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121 Views
Registered: ‎06-10-2018

Vivado synthesis Strategy: Flow Perfoptimized high.

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I have developed a filter design running at 100MHz frequency constraint.But after implementation i got WNS (worst negative slack)  -0.09ns .For getting positive slack to fulfill my constraint, i used synthesis strategy option "Flow perfoptimized high ". As a result i met my constraint  but lookup tables were increased somehow from 5500 to 6500.My questions are:

What this synthesis strategy does ?

Why my lookup tables increased?

Screenshot_2018-12-04-10-39-40-09.png

 

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Moderator
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31 Views
Registered: ‎07-21-2014

Re: Vivado synthesis Strategy: Flow Perfoptimized high.

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zohaib-ramzan@311.00

Sure.

LUT combining: Suppose the design has a LUT2 and a LUT1 in the combo path and tool sees a possibility of combining them into one LUT3, with Flow Perfoptimized high tool will not perform such operations.

Fanout: Vivado uses 10K as a fanout limit for a driver/net before performing any replications. Which means if a flop drives 25K loads then with default strategy there will be total 1-2 replication of the driver, but in case of Flow Perfoptimized high the replications would be 60+.

Thanks
anusheel

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Moderator
Moderator
61 Views
Registered: ‎07-21-2014

Re: Vivado synthesis Strategy: Flow Perfoptimized high.

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zohaib-ramzan@311.00

Flow_Perfoptimized_high will not perform lut combining and also the fanout limit will be set to 400 which may also result in extra logic. 

Thanks
Anusheel

 

 

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Registered: ‎06-10-2018

Re: Vivado synthesis Strategy: Flow Perfoptimized high.

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Can you describe your point in simpel elaborative words?
I could not get what you said earlier.
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Moderator
Moderator
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Registered: ‎07-21-2014

Re: Vivado synthesis Strategy: Flow Perfoptimized high.

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zohaib-ramzan@311.00

Sure.

LUT combining: Suppose the design has a LUT2 and a LUT1 in the combo path and tool sees a possibility of combining them into one LUT3, with Flow Perfoptimized high tool will not perform such operations.

Fanout: Vivado uses 10K as a fanout limit for a driver/net before performing any replications. Which means if a flop drives 25K loads then with default strategy there will be total 1-2 replication of the driver, but in case of Flow Perfoptimized high the replications would be 60+.

Thanks
anusheel