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hkh80652005
Visitor
Visitor
5,428 Views
Registered: ‎07-22-2009

help, how to add a constraint that only use 3 input in 4-input LUT fpga.

fpga is Virtex II, I want all of the used LUT only use 3 out of 4 inputs. but I don't know how to give this constraint in XTS. do someone know this?thanks very much!

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bassman59
Historian
Historian
5,415 Views
Registered: ‎02-25-2008


hkh80652005 wrote:

fpga is Virtex II, I want all of the used LUT only use 3 out of 4 inputs. but I don't know how to give this constraint in XTS. do someone know this?thanks very much!


Uhhh, why?

----------------------------Yes, I do this for a living.
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hkh80652005
Visitor
Visitor
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Registered: ‎07-22-2009

thans for your reply. I want to reserve some resources for every LUT, so I want to use only 3 input for every 4-input LUT.
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drjohnsmith
Teacher
Teacher
5,396 Views
Registered: ‎07-09-2009

why would you want to reserve resource ?

 

it's no use to you, you can't sell it or anything !

 

what are your views on the action of the synthesiser ?

 

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bassman59
Historian
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5,395 Views
Registered: ‎02-25-2008


hkh80652005 wrote:
thans for your reply. I want to reserve some resources for every LUT, so I want to use only 3 input for every 4-input LUT.

This makes absolutely no sense. None at all.

 

What will likely happen if you are able to do this is that you will use more LUTs, which defeats the purpose of saving resources.

 

In fact, the Spartan 6 and Virtex 6 devices have a 6-input LUT because the wider-input LUTs are more efficient and as such can reduce the number of LUTs required by a design.

----------------------------Yes, I do this for a living.
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goran
Xilinx Employee
Xilinx Employee
5,380 Views
Registered: ‎08-06-2007

Hi,

 

There is no way to tell the synthesizer to only use 3 out of 4 inputs.

The only way you can do this is by skipping the synthesizer and manually insert LUT3 primitives for the whole design.

That is something that I don't even wish on my enemy, so just don't go there unless your design is less than 20 LUTs.

 

Göran

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