WM8281ECSSLASHR datasheet(279/392 Pages) CIRRUS

279 / 392 page

WM8281

Rev 4.0

279

A similar procedure applies for the deriviation of the FLL Synchroniser parameters - assuming that

this function is used.

The FLLn_SYNC_FRATIO field selects the frequency division ratio of the FLL synchroniser input. The

FLLn_GAIN and FLLn_SYNC_DFSAT fields are used to optimise the FLL, according to the input

frequency. These fields should be set as described in Table 105.

Note that the FLLn_SYNC_FRATIO register coding is not the same as the FLLn_FRATIO register.

SYNCHRONISER

FREQUENCY FSYNC

FLLn_SYNC_FRATIO

FLLn_SYNC_GAIN

FLLn_SYNC_DFSAT

1MHz - 13.5MHz

0h (divide by 1)

4h (16x gain)

0 (wide bandwidth)

256kHz - 1MHz

1h (divide by 2)

2h (4x gain)

0 (wide bandwidth)

128kHz - 256kHz

2h (divide by 4)

0h (1x gain)

0 (wide bandwidth)

64kHz - 128kHz

3h (divide by 8)

0h (1x gain)

1 (narrow bandwidth)

Less than 64kHz

4h (divide by 16)

0h (1x gain)

1 (narrow bandwidth)

Table 105 Selection of FLLn_SYNC_FRATIO, FLLn_SYNC_GAIN, FLLn_SYNC_DFSAT

The FLL operating frequency, FVCO, is the same frequency calculated as described above.

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

N.K (Sync) = FVCO / (FLLn_SYNC_FRATIO x 3 x FSYNC)

Note that, in the above equations:

FSYNC is the synchroniser input frequency, after division by FLLn_SYNCCLK_DIV, where

applicable.

FLLn_SYNC_FRATIO is the FVCO clock ratio (1, 2, 4, 8 or 16).

The value of N (Sync) is held in the FLLn_SYNC_N register field.

The value of K (Sync) is determined by the ratio FLLn_SYNC_THETA / FLLn_SYNC_LAMBDA.

The FLLn_SYNC_N, FLLn_SYNC_THETA and FLLn_SYNC_LAMBDA fields are all coded as

integers (LSB = 1).

In Fractional Mode (FLLn_THETA > 0), the register fields FLLn_THETA and FLLn_LAMBDA can be

calculated as described below.

Note that an equivalent procedure is also used to derive the FLLn_SYNC_THETA and

FLLn_SYNC_LAMBDA register values from the corresponding synchroniser parameters.

Calculate GCD(FLL) using the ‘Greatest Common Denominator’ function:

GCD(FLL) = GCD(FLLn_FRATIO x FREF, FVCO / 3)

where GCD(x, y) is the greatest common denominator of x and y

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

Next, calculate FLLn_THETA and FLLn_LAMBDA using the following equations:

FLLn_THETA = ((FVCO / 3) - (FLL_N x FLLn_FRATIO x FREF)) / GCD(FLL)

FLLn_LAMBDA = (FLLn_FRATIO x FREF) / GCD(FLL)

Note that, in Fractional Mode, the values of FLLn_THETA and FLLn_LAMBDA must be co-prime (ie.

not divisible by any common integer). The calculation above ensures that the values will be co-prime.

The value of K must be a fraction less than 1 (ie. FLLn_THETA must be less than FLLn_LAMBDA).

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