WM8941 Datasheet, PDF (65/96 Pages) Wolfson Microelectronics plc – Mono CODEC with Speaker Driver and Video Buffer

Pre Production
WM8941
If the PLL frequency is an exact integer (5,6,7,8,9,10,11,12) then FRAC_EN can be set to 0 for low
power operation.
INPUT CLOCK
(F1)
11.2896MHz
DESIRED PLL
OUTPUT (F2)
90.3168MHz
DIVISION FRACTIONAL INTEGER SDM
REQUIRED (R) DIVISION (K) DIVISION (N)
8
0
8
0
12.2880MHz
98.3040MHz
8
0
8
0
Table 49 PLL Modes of Operation (Integer N mode)
FRACTIONAL K MODE
The Fractional K bits provides K[23:0] provide finer divide resolution for the PLL frequency ratio (up
to 1/2²⁴). If these are used then FRAC_EN must be set. The relationship between the required
division X, the fractional division K[23:0] and the integer division N[3:0] is:
K = 2²⁴ ( R – N)
where 0 < (R – N) < 1 and K is rounded to the nearest whole number.
EXAMPLE:
PLL input clock (f1) is 12MHz and the required clock (SYSCLK) is 12.288MHz.
R should be chosen to ensure 5 < N < 13. There is a fixed divide by 4 in the PLL and a selectable
divider (MCLKDIV[3:0]) after the PLL which should be set to divide by 2 to meet this requirement.
Enabling the divide by 2 sets the required f2 = 4 * 2 * 12.288MHz = 98.304MHz.
R = 98.304 / 12 = 8.192
N = int R = 8
K = int (2²⁴ x (8.192 – 8)) = 3221225 = 3126E9h
So N[3:0] will be 8h and K[23:0] will be 3126E9h to produce the desired 98.304MHz clock.
The PLL performs best when f2 is around 90MHz. Its stability peaks at N=8. Some example settings
are shown in Table 56.
MCLK
(MHz)
DESIRED
OUTPUT
(MHz)
F2
(MHz)
PRESCALE
DIVIDE
12
11.2896 90.3168
1
12
12.2880 98.3040
1
13
11.2896 90.3168
1
13
12.2880 98.3040
1
14.4 11.2896 90.3168
1
14.4 12.2880 98.3040
1
19.2 11.2896 90.3168
2
19.2 12.2880 98.3040
2
19.68 11.2896 90.3168
2
19.68 12.2880 98.3040
2
19.8 11.2896 90.3168
2
19.8 12.2880 98.3040
2
24
11.2896 90.3168
2
24
12.2880 98.3040
2
26
11.2896 90.3168
2
26
12.2880 98.3040
2
27
11.2896 90.3168
2
27
12.2880 98.3040
2
Table 50 PLL Frequency Examples
POSTSCALE
R
DIVIDE
(MCLKDIV)
2
7.5264
2
8.192
2
6.947446
2
7.561846
2
6.272
2
6.826667
2
9.408
2
10.24
2
9.178537
2
9.990243
2
9.122909
2
9.929697
2
7.5264
2
8.192
2
6.947446
2
7.561846
2
6.690133
2
7.281778
N
(Hex)
7
8
6
7
6
6
9
A
9
9
9
9
7
8
6
7
6
7
K
(Hex)
86C226
3126E9
F28BD4
8FD525
45A1CA
D3A06E
6872B0
3D70A3
2DB492
FD809F
1F76F8
EE009E
86C226
3126E9
F28BD4
8FD525
BOAC93
482296
w
PP, Rev 3.3, December 2007
65