wm8918 Wolfson Microelectronics plc, wm8918 Datasheet - Page 100

• Manufacturer Part Number: wm8918
• Description: Ultra Low Power Dac For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

WM8918
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In order to follow the above requirements for F
according to the desired output F
range 90-100MHz. The available divisions are integers from 4 to 64. Some typical settings of
FLL_OUTDIV are noted in Table 64.
Table 64 Selection of FLL_OUTDIV
The value of FLL_FRATIO should be selected as described in Table 65.
Table 65 Selection of FLL_FRATIO
In order to determine the remaining FLL parameters, the FLL operating frequency, F
calculated, as given by the following equation:
The value of FLL_N and FLL_K can then be determined as follows:
See Table 66 for the coding of the FLL_OUTDIV and FLL_FRATIO fields.
Note that F
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
integer value, as illustrated in the following example:
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
The register fields that control the FLL are described in Table 66. Example settings for a variety of
reference frequencies and output frequencies are shown in Table 68.
VCO
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
1MHz - 13.5MHz
256kHz - 1MHz
128kHz - 256kHz
64kHz - 128kHz
Less than 64kHz
REFERENCE FREQUENCY F
, is within its recommended limits of 90-100 MHz.
OUTPUT FREQUENCY F
If N.K = 8.192, then K = 0.192
Multiplying K by 2
Apply rounding to the nearest integer = 12583 (decimal) = 3127 (hex)
F
N.K = F
REF
VCO
is the input frequency, after division by FLL_CLK_REF_DIV, where applicable.
= (F
VCO
OUT
/ (FLL_FRATIO x F
x FLL_OUTDIV)
16
gives 0.192 x 65536 = 12582.912 (decimal)
OUT
OUT
REF
. The divider, FLL_OUTDIV, must be set so that F
REF
011111 (divide by 32)
011000 (divide by 24)
001111 (divide by 16)
000111 (divide by 8)
000100 (divide by 5)
000011 (divide by 4)
0h (divide by 1)
1h (divide by 2)
2h (divide by 4)
3h (divide by 8)
4h (divide by 16)
)
VCO
, the value of FLL_OUTDIV should be selected
FLL_OUTDIV
FLL_FRATIO
PD, Rev 4.0, September 2010
16
and treating FLL_K as an
Production Data
VCO
VCO
, must be
is in the
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