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Other CAD Tools >> Physical Verification, Extraction and Analysis >> Assura lvs problem under linux
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Message started by stephen on Feb 13^th, 2005, 11:47am

Title: Assura lvs problem under linux
Post by stephen on Feb 13^th, 2005, 11:47am

I often parameter mismatches when doing LVS with Assura under Linux (Fedora core 2). It keeps reporting parameter mismatch errors of a very tiny percent (like 1e-7 percent). I don't think it's a problem of my layout. Instead, it should be the problem of Assura handling floating-point number incorrectly under linux.

Any ideas?
Thanks.

Title: Re: Assura lvs problem under linux
Post by Andrew Beckett on Feb 13^th, 2005, 9:34pm

Most rule decks would do LVS comparison with a tolerance to avoid this sort of thing (not just for Assura, but for all physical verification tools). Floating point rounding errors are inevitable with IEEE floating point arithmetic, and so doing exact comparison of calculations done from different source data is asking for trouble.

Andrew.

Title: Re: Assura lvs problem under linux
Post by stephen on Feb 14^th, 2005, 8:39am

I see your point. But the same version of cadence works all right under unix without this kind of error. Also we have never met this kind of error before in Diva.

Andrew Beckett wrote on Feb 13^th, 2005, 9:34pm:

Title: Re: Assura lvs problem under linux
Post by Andrew Beckett on Feb 14^th, 2005, 9:12am

Either way, I wouldn't consider that a tool bug. There are quite likely to be differences in floating point calculations between different hardware platforms. By luck you've not hit this problem before (probably because all the testing was done on another platform), but it is dangerous to rely on exact matching of floating point numbers, IMHO.

I've not seen any reports of similar problems with Assura, so I doubt it's a tool bug.

Regards,

Andrew.

Title: Re: Assura lvs problem under linux
Post by stephen on Feb 14^th, 2005, 9:43am

I guess the right way to do is to modify the rule to add a little tolerance. I checked the rule and it seems I have to modify many lines, like resistance tolerance, mos tolerance, capacitor tolerance, etc. Is there any general tolerance which controls all the devices?

Andrew Beckett wrote on Feb 14^th, 2005, 9:12am:

Title: Re: Assura lvs problem under linux
Post by Paul on Feb 15^th, 2005, 11:39pm

Stephen,

usually resistor and capacitor tolerances are not set to 0, because the schematic tool and the simulator have a more elaborate way of calculating the value (like taking into account resistivity of the poly contacts etc.). So you probably only have to change the transistor tolerances which are set to 0. The number of concerned rules should not exceed 10 in a mixed-mode technology. Be careful not to reduce tolerances related to transistors of standard cells, they usually do not have perfect match and this could lead to a large number of "false" errors.

Good luck

Paul