

must be enabled by setting FLL_FRAC = 1. The FLL settings must be adjusted, if necessary, to

produce a non-integer value of N.K.

The FLL output frequency is generated according to the following equation:

FOUT = (FVCO / FLL_OUTDIV)

The FLL operating frequency, FVCO is set according to the following equation:

FVCO = (FREF x N.K x FLL_FRATIO)

FREF is the input frequency, as determined by FLL_CLK_REF_DIV.

FVCO must be in the range 90-100 MHz. Frequencies outside this range cannot be supported.

Note that the output frequencies that do not lie within the ranges quoted above cannot be guaranteed

across the full range of device operating temperatures.

In order to follow the above requirements for FVCO, the value of FLL_OUTDIV should be selected

according to the desired output FOUT, as described in Table 53.

OUTPUT FREQUENCY FOUT

FLL_OUTDIV

2.8125 MHz

– 3.125 MHz

4h (divide by 32)

5.625 MHz

– 6.25 MHz

3h (divide by 16)

7.5 MHz - 8.333 MHz

7h (divide by 12)

11.25 MHz

– 12.5 MHz

2h (divide by 8)

15 MHz - 16.667 MHz

6h (divide by 6)

22.5 MHz

– 25 MHz

1h (divide by 4)

45 MHz

– 50 MHz

0h (divide by 2)

Table 53 Selection of FLL_OUTDIV

The value of FLL_FRATIO should be selected as described in Table 54.

REFERENCE FREQUENCY FREF

FLL_FRATIO

1MHz - 13.5MHz

0h (divide by 1)

256kHz - 1MHz

1h (divide by 2)

128kHz - 256kHz

2h (divide by 4)

16kHz - 128kHz

3h (divide by 8)

Less than 16kHz

4h (divide by 16)

Table 54 Selection of FLL_FRATIO

In order to determine the remaining FLL parameters, the FLL operating frequency, FVCO, must be

calculated, as given by the following equation:

FVCO = (FOUT x FLL_OUTDIV)

The value of FLL_N and FLL_K can then be determined as follows:

N.K = FVCO / (FLL_FRATIO x FREF)

Note that FREF is the input frequency, after division by FLL_CLK_REF_DIV, where applicable.

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