WM8948ECSSLASHR datasheet(82/189 Pages) WOLFSON

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WM8948

Production Data

PD, May 2011, Rev 4.1

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 52.

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 52 Selection of FLL_OUTDIV

The value of FLL_FRATIO should be selected as described in Table 53.

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 53 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 54. Example settings for a variety of

reference frequencies and output frequencies are shown in Table 55.

WM8948ECS/R Datenblatt(PDF) 82 Page - Wolfson Microelectronics plc

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