Production Data
WM8912
In order to follow the above requirements for FVCO, the value of FLL_OUTDIV should be selected
according to the desired output FOUT. The divider, FLL_OUTDIV, must be set so that FVCO is in the
range 90-100MHz. The available divisions are integers from 4 to 64. Some typical settings of
FLL_OUTDIV are noted in Table 46.
OUTPUT FREQUENCY FOUT
2.8125 MHz - 3.125 MHz
3.75 MHz - 4.1667 MHz
5.625 MHz - 6.25 MHz
11.25 MHz - 12.5 MHz
18 MHz - 20 MHz
FLL_OUTDIV
011111 (divide by 32)
011000 (divide by 24)
001111 (divide by 16)
000111 (divide by 8)
000100 (divide by 5)
000011 (divide by 4)
22.5 MHz - 25 MHz
Table 46 Selection of FLL_OUTDIV
The value of FLL_FRATIO should be selected as described in Table 47.
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
64kHz - 128kHz
Less than 64kHz
Table 47 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:
F
VCO = (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
)
See Table 48 for the coding of the FLL_OUTDIV and FLL_FRATIO fields.
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 216 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 216 gives 0.192 x 65536 = 12582.912 (decimal)
Apply rounding to the nearest integer = 12583 (decimal) = 3127 (hex)
For best performance, FLL Fractional Mode should always be used. Therefore, if the calculations
yield an integer value of N.K, then it is recommended to adjust FLL_OUTDIV in order to obtain a
non-integer value of N.K. Care must always be taken to ensure that the FLL operating frequency,
F
VCO, is within its recommended limits of 90-100 MHz.
The register fields that control the FLL are described in Table 48. Example settings for a variety of
reference frequencies and output frequencies are shown in Table 50.
PD, Rev 4.0, September 2010
71
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