WM8281ECSSLASHR datasheet(277/392 Pages) CIRRUS

277 / 392 page

WM8281

Rev 4.0

277

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

FOUT = (FVCO / FLLn_OUTDIV)

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

FVCO = (FREF x 3 x N.K x FLLn_FRATIO)

FREF is the input frequency, as determined by FLLn_REFCLK_DIV.

When the FLL output is selected as the SYSCLK or ASYNCCLK source, then FVCO must be exactly

294.912MHz (for 48kHz-related sample rates) or 270.9504MHz (for 44.1kHz-related sample rates).

Note that the output frequencies that do not lie on or between the frequencies quoted above cannot

be guaranteed across the full range of device operating conditions.

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

according to the desired output FOUT. The divider, FLLn_OUTDIV, must be set so that FVCO is in the

range 270MHz to 295MHz. The available divisions are integers from 2 to 7. Some typical settings of

FLLn_OUTDIV are noted in Table 102.

OUTPUT FREQUENCY FOUT

FLLn_OUTDIV

45 MHz to 52 MHz

110 (divide by 6)

67.5 MHz to 78 MHz

100 (divide by 4)

90 MHz to 104 MHz

011 (divide by 3)

135 MHz to 150 MHz

010 (divide by 2)

Table 102 Selection of FLLn_OUTDIV

The FLLn_FRATIO field selects the frequency division ratio of the FLL input. The FLLn_GAIN field is

used to optimise the FLL, according to the input frequency. As a general guide, these fields should be

selected as described in Table 103. (Note that additional guidelines also apply, as described below.)

REFERENCE

FREQUENCY FREF

FLLn_FRATIO

FLLn_GAIN

1MHz - 13.5MHz

0h (divide by 1)

4h (16x gain)

256kHz - 1MHz

1h (divide by 2)

2h (4x gain)

128kHz - 256kHz

3h (divide by 4)

0h (1x gain)

64kHz - 128kHz

7h (divide by 8)

0h (1x gain)

Less than 64kHz

Fh (divide by 16)

0h (1x gain)

Table 103 Selection of FLLn_FRATIO and FLLn_GAIN

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 FLLn_OUTDIV)

The value of N.K can then be determined as follows:

N.K = FVCO / (FLLn_FRATIO x 3 x FREF)

Note that, in the above equations:

FLLn_OUTDIV is the FOUT clock ratio.

FREF is the input frequency, after division by FLLn_REFCLK_DIV, where applicable.

FLLn_FRATIO is the FVCO

clock ratio (1, 2, 3 … 16).

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