ICS5342 Datasheet, PDF (19/36 Pages) Integrated Circuit Systems – 16-Bit Integrated Clock-LUT-DAC

ICS5342
GENDAC
out numbers with a calculator will produce a workable
combination. Multiplying possible values of (N1+2) by
the desired ratio will indicate approximately the value of
M. This method is shown in the example below. A pro-
gram could be written to try all possible combinations of
(M+2) and (N1+2) (3937 possible combinations). Dis-
card those outside the error band, and select from those
remaining by giving preference to ratios which use lower
values of (M+2). Lower values of (M+2) and (N1+2)
provide better noise rejection in the phase locked loop.
C. Setting (N1+2) = 3,4, ...12, 13 and performing some
simple calculations yields the following table: (Note that
N1 cannot be 0).
The ratio 83/9 is closest. Thus:
(N2+2) = 9
N2=7
(M+2) = 83
M = 81
Example: Suppose you have a 14.318 MHz reference crystal
and want an output frequency of 66 MHz. You want to limit
the VCO frequency to 240 Mhz and have an error of no great-
er than 0.5%. What are the values of the PLL data registers?
A. 66*8 = 528 > 250 — VCO speed too high
66*4 = 264 > 250 — VCO speed too high
66*2 = 132 < 250 — VCO speed OK, N2 = 2, N2 code =
01 from the Post Divide Code table in the PLL Data
Registers section.
B. 132/14.31818 = 9.219 This is the desired frequency mul-
tiplication ratio.
The M-byte PLL parameter word is simply 81 in binary, plus
bit 7 (which must be set to 0), or 01010001. The N-byte PLL
parameter word is N2 code (01) concatenated with 5 bits of
N2 in binary (00111), or 00100111. Once again, bit 7 must be
zero.
The combination with the least frequency error was chosen,
but several other combinations are within the 0.5% tolerance.
Because the lowest value of (M+2) offers the best damping,
the 37/4 combination will have the best power supply rejec-
tion. This results in lower jitter due to external noise.
Example Calculation of PLL Data Register Values
(N1 + 2)
3
4
5
6
7
8
9
10
11
12
13
(N1 + 2) *9.219
27.657
36.876
46.095
55.314
64.533
73.752
82.971
92.19
101.409
110.628
119.847
rounded (=M + 2)
28
37
46
55
65
74
83
92
101
111
120
Actual Ratio
9.33
9.25
9.20
9.17
9.29
9.25
9.22
9.20
9.18
9.25
9.23
Percent Error
-1.23
-0.34
0.21
0.57
-0.72
-0.34
-0.03
0.21
0.40
-0.34
-0.13
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