WM8945ECSSLASHR datasheet(76/169 Pages) WOLFSON

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WM8945

Production Data

PD, May 2011, Rev 4.1

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 51.

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)

11.25 MHz – 12.5 MHz

2h (divide by 8)

22.5 MHz – 25 MHz

1h (divide by 4)

45 MHz – 50 MHz

0h (divide by 2)

Table 51 Selection of FLL_OUTDIV

The value of FLL_FRATIO should be selected as described in Table 52.

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 52 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.

In FLL Fractional Mode, the fractional portion of the N.K multiplier is held in the FLL_K register field.

This field is coded as a fixed point quantity, where the MSB has a weighting of 0.5. Note that, if

desired, the value of this field may be calculated by multiplying K by 2^16 and treating FLL_K as an

integer value, as illustrated in the following example:

If N.K = 8.192, then K = 0.192.

Multiplying K by 2^16 gives 0.192 x 65536 = 12582.912 (decimal) = 3126 (hex).

For best FLL performance, the FLL fractional mode is recommended. Therefore, if the calculations

yield an integer value of N.K, then it is recommended to adjust FLL_FRATIO in order to obtain a non-

integer value of N.K. Care must always be taken to ensure that the FLL operating frequency, FVCO, is

within its recommended limits of 90-100 MHz.

The register fields that control the FLL are described in Table 53. Example settings for a variety of

reference frequencies and output frequencies are shown in Table 54.

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