FPGA Central

Hi all,

Lets assume I'm using a Xilinx Virtex device and I have a VHDL design
that includes the following

a<=b+c;

Will the design tools (I happen to be using Foundation 2.1i) infer a
"simple" adder or will the tools automatically infer an adder that uses
the dedicated carry look ahead logic?? Will that logic be placed
appropriately (i.e. like the ACC and ADD standard components that use
the RLOC constraint)?

Thanks for your help.

Hi Kload!


> Lets assume I'm using a Xilinx Virtex device and I have a VHDL design
> that includes the following
>
> a<=b+c;
>
> Will the design tools (I happen to be using Foundation 2.1i) infer a
> "simple" adder or will the tools automatically infer an adder that uses
> the dedicated carry look ahead logic??

With no synthesis costraints: carry-ribble-adder, because it's the smallest.

With speed-constraints: Depending on the synthesis tool and target
library. Often Carry-Lookahead.

With something like "synthesis pragmas" (supported by Synopsys) you can
manually choose the type of adder.



Ralf

Well, if you have the tools, why don't you have a look at the results?
Just use the Floorplanner or the FPGA Editor.

If nothing helps, try a more recent version. You may wish to have a look
at the xilinx home page.

BR Chris

Kload wrote:
> Hi all,
>
> Lets assume I'm using a Xilinx Virtex device and I have a VHDL design
> that includes the following
>
> a<=b+c;
>
> Will the design tools (I happen to be using Foundation 2.1i) infer a
> "simple" adder or will the tools automatically infer an adder that uses
> the dedicated carry look ahead logic?? Will that logic be placed
> appropriately (i.e. like the ACC and ADD standard components that use
> the RLOC constraint)?
>
> Thanks for your help.
>