[ibis-quality] Re: IV curve sweep range
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <ibis-quality@xxxxxxxxxxxxx>
- Date: Tue, 12 Jul 2005 13:59:04 -0700
My thoughts about this have been along the lines of the amount of current
that has to be sunk by the buffer that is seeing the reflection. If you are
working with a 50 Ohm system the maximum current that could be imposed on
the buffer relates to the voltage swing and the impedance of the
transmission line. For a 5 Volt system and 50 Ohms line it sounds like 100
mA. If it was a RAMBUS system of 3.3V and 27 Ohms its 122 mA. A 1.5V swing
and 50 Ohms is 30 mA., etc. If we limit the tables to say 250 mA we should
never see a reflection that exceeds the currents available in the net.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: ibis-quality-bounce@xxxxxxxxxxxxx
[mailto:ibis-quality-bounce@xxxxxxxxxxxxx]On Behalf Of Robert Haller
Sent: Tuesday, July 12, 2005 1:03 PM
To: ibis-quality@xxxxxxxxxxxxx
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@xxxxxxxxxxxxx
>www.teraspeed.com
>
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>401-284-1827
>
>-----Original Message-----
>From: ibis-quality-bounce@xxxxxxxxxxxxx
>[mailto:ibis-quality-bounce@xxxxxxxxxxxxx]On Behalf Of Mirmak, Michael
>Sent: Tuesday, July 12, 2005 12:00 PM
>To: ibis-quality@xxxxxxxxxxxxx
>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@xxxxxxxxxxxxx
>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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