RE: [IBIS-Users] Re: IV curve sweep range

Hi, Arpad,

This is an interesting point.  Here are my $0.02 of ideas:

1) An SI tool might need to extrapolate outside the table range when setting
the DC operating point,  since the simulator can try any point in the entire
[-Vcc,2*Vcc] range while trying to find the operating point.

2) A signal can "double" on reflection.  For example, an I/O operating at
1.2V with a 0.8V swing would need to cover from 1.2-(2*0.8) to 1.2+(2*0.8),
or -0.4V to 2.8V.  This might be a difficult rule for the parser to check.

Best regards,
Lynne


-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Aubrey_Sparkman@dell.com
Sent: Tuesday, July 12, 2005 2:26 PM
To: arpad.muranyi@intel.com; ibis-users@eda.org; ibis@eda.org
Subject: RE: [IBIS-Users] Re: IV curve sweep range

I would like to see the sweep range linked to the allowed or expected signal
swing.  If the buffer can be used in a "less than perfectly terminated"
topology where rising or falling overshoot outside 0 to Vcc can happen, then
the sweep should be REQUIRED to cover that range.  If the signal swing will
always be well within 0 to Vcc, then a reduced sweep range could be allowed.

my $.02

Aubrey Sparkman
Enterprise Engineering Signal Integrity Team Dell, Inc. 
Aubrey_Sparkman@Dell.com
(512) 723-3592

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Muranyi, Arpad
Sent: Tuesday, July 12, 2005 3:45 PM
To: ibis-users@eda.org; ibis@eda.org
Subject: [IBIS-Users] Re: IV curve sweep range

Hello everyone,

I hope you don't mind that I brought this discussion to the IBIS and IBIS
users reflectors, but I wanted to save Mike a bunch of forwarding these
messages to and from me (since I don't want to sign up to the IBIS quality
list at the moment).
Plus the subject ultimately is a spec related one, so it may not hurt to
discuss it in these lists.

I agree with Tom, that those original IBIS ranges are still useful.  The
point I was trying to make is that IBIS REQUIRES those ranges, period.
True, the parser doesn't flag models now which do not obey these
requirements of the IBIS specification, but technically these models are out
of spec.

Also true, the model maker can always do a smaller range, and then add two
points on the two ends, just to satisfy the spec requirements, but if the
table has 100 points already, the model make will have to figure out which
other two points they have to remove before they can add the points at the
ends.

Forcing the model maker to generate models for the -Vcc to 2Vcc range may
reduce the quality of the model, because many points may be used up in those
areas of the curves which never get used, and at times it may not be
possible to generate those points due to convergence issues.

The comment from Bob about the current limiting at 1A or 10A seems to be a
related, but different issue.  I would like to see that the IBIS
specification is fixed so that it doesn't REQUIRE the -Vcc to 2Vcc range.
But I agree, this may be a difficult one, because if we just eliminate the
rule, we may start getting a bunch of useless models, if people don't know
what ranges they should provide.  So what should the spec say then?  Spell
out some STRONGLY RECOMMENDED rules?

Thanks,

Arpad
==================================================================


-----Original Message-----
From: ibis-quality-bounce@freelists.org
[mailto:ibis-quality-bounce@freelists.org] On Behalf Of Robert Haller
Sent: Tuesday, July 12, 2005 1:03 PM
To: ibis-quality@freelists.org
Subject: [ibis-quality] Re: IV curve sweep range

Tom,  Michael
Agreed that sweeping over the entire range makes sense in come cases and

it is excessive in others.
But in order to avoid excess currents and meet the spec in its presently

approved state,
the committee reached consensus that limiting the current at something
resonable (1Amp) below the hard ibis check ceiling of 10 Amps seemed
acceptable. We want to minimize warnings from the parser and most devices
(that people are simulating with IBIS)  operate below an 1 amp.

The point we were trying to make was when sweep terminated devices (i.e.

ODT) it is important to sweep across the entire 'legal' range to avoid
simulators interpolating or extrapolating.

I have persoannly always had an issue with the excessivly wide range and

would support a bird that  reduces it to something reasonable. The rub is
getting consensus in Open forum on what is reasonable with all of the
exceptions....

my $.02
Bob
 
Tom Dagostino wrote:

>There are still a lot non busses that people need to simulate, not all
of
>them will be terminated.  In addition some people simulate open
connectors
>which may have non terminated lines by definition.  So, yes there are
places
>where it does not make sense to do the -Vcc to 2*Vcc range but there
are
>cases still where it makes sense.
>
>Tom Dagostino
>Teraspeed(R) Labs
>13610 SW Harness Lane
>Beaverton, OR 97008
>503-430-1065
>tom@teraspeed.com
>www.teraspeed.com
>
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>401-284-1827
>
>-----Original Message-----
>From: ibis-quality-bounce@freelists.org 
>[mailto:ibis-quality-bounce@freelists.org]On Behalf Of Mirmak, Michael
>Sent: Tuesday, July 12, 2005 12:00 PM
>To: ibis-quality@freelists.org
>Subject: [ibis-quality] IV curve sweep range
>
>
>Forwarded from Arpad Muranyi...
>
>- MM
>
>-----Original Message-----
>From: Muranyi, Arpad
>Sent: Tuesday, July 12, 2005 11:20 AM
>To: ibis-quality-bounce@freelists.org
>Cc: Mirmak, Michael
>Subject: IV curve sweep range
>
>Hello everyone,
>
>I am not monitoring this email reflector, and I don't plan on 
>subscribing, but Mike Mirmak forwarded me a message regarding the IV 
>curve sweep range discussion, to which I would like to add a few 
>comments.  I don't know what exactly is being proposed, I am just going

>to mention my views here.
>
>The problem I see is that IBIS requires a sweep range of -Vcc to 2*Vcc.
>This comes from the days when most buffers switched rail to rail and 
>most buses were unterminated.  The reason for this range stems from the

>T-line reflection theory, that the signal can double at the end of the 
>line.  If we had a strong driver, which could drive close to the rail, 
>then the doubling of this signal could span a range close to -Vcc or
>+2*Vcc assuming no clamping effects at the end of the line.
>
>The problem is that most modern buses and drivers are not operating
like
>this any more.  For one, the signal swing doesn't go rail-to-rail any 
>more, and second, most buses are
terminated.
>Even if we ignore the termination question, the reduced-swing signaling

>alone warrants a reconsideration of the IBIS rule.
>
>One of my favorite examples is GTL.  The signal goes from a low level 
>between 0-0.5V to a high level of 1.5V.  If I took 0-1.5, even if I 
>doubled the signal swing on the top and bottom, I would get a range of 
>-1.5V to 3.0V.  Aside from the fact that the bus is terminated and we 
>will never see a doubling at the end of the T-line, compare this range 
>with what IBIS would require if this buffer was powered by 3.3V:  it 
>would be -3.6V to 6.6V, almost double!  This clearly doesn't make
sense.
>
>I would propose that we should write a BIRD on this, and change the
IBIS
>specification so that it would not REQUIRE the range of -Vcc to 2*Vcc.
>I am not sure what it should say yet, but I would definitely NOT favor 
>that the parser should be changed to check the ranges in the IBIS files

>against the IBIS rules.  I would much prefer to have a rule that is 
>based on signaling, though I have to admit that this may be somewhat 
>more complicated to spell out.
>
>Please comment,
>
>Arpad
>===========================================================


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