WM8998 Datasheet, PDF (222/307 Pages) Wolfson Microelectronics plc – High Performance Audio Hub CODEC

WM8998
The FLL operating frequency, FVCO is set according to the following equation:
FVCO = (FREF x N.K x FLLn_FRATIO)
Production Data
FREF is the input frequency, as determined by FLLn_REFCLK_DIV.
FVCO must be in the range 90MHz to 100MHz. Frequencies outside this range cannot be supported.
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 90MHz to 100MHz. The available divisions are integers from 2 to 7. Some typical settings of
FLLn_OUTDIV are noted in Table 91.
OUTPUT FREQUENCY FOUT
22.5 MHz to 26 MHz
45 MHz to 50 MHz
Table 91 Selection of FLLn_OUTDIV
FLLn_OUTDIV
100 (divide by 4)
010 (divide by 2)
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 92. (Note that additional guidelines also apply, as described below.)
REFERENCE
FREQUENCY FREF
1MHz - 13.5MHz
FLLn_FRATIO
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 92 Selection of FLLn_FRATIO and FLLn_GAIN
FLLn_GAIN
4h (16x gain)
2h (4x gain)
0h (1x gain)
0h (1x gain)
0h (1x 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 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).
If the above equations produce an integer value for N.K, then the value of FLLn_FRATIO should be
adjusted to the nearest odd number division (eg. divide by 3), and the value of N.K re-calculated. A
non-integer value of N.K is recommended for best performance of the FLL.
After the value of FLLn_FRATIO has been determined, the input frequency, FREF, must be compared
with the maximum frequency limit noted in Table 93. If the input frequency (after division by
FLLn_REFCLK_DIV) is higher than the applicable limit, then the FLLn_REFCLK_DIV division ratio
should be increased, and the value of N.K re-calculated.
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PD, October 2014, Rev 4.0
222