( ESNUG 240 Item 1 ) ---------------------------------------------- [5/9/96]
Subject: ( ESNUG 239 #7 ) It's Not Wise To Synthesize ROMs
> You should not be trying to synthesize ROM or RAM from RTL. Small register
> files, maybe, but for anything beyond a few hundred gates, you may be MUCH
> better off with a memory generated by your Silicon vendor. Many provide
> memory compilers that generate the specific configuration you want from a
> set of architechtural choices.
From: neuman@lsil.com (Darren Neuman)
John, We have had a lot of luck with synthesized ROM's. However, there are
some guidelines to follow when choosing whether to synthesize or not:
1) Do not synthesize anything which may change the ROM code. This is
particularly applicable to ucode which may be updated in a later rev.
2) Smaller ROM's synthesize better. Larger ROM's do not. The useful cutoff
point is dependant upon a number of items: routing layers, ROM contents.
3) It's good if a ROM's contents are predominantly 0's or 1's. We've had
some DSP coefficient ROM's which was 2/3 0's in the ROM code (it was
512 x 25 bits) and it was actually smaller in synthesized *gates* in
layout than in ROM form (3 layer metal)!
The code was a huge "case" statement. Not elegant, but it works.
- Darren Neuman
LSI Logic
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