Subject: Does IBIS describe output transition which both MOS turned on?

From: John Lin - TAO <[email-masked]>
Date: Wed, 13 May 1998 10:54:34 +0800

Dear IBIS fans/SI Experts,

As I know  for a CMOS device, during output transition, both PMOS and
NMOS will turn on in a short period of time. I wonder if an IBIS model
describes this behavior. If yes  then how it does. Will the behavior
affect the accuracy of simulation with IBIS model?

Based on what I know the V-I curve for PULL UP or DOWN is obtained by
turning one MOS off and then swift the V-fixture to get Current reading.
It seems to me that the V_I curve doesn't contain the information to
describe the transition, both PMOS and NMOS turned on.

Any comment?

Thanks,

JOHNLIN
CAE Engineer of EDA Department
Digital Equipment Corp. Taiwan Branch
Email: [email-masked] <mailto:[email-masked]> 
TEL: 1-886-3-3900000 ext. 2152

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From: Vinu Arumugham <[email-masked]>
Date: Tue, 12 May 1998 22:56:12 -0700

John,

IBIS models also have V/T curves.  These should cover overlapped
conduction.

Vinu

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From: Jean-Claude Perrin <[email-masked]>
Date: Wed, 13 May 1998 08:52:37 -0700

Vinu,

   This is not really true. The V/T curve in the IBIS model definition does not
give any information about the PMO/CMOS overlapped current.
The V/T describes the behavior of the buffer  output versus frequency but does
not give any information on the current passing internaly through the output
transistors.
To know the value of the through current it is necessary to know the "Ron"
variation of the two transistors during the transition periode. The value of
this current depends upon the equivalent impedance of the two transistors,
impedance which is connected between Vdd and Vss power supply.  
 
 Jean Claude Perrin

------------------
Original text

From: Vinu Arumugham <[email-masked]>, on 5/12/98 10:56 PM:
To: John Lin - TAO <[email-masked]>
Cc: "'IBIS_USER'" <[email-masked]>, "'SI_LIST'" <[email-masked]>

John,

Vinu

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From: "D. C. Sessions" <[email-masked]>
Date: Wed, 13 May 1998 07:41:02 -0700

Jean-Claude Perrin wrote:

[wrt: IBIS v/t curves modelling crowbar current in CMOS]

> impedance which is connected between Vdd and Vss power supply.

The IBIS v/t curves (four, note: one each with the load to power
and one with the load to ground for both rising and falling edges)
give the full picture.  The turnon of the pullup device is given
by the rising edge/grounded load curve; the turnoff of the pullup
is given by the falling edge/grounded load curve.  The turnon of
the pulldown device is given by the rising edge/pullup load curve;
the turnoff of the pulldown is given by the falling edge/pullup
load curve.  Crowbar current on the rising edge is just the
overlap between the pullup turnon and pulldown turnoff, and on
the falling edge between the pulldown turnon and pullup turnoff.

Also, the crowbar current in CMOS outputs (esp. tristate ones, and for
practical purposes that means all of them) is very low by design.  At
least the ones I design are, and I have yet to see any others that act
differently.  Unlike internal gates output drivers have separate paths
for turning on the pullup, turning off the pulldown, turning on the
pulldown, and turning off the pulldown.  As a result it's easy to turn
the driver devices OFF faster than ON, and since crowbar current not
only wastes power but slows down the buffer I have a hard time imagining
a competent designer shipping a driver that has more than trivial
crowbar current.

-- 
D. C. Sessions
[email-masked]

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From: Fred Balistreri <[email-masked]>
Date: Wed, 13 May 1998 10:06:08 -0700

> a competent designer shipping a driver that has more than trivial

Excuse my ignorance D.C. but I thought the V/T curves are a function of
voltage vs time during the respective tr/tf periods. By itself this
represents a behavior for the given load and the given load only. In the
past the V/T curves were used by the simulation vendors as a means to
coorelate the data. In other words one could look at the v/t data and
make it match IBIS under that load. Now enter 4 v/t curves. We can use
the data to test and make that match IBIS. But now the final output wrt
v/t will depend on the vendors interpretation, algorithms, topology,
and the phase of the moon. There seems to be a lot of decisions that
need to be made left up for grabs. Although my company attends the 
IBIS meetings I do not. Is there talk of publishing final V/T curves 
for say a resistively unloaded device for example. This would serve as
a means of coorelation for the vendors sake. 

