Re: [SI-LIST] : Does IBIS describe output transition which both MOS turned on?

Fred Balistreri (fred@apsimtech.com)
Wed, 13 May 1998 17:08:11 -0700

D. C. Sessions wrote:
>
> Fred Balistreri wrote:
> > D. C. Sessions wrote:
> > > Jean-Claude Perrin wrote:
>
> > > [wrt: IBIS v/t curves modelling crowbar current in CMOS]
> > >
> > > > 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.
> > >
> > > 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.
>
> > 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.
>
> Voltage vs. time *into a load*, and the timescales are
> supposed to match up.
>
> > 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 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.
>
> > One problem I see is that the v/t information by itself does not contain
> > 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
> > 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,
>
> 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
> > see such a simple solution to this problem. Real devices seem to be much
> > 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.
>
> --
> D. C. Sessions
> dc.sessions@tempe.vlsi.com

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.

Best Regards,