ICS ICS-130, Руководство по эксплуатации

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DOC E10523 Rev.D

Interactive Circuits And Systems Ltd. 2000
46
The values of the P and Q parameters must be selected so that f
VCO
remains between 50
MHz and 150 MHz, inclusive. The value programmed to the M field programs a division
register to allow sub-multiples of the VCO frequency to be obtained at the output; the
maximum divisor possible is 128.
The Index field (I) is used to preset the VCO to an appropriate range. The value for this
field should be chosen from the following table. (Note that this table is referenced to f
VCO
rather than to the desired output frequency.)
I f
vco
(MHz)
0000 50 – 80
1000 80 - 150
If the desired VCO frequency is exactly 80 MHz, then either index value may be used
(since both limits are tested). However, the manufacturer recommends using the setting
corresponding to the higher frequency range.
6.2.5 VCO Programming Constraints
There are three primary programming constraints the user must be aware of:
1: 50MHz <= Fvco <= 150MHz
2: 1 <= P <= 127
3: 1 <= Q <= 127
The constraints have to do with the trade-offs between optimum speed with lowest noise,
VCO stability and factors affecting the loop equation.
6.2.6 Program Register Example
The following is an example of how to calculate a clock programming word:
Derive the proper programming word for 12.8 MHz clock frequency.
Since 12.8 MHz < 50 MHz, quadruple it to 51.2 MHz
Set M to 010
2
Set I to 0000
2