( ESNUG 244 Item 3 ) ---------------------------------------------- [7/25/96]
From: "Russell Ray" <rray@msai.mea.com>
Subject: The Best Way I've Found To Synthesize ROMs Using Synopsys
John,
I've been investigating synthesizing ROMs further and have found some
interesting facts. Yes, big ROMs are not nice in Synthesis. They take a
large amount of time to sythesize and routing can be very difficult in the
dense area produced.
However, ROMs are not yet available in the process we are targeting so we
still had to press on. I found that using a PLA input for ROM code gave the
fastest and smallest design (for my library of course). The only constraint
was "set_max_area = 0". When I tried to force a timing constraint my area
blew up and the timing was never met. By only setting the area I was able
to get the smallest and fastest implementation. We also tried large "case"
statements but they always came in slower than the PLA. We also tried to
optimize the PLA terms before passing to Synopsys. This also produced larger
and slower ROMs than giving it all of the terms.
- Russell Ray
Mitsubishi Semiconductor America, Inc.
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