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Virtex-II DCM frequency synthesizer

I'm using Virtex-II DCM frequency synthesizer to get 52MHz clock from

16MHz. Two questions: 1) Since my input frequency is outside the minimum operating range in the low frequency mode(24MHz), I cannot use CLKFB feedback. Is there any easy way to override this restriction. 2) I tried doing this freq. conversion without CLKFB using CLKFX_DIVIDE = 4, CLKFX_MULTIPLY=13 parameters. Output is connected to CLKFX. Input is connected directly from external source. I'm measuring 64MHz on that output instead of 52MHz. Is there anything wrong with those particular parameters?

Thanks, Eugene.

Reply to
Evgeni
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"Evgeni" ??????: snipped-for-privacy@posting.google.com...

double your clock outside

it seems your parameters are not recognized by the implementation, and the default parameters (4/1) are used. check your constraints.

Reply to
Jay

Reply to
Peter Alfke

Evgeni,

No, you can not use CLKFB below 24 MHz.

The M and D value is not appearing to be set at all (default is X4).

Aust> I'm using Virtex-II DCM frequency synthesizer to get 52MHz clock from

Reply to
Austin Lesea

DCM's can be a bundle of fun if you don't handle their post-reset behavior properly. You have to check the "LOCKED" pin and, if lock is not achieved, toggle the "RST" pin.

As an example, on a working design, I hold "RST" high for about 10us and check "LOCKED" about 200us later. If lock is not achieved the process repeats. The reset tree going into the rest of the design is held in reset until the DCM's are healthy. Check the data sheets for lock time information, it isn't instantaneus.

O yeah, critical in the above is to use a "garbage" clock to time the DCM reset/monitoring function. Don't make the mistake of using a clock coming out of a DCM that is being reset or you'll go around in circles for a while. Who in their right mind would do that, right? :-(

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Martin Euredjian

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