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Old 04-21-2006, 04:46 PM
Brannon
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Default XST duplicate register option does not work

I have an application that has 300 pins on a tri-stated bus. They have
a tight offset constraint for the output time. Driving the T on the
OBUFT for all those off the same line fails time spec because the
fanout (of 300) stretches the time a little too long. In Synplicity,
you turn on the duplicate register option, and it will duplicate the
last register on the T line 300 times so that the timespec will pass.
XST is clueless. It seems to not even care that it is failing timespec;
it does no register duplication to fix failed OFFSET constraints. I had
to duplicate the register myself by declaring a bunch of them. Anyone
else seen the same issue?

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Old 04-21-2006, 07:37 PM
Gabor
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Default Re: XST duplicate register option does not work


Brannon wrote:
> I have an application that has 300 pins on a tri-stated bus. They have
> a tight offset constraint for the output time. Driving the T on the
> OBUFT for all those off the same line fails time spec because the
> fanout (of 300) stretches the time a little too long. In Synplicity,
> you turn on the duplicate register option, and it will duplicate the
> last register on the T line 300 times so that the timespec will pass.
> XST is clueless. It seems to not even care that it is failing timespec;
> it does no register duplication to fix failed OFFSET constraints. I had
> to duplicate the register myself by declaring a bunch of them. Anyone
> else seen the same issue?


I've done the same thing myself. I noticed that XST will duplicate a
register
to go into an IOB if it is also used internally (one register for
feedback
and another going out), but I haven't seen it duplicate registers to
reduce fanout. If you also have a fast clock, be aware that the signal
driving your 300 OBUFT D-FF's may have a tough time meeting the
period spec...

Good luck,
Gabor

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