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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.

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.

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

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)

(N1 + 2) *9.219

rounded (=M + 2)

Actual Ratio

Percent Error

27.657

9.33

-1.23

36.876

9.25

-0.34

46.095

9.20

0.21

55.314

9.17

0.57

64.533

9.29

-0.72

73.752

9.25

-0.34

82.971

9.22

-0.03

92.19

9.20

0.21

101.409

101

9.18

0.40

110.628

111

9.25

-0.34

119.847

120

9.23

-0.13

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