Re: BIRD 42.3

In other words, different applications require different
device models. Even one given circuit will require
different models depending, as in the example, on whether
the time between output transitions is greater than or less
than the settling time. True?

> > (1) You have provided an interesting demonstration
> > case where a two-waveform based IBIS model may be needed.
> >
> > If you terminate the load to Vcc, then there will be
> > no difference in theory. However, if you terminate the
> > output to some other voltage, you will see a difference.
> >
> > (2) It is also interesting that you pose the problem
> > in terms of conductance variations (which could
> > be implied by the IBIS architecture.) It could
> > have been presented in terms of resistance
> > variations. However, there would be an implied
> > difference. Pulldown one, for example goes
> > linearly from 0 to 20 mS conductance. If it
> > were expressed as resistance, it would go
> > non-linearly from infinite to 100 ohms to 50 ohms
> > resistance.
> >
> > (3) I understand that there are other real world
> > effects that can enter into the bounce analysis
> > that may not show up in the IBIS model processing.
> > I would still think that a delta current could
> > still be used for some of these effects that
> > are caused just by the device switching itself.
> >
> >
> > As long ago promised, here's a talking case for BIRD42.3
> > For simplicity, I've restricted the case to a very simple
> > falling-edge behavioral model. The driver in question
> > is a 50-ohm switched resistor, with the following
> > time/conductance tables. The pulldown driver is common,
> > but there are two different pullup responses: All
> > listed conductances are piecewise linear.
> >
> > Time | Pulldown | Pullup #1 | Pullup #2 |
> > | Conductance | Conductance | Conductance |
> > --------+-------------+-------------+-------------+
> > 0 ps | 0 mS | 20 mS | 20 mS |
> > 500 ps | 0 mS | 20 mS | 20 mS |
> > 1000 ps | 0 mS | 10 mS | 20 mS |
> > 1500 ps | 0 mS | 0 mS | 0 mS |
> > 2000 ps | 10 mS | 0 mS | 0 mS |
> > 2500 ps | 20 mS | 0 mS | 0 mS |
> > 3000 ps | 20 mS | 0 mS | 0 mS |
> >
> > As is typical for real-world drivers, these are
> > break-before-make.
> >
> > *IF* the load had time to settle completely to zero
> > current before the next edge, there would be no
> > difference between them; however, in real high-speed
> > switching even unterminated lines take too long to
> > settle and thus the pullup is still sourcing current
> > when this schedule starts. Obviously, a terminated
> > line is even worse; either way the turnoff of the
> > pullup has important consequences.
> >
> > IMNSHO, this effect is far more important than the
> > (extremely rare) case of major crowbar current,
> > and any solution to this will necessarily also take
> > care of those few instances where crowbar current
> > is actually significant.
 
Received on Fri Oct 3 12:27:14 1997