WM8912 datasheet(70/129 Pages) CIRRUS

70 / 129 page

WM8912

Production Data

PD, Rev 4.1, February 2013

FREQUENCY LOCKED LOOP (FLL)

The integrated FLL can be used to generate SYSCLK from a wide variety of different reference

sources and frequencies. The FLL can use either MCLK, BCLK or LRCLK as its reference, which may

be a high frequency (eg. 12.288MHz) or low frequency (eg. 32,768kHz) reference. The FLL is tolerant

of jitter and may be used to generate a stable SYSCLK from a less stable input signal. The FLL

characteristics are summarised in “Electrical Characteristics”.

Note that the FLL can be used to generate a free-running clock in the absence of an external

reference source. This is described in the “Free-Running FLL Clock” section below.

The FLL is enabled using the FLL_ENA register bit. Note that, when changing FLL settings, it is

recommended that the digital circuit be disabled via FLL_ENA and then re-enabled after the other

recommended the FLL be reset by setting FLL_ENA to 0.

The FLL_CLK_REF_SRC field allows MCLK, BCLK or LRCLK to be selected as the input reference

clock.

The field FLL_CLK_REF_DIV provides the option to divide the input reference (MCLK, BCLK or

LRCLK) by 1, 2, 4 or 8. This field should be set to bring the reference down to 13.5MHz or below. For

best performance, it is recommended that the highest possible frequency - within the 13.5MHz limit -

should be selected.

The field FLL_CTRL_RATE controls internal functions within the FLL; it is recommended that only the

default setting be used for this parameter. FLL_GAIN controls the internal loop gain and should be set

to the recommended value quoted in Table 48.

The FLL output frequency is directly determined from FLL_FRATIO, FLL_OUTDIV and the real

number represented by FLL_N and FLL_K. The field FLL_N is an integer (LSB = 1); FLL_K is the

fractional portion of the number (MSB = 0.5). The fractional portion is only valid when enabled by the

field FLL_FRACN_ENA.

It is recommended that FLL_FRACN_ENA is enabled at all times. Power consumption in the FLL is

reduced in integer mode; however, the performance may also be reduced, with increased noise or

jitter on the output.

If low power consumption is required, then FLL settings must be chosen when N.K is an integer (ie.

FLL_K = 0). In this case, the fractional mode can be disabled by setting FLL_FRACN_ENA = 0.

For best FLL performance, a non-integer value of N.K is required. In this case, the fractional mode

must be enabled by setting FLL_FRACN_ENA = 1. The FLL settings must be adjusted, if necessary,

to produce a non-integer value of N.K.

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 48 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.

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