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Comp.Arch.FPGA | XST removes duplicate logic no matter what

There are 5 messages in this thread.

You are currently looking at messages 0 to 5.

XST removes duplicate logic no matter what - MM - 2009-03-31 17:10:00

I've been fighting XST not to remove duplicate
logic I put on purpose to 
decrease fanout on some nets and I can't find a set of attributes, which 
would work... I tried "keep" and "noreduce" in combination with
MAX_FANOUT, 
but XST(8.2.03i) seems to just ignore them all. Does anyone know how to 
force the damn thing to keep the duplicate logic?


Thanks,
/Mikhail
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Re: XST removes duplicate logic no matter what - gabor - 2009-03-31 17:28:00

On Mar 31, 5:10=A0pm, "MM"
<mb...@yahoo.com> wrote:
> I've been fighting XST not to remove duplicate logic I put on purpose to
> decrease fanout on some nets and I can't find a set of attributes, which
> would work... I tried "keep" and "noreduce" in combination with
MAX_FANOU=
T,
> but XST(8.2.03i) seems to just ignore them all. Does anyone know how to
> force the damn thing to keep the duplicate logic?
>
> Thanks,
> /Mikhail

If you run from the ISE GUI, set the following in the Xilinx-specific
options tab of the Synthesis properties:

Register Duplication: checked
Equivalent Register Removal: unchecked

Then in the HDL Options tab uncheck "Resource Sharing".

This should allow XST to duplicate registers as necessary to
reduce fanout as well as to push registers into IOBs if
requested.

Regards,
Gabor
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Re: XST removes duplicate logic no matter what - Mike Treseler - 2009-03-31 17:31:00

MM wrote:
> I've been fighting XST not to remove duplicate logic I put on purpose to 
> decrease fanout on some nets

There is a max_fanout constraint,
but I would just constrain Fmax
and let synthesis work out the details.

     -- Mike Treseler
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Re: XST removes duplicate logic no matter what - MM - 2009-03-31 17:38:00

"Mike Treseler"
<m...@gmail.com> wrote in message 
news:4...@gmail.com...
>
> There is a max_fanout constraint,
> but I would just constrain Fmax
> and let synthesis work out the details.

I am in the fine tuning mode. The design is big and I am trying to achieve 
closure on just a few last nets.

/Mikhail
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Re: XST removes duplicate logic no matter what - MM - 2009-03-31 17:53:00

"gabor" <g...@alacron.com> wrote
in message 
news:0...@v6g2000vbb.googlegroups.com...

>If you run from the ISE GUI, set the following in the Xilinx-specific
>options tab of the Synthesis properties:
>
>Register Duplication: checked
>Equivalent Register Removal: unchecked
>
>Then in the HDL Options tab uncheck "Resource Sharing".


Thanks. Giving this a try. It must have been Equivalent Register Removal 
setting! I wish the tools would produce meaningful warnings about the user 
constraints been overridden!

/Mikhail