Subject: RE: [IBIS-Users] Question about warning message
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Mon Nov 18 2002 - 13:55:04 PST
The problem is that this part of the spec is very old, it was
written in IBIS 1.1. In those days we used mostly unterminated
busses, and most buffers switched rail to rail. Given that
it was simple to say we need to cover -Vcc to 2*Vcc with the
IV tables.
However, this is not true any more. Most of our buffers are
small signal swing, most of our signals are parallel terminated,
etc... Consider the example of a 3.3V GTL buffer. The signal
goes from about 0.5V to 1.5V, a 1V swing. Even if it was
unterminated, the signal would only be driven from 0 to 1.5V.
Adding the effects of FULL reflection, the doubling would
give us a theoretical maximum swing of -1.5 to 3.0V. However,
according to the spec, because the power supply to the buffer
is 3.3V, the range "must" be -3.3V to 6.6V. For this signal,
which couldn't possibly go beyond -1.5V to 3.0V it doesn't
make sense to require a table that covers -3.3V to 6.6V.
So the IBIS specification should be changed, in my opinion,
or this rule needs to be handled with a grain of salt...
Please note, I am not signing up for writing a BIRD...
Arpad Muranyi
Itel Corporation
================================================================
-----Original Message-----
From: Flora, Matthew [mailto:matthew_flora@mentorg.com]
Sent: Monday, November 18, 2002 1:31 PM
To: Fred Balistreri
Cc: 'ibis-users@eda.org'
Subject: RE: [IBIS-Users] Question about warning message
Dear Fred,
My apologies. I didn't remember the spec saying "must". I thought it said "should". My mistake.
Cheers,
Matthew Flora
> -----Original Message-----
> From: Fred Balistreri [mailto:fred@apsimtech.com]
> Sent: Monday, November 18, 2002 1:09 PM
> To: Flora, Matthew
> Cc: 'ibis-users@eda.org'
> Subject: Re: [IBIS-Users] Question about warning message
>
>
> The reason IBIS models go from -Vcc to 2Vcc is because that is what it
> says in the spec. The following is a direct quote from the spec,
> immediately
> following the section that describes the testing voltage range:
>
> "These voltage ranges must be spanned by the IBIS data. Data derived
> from
> lab measurements may not be able to span these ranges as such
> and so may
> need to be extrapolated to cover the full range. This data must not be
> left for the simulator to provide."
>
> If we don't want people to follow the spec then I would argue that the
> spec needs modification. More importantly because of the spec
> ibis tools
> all
> conform to the voltage testing as prescribed, included s2ibis which a
> lot
> of people use. So I don't think the statement about people blindly
> following
> the voltage requirement is true. It may be more of the case that
> historically
> this is what we have done, which can be traced back to where it all
> started,
> the specification itself.
>
> Best Regards,
> Fred Balistreri
> fred@apsimtech.com
>
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