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Old 11-22-2005, 07:26 PM
Posts: n/a
Default Re: Disabling Xilinx clock enable usage...

johnp wrote:
> I'm working on a high speed design in a Xilinx V2Pro and I'm running
> into a timing
> problem. Instead of packing logic into LUTs, XST wants to use the
> Enable
> signal in the CLB. To use the Enable, it needs to use an extra LUT to
> create
> the Enable signal, so I get routing delays and an extra CLB delay.
> Here's some sample code:
> req [3:0] sig4;
> wire [3:0] sig3;
> always @(posedge clk)
> if (sig1 & ~sig2)
> sig4 <= sig3;
> Xilinx could fit this into 4 CLBs total by simply using the 4 LUTs and
> the 4 flip-flops.
> Each LUT would handle one bit of sig4.
> Instead, XST uses a LUT to create (sig1 & ~sig2), then feeds that
> output to the
> Enable pins on 4 flip-flops. I now get the delay through the LUT and
> routing delays
> to my flip-flops.
> Any way to tell XST to not use the Enable signal and force it to use
> the LUTs for
> this section of logic?

Your coding style exactly matches the template for clock enable

always @(posedge clk)
if (condition)
sig4 <= sig3;

You can work around this by coding the 'no change' action as an
explicit feedback, as follows:

always @(posedge clk)
sig4 <= (sig1 & ~sig2) ? sig3 : sig4;

(There really should be an attribute to support this.)


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