One problem I see is that the v/t information by itself does not contain
the current information. That's buried in the I/V information. In fact
we can only get the answer for the given loads not dynamically. Your
presumption seems to be based on pullup/pulldown simple topologies. In
fact I can tell you we are dealing with very complex I/O stages with
feed backs, multiple current switching techniques, and the like. I don't
see such a simple solution to this problem. Real devices seem to be much
more complex and dynamic than IBIS can support. I am encouraged after
reading 3.1 spec and the formation of an independent IBIS users group
though. 

Best Regards,

-- 
Fred Balistreri
[email-masked]

http://www.apsimtech.com

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From: "D. C. Sessions" <[email-masked]>
Date: Wed, 13 May 1998 15:13:07 -0700

> voltage vs time during the respective tr/tf periods.

Voltage vs. time *into a load*, and the timescales are
supposed to match up.

>                                                      By itself this

One reason for loading the driver during derivation of v/t curves is
that outputs typically go through a very high-impedance state when
both devices are OFF.  In this time other signal sources such as
gate coupling can have a large effect on the output voltage despite
the fact that the signal source doesn't have the v/i characteristics
of the IBIS driver.

Without going into the other uses of the v/t curves, they *can* be used
to answer the present question, PROVIDED that the test load is heavy
enough to be meaningful  (A 10Kohm load isn't going to tell us much
about the characteristics of a driver with an Rdson of twelve ohms.)
The reason that this is so straightforward is that the crowbar
current only flows for a brief time when both devices are nearly OFF
anyway, so they are in deep saturation and thus the drain currents
are pretty much independent of voltage.

> the current information.

Sure they do, it's just Ohm's Law: Id = Vout/Rload

>                          That's buried in the I/V information. In fact
> we can only get the answer for the given loads not dynamically.

Without feedback, Norton models seem to work pretty well.

>                                                                 Your
> feed backs,

Eeeeyeew!  Feedback!  Feedback is evil.  Bad driver, BAD.
Go to bed without stability.  Naughty, naughty, naughty.

Well, OK, that's overstating it.  Sometimes feedback is
not only useful but necessary; it still makes behavioral
modelling (a la IBIS) pretty well impossible.  IMNSHO, this
is more annoying than crippling since feedback has other
properties that limit its usefulness in applications where
signal integrity is a major concern, so giving up behavioral
modelling of feedback-controlled drivers is (again, IMO) no
great loss.

>             multiple current switching techniques, and the like. I don't
> more complex and dynamic than IBIS can support.

Well, that's certainly true -- for instance, IBIS can't really deal
with JEDEC flexible-impedance drivers, and I'm writing THOSE into
the IEEE 1394b spec.  Great fun.  Still, we need to keep in mind that
the more exotic variants that you and I deal with are of very narrow
application.  Most people don't encounter anything but the vanilla
CMOS device, and for *that* the simple v/t analysis I presented is
quite adequate.  Actually, even for the exotica you mentioned it
will give a pretty good idea of the crowbar current.  Oddly enough,
the flex-z stuff is one of the ONLY cases it won't do well.

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From: Chris Rokusek <[email-masked]>
Date: Wed, 13 May 1998 20:39:35 -0700

Okay, can't resist here's my two cents...

Fred Balistreri wrote:
> Id is not crowbar. So you still have to go back to the I/V data and dig
> it out as I already mentioned. V/t curves were originally intended to
> model the ac or switching characteristics which IBIS was (is?) lacking.
> Sure the information is there so is ground bounce to some degree.
> Remember the 4 v/t curve information only got added recently, but the
> single v/t curve has been in IBIS for quite some time. Chances are the
> users haven't seen a 4 v/t model yet.

Since IBIS V2.1 (circa 1995) there has been a limit of 100 waveform
tables per model allowing many "arbitrary" loading conditions.  One of
the reasons multiple waveforms were allowed right from the start was to
support a mechanism for estimating VCC, GND & the dreaded Crowbar
currents.

I've seen many 4 v/t models for quite a while but I also still see many
1 v/t models or multiple X v/t waveforms to useless loads.

***
*** MODEL BUILDERS PLEASE READ ON... 
***

More recently an IBIS model building cookbook that details recommended
loading conditions was posted for public download:

   http://www.eia.org/eig/ibis/tools.htm

   Click "IBIS Cookbook for V2.1 (Oct 13th '97)"

Best Regards,

Chris Rokusek
Viewlogic Systems