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Old 11-22-2005, 07:19 PM
Posts: n/a
Default Disabling Xilinx clock enable usage...

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
signal in the CLB. To use the Enable, it needs to use an extra LUT to
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?


John Providenza

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