WM8945
Production Data
The FLL operating frequency, FVCO is set according to the following equation:
FVCO = (FREF x N.K x FLL_FRATIO)
F
REF is the input frequency, as determined by FLL_CLK_REF_DIV.
VCO must be in the range 90-100 MHz. Frequencies outside this range cannot be supported.
F
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 51.
OUTPUT FREQUENCY FOUT
2.8125 MHz – 3.125 MHz
5.625 MHz – 6.25 MHz
FLL_OUTDIV
4h (divide by 32)
3h (divide by 16)
2h (divide by 8)
1h (divide by 4)
0h (divide by 2)
11.25 MHz – 12.5 MHz
22.5 MHz – 25 MHz
45 MHz – 50 MHz
Table 51 Selection of FLL_OUTDIV
The value of FLL_FRATIO should be selected as described in Table 52.
REFERENCE FREQUENCY FREF
1MHz – 13.5MHz
FLL_FRATIO
0h (divide by 1)
256kHz – 1MHz
1h (divide by 2)
2h (divide by 4)
3h (divide by 8)
4h (divide by 16)
128kHz – 256kHz
16kHz – 128kHz
Less than 16kHz
Table 52 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:
N.K = FVCO / (FLL_FRATIO x FREF
)
Note that FREF is the input frequency, after division by FLL_CLK_REF_DIV, where applicable.
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 2^16 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 2^16 gives 0.192 x 65536 = 12582.912 (decimal) = 3126 (hex).
For best FLL performance, the FLL fractional mode is recommended. Therefore, if the calculations
yield an integer value of N.K, then it is recommended to adjust FLL_FRATIO in order to obtain a non-
integer value of N.K. Care must always be taken to ensure that the FLL operating frequency, FVCO, is
within its recommended limits of 90-100 MHz.
The register fields that control the FLL are described in Table 53. Example settings for a variety of
reference frequencies and output frequencies are shown in Table 54.
PD, May 2011, Rev 4.1
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
