WM8904ECSSLASHR datasheet(109/188 Pages) WOLFSON

109 / 188 page

Pre-Production

WM8904

PP, Rev 3.3, September 2012

109

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)

See Table 71 for the coding of the FLL_OUTDIV and FLL_FRATIO fields.

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. The divider, FLL_OUTDIV, must be set so that FVCO is in the

range 90-100MHz. The available divisions are integers from 4 to 64. Some typical settings of

FLL_OUTDIV are noted in Table 69.

OUTPUT FREQUENCY FOUT

FLL_OUTDIV

2.8125 MHz - 3.125 MHz

011111 (divide by 32)

3.75 MHz - 4.1667 MHz

011000 (divide by 24)

5.625 MHz - 6.25 MHz

001111 (divide by 16)

11.25 MHz - 12.5 MHz

000111 (divide by 8)

18 MHz - 20 MHz

000100 (divide by 5)

22.5 MHz - 25 MHz

000011 (divide by 4)

Table 69 Selection of FLL_OUTDIV

The value of FLL_FRATIO should be selected as described in Table 70.

REFERENCE FREQUENCY FREF

FLL_FRATIO

1MHz - 13.5MHz

0h (divide by 1)

256kHz - 1MHz

1h (divide by 2)

128kHz - 256kHz

2h (divide by 4)

64kHz - 128kHz

3h (divide by 8)

Less than 64kHz

4h (divide by 16)

Table 70 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)

See Table 71 for the coding of the FLL_OUTDIV and FLL_FRATIO fields.

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

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