WM8993 datasheet(131/229 Pages) WOLFSON

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Production Data

WM8993

PD, November 2010, Rev 4.0

131

used in both Integer and Fractional Modes. The fractional portion, K, is only valid in Fractional Mode

when enabled by the field FLL_FRAC. It is recommended that FLL_FRAC is enabled at all times.

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 216 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 216 gives 0.192 x 65536 = 12582.912 (decimal)

Apply rounding to the nearest integer = 12583 (decimal) = 3127 (hex)

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 87.

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)

Table 87 Selection of FLL_OUTDIV

The value of FLL_FRATIO should be selected as described in Table 88.

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 88 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:

WM8993 Datasheet(PDF) 131 Page - Wolfson Microelectronics plc

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