WM8912 Datasheet, PDF (71/128 Pages) Wolfson Microelectronics plc – Ultra Low Power DAC with Headphone Driver for Portable Audio Applications

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
22.5 MHz - 25 MHz
Table 46 Selection of FLL_OUTDIV
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)
The value of FLL_FRATIO should be selected as described in Table 47.
REFERENCE FREQUENCY FREF
1MHz - 13.5MHz
256kHz - 1MHz
128kHz - 256kHz
64kHz - 128kHz
Less than 64kHz
Table 47 Selection of FLL_FRATIO
FLL_FRATIO
0h (divide by 1)
1h (divide by 2)
2h (divide by 4)
3h (divide by 8)
4h (divide by 16)
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)
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,
FVCO, 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.
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PD, Rev 4.0, September 2010
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