DCM question

Sep 16, 2005 4 Replies
Z

I have a question that is more theoretical than practical:



The DCM uses the feedback loop so that it can create a 360 (= 0) degree phaseshift from CLKIN to CLK0, CLK90 etc. (or that is how I understood it)



But why is this loop nessecary to create this behaviour?



I know it's a bit vague, but I didn't find to much detailed info on it...



Can someone please explain me the working of this system, or give me a link to a place on the net where I can find it?



I have the standard VirtexII-Pro manual, but it doesn't say much about the whole system itself...



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Z

Maybe that´s what you want...

formatting link

It is "only" a presentation, but very complete

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M

In case this is a homework problem, I'll pose a series of leading question to you:

What does the DCM do with the feedback path? How else could you create a zero phase shift from CLKIN to CLK0 without using feedback? For what reasons would an implementation without feedback _sometimes_ produce zero phase shift?

Marc

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J

How else would the silicon know what 360 degrees is versus 300 or 450 degrees? If you want a continuous, stable output to look like the input to the DCM, the phase shift has to be precisely a multiple of 360 degrees. That's 360 degrees for the input to the output - the two need to be compared.

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B

For a well written explanation of one of the uses of DCM, I would go to Xilinx Application note 462 XAPP462 and begin reading around page 25, Eliminating Clock Skew.

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