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?
>
> That's one way. (For those joining late: Mr. Schumacher refers
> to the problem where an output is still sourcing current into
> an unsettled line when it switches LOW. The pullup turnoff
> will produce one output step, followed some time later by the
> pulldown turnon step.)
>
> Far more serious, I would tink, is the situation in terminated
> lines (eg, SSTL) where the line NEVER quiesces to a zero-current
> state. For a whole lot of reasons, the output will almost
> always do a two-step: first to the OFF state and then to the
> reverse state. The difference between this and the academic
> smooth-transition model is pretty significant.

So, to get a correct IBIS driver model for a given situation,
a user must either

(a) ask the device vendor to run the vendor's SPICE model
    driving the user's desired circuit at the desired switching
    rate, capture the i-v curve for that circuit and package the
    curve as an IBIS model, or

(b) ask a model developer to attach a real device to the
    desired circuit, switch it at the desired rate, capture
    the i-v curve, and package the curve as an IBIS model.

If best- and worst-case temp/process/SSO models are desired then
the above must be repeated for each condition. Repeat the above
for each additional desired switching rate of the circuit; and
again for each new circuit which the device might be used to
drive. That entire family of curves then becomes one IBIS model
for one user.

A user might have the vendor/developer capture the waveforms
at all the pins of the circuit and save himself the trouble
of repeating the simulation/measurement. Prudence requires
executing an NDA with the device supplier/third party model
developer before handing over circuit designs and operating
conditions.

 
Received on Mon Oct 6 07:28:40 1997