WM8581
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PP Rev 1.0 March 2006
50
POSTSCALE_A
PLLACLK FREQUENCY
0
256fs
1
128fs
Table 43 PLL S/PDIF Receiver Mode Clock Divider Configuration
PLL CONFIGURATION EXAMPLE
Consider the situation where the oscillator clock (OSCCLK) input frequency is fixed at 12MHz and the
required PLLBCLK frequency is 12.288MHz.
1.
Calculate the f
2
, FREQMODE_B and POSTSCALE_B Values
The PLL is designed to operate with best performance when the f
2
clock is between 90 and 100MHz. The
necessary PLLBCLK frequency is 12.288MHz. Choose POSTSCALE_B and FREQMODE_B values to set
the f
2
frequency in the range of 90 to 100MHz. In this case, the default values (POSTSCALE_B = 0 and
FREQMODE_B[1:0] = 10) will configure the f
2
to PLLBCLK divider as 8 and hence will set the f
2
frequency at
98.304MHz; this value is within the 90 to 100MHz range and is hence acceptable.
•
POSTSCALE_B
=
0
•
FREQMODE_B [1:0] = 10b
•
f
2
= 98.304MHz
2.
Calculate R Value
Using the relationship: R = (f
2
÷ f
1
), the value of R can be calculated.
•
R
=
(f
2
÷ f
1
)
•
R
=
(98.304
÷ 12)
•
R
=
8.192
3.
Calculate PLLB_N Value
The value of PLLB_N is the integer (whole number) value of R, ignoring all digits to the right of the decimal
point. In this case, R is 8.192, hence PLLB_N is 8.
4.
Calculate PLL_K Value
The PLLB_K value is simply the integer value of (2
22
(R-PLLB_N)).
•
PLLB_K = integer part of (2
22
x (8.192 – 8))
•
PLLB_K = integer part of 805306.368
•
PLLB_K = 805306 (decimal) / C49BA (hex)
A number of example configurations are shown in Table 44. Many other configurations are possible; Table 44
shows only a small number of valid possibilities. As both PLLs are identical, the same configuration
procedure applies for both.