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Registered: ‎04-06-2021

why Xilinx ISE 14.7 and Vivado 2020.2 showing different resource utilization for same FPGA (artix 7 xc7a100Tlcsg324)

hello everyone,

I have designed a circuit using vhdl language. When I synthesis that design using Vivado 2020.2 and xilinx ISE 14.7

both tools showing different utilization for the same FPGA boards. 

My utilization for Artix 7 (xc7a100Tlcsg324) in Vivado 2020.2

Slice LUTS --  179

Slice Registers -- 120

My utilization for Artix 7 (xc7a100Tlcsg324) in Xilinx ISE 14.7

Slice LUTS --  291

Slice Registers -- 113


I don't why ?? Can anyone please help me to understand??



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2 Replies
Registered: ‎02-08-2021


Yes, this kind of result is expected.

ISE uses different routing algorithms compared to Vivado.

In Vivado, we can see more optimized placement & routing, hence optimized resource utilization.

Also, if the timing constraints are different for ISE & Vivado projects, the resources may vary as the tool tries to place & route the design so as to meet the timing constraint.

Registered: ‎07-09-2009

As @hpbhat says , its mainly down to them being two totally different tools.

A few key differences,

   ISE works on assumption all clocks are independent unless you say, whilst Vivado is the opposite way around, 

          Thinking about that , I might have that the wrong way round, but some one will correct us.

Both tools have different algorithms, Vivado is meant to be 100 % deterministic, whilst ISE uses a form of  artificial annealing , i.e. a bit of a randomness,

Both tools, work till they meet the timing constraints, and then stop.

    So depending how ISE and Vivado start compiling your code, they might well come to a different first answer that meets your timing constraints.

Its also a truism, that in ISE days, people used to at most put a constraint on the clock frequencies  in

    whilst in Vivado, you need to constrain the clocks and any relationships between them, so the constraints are different,

Out of interest, I have a few designs, some are bigger in Vivado than ISE, some are smaller,

    All meet constraints, so I do not worry,


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