Re: [SI-LIST] : Re: Is feedback really EVIL?

D. C. Sessions (dc.sessions@vlsi.com)
Thu, 14 May 1998 12:01:00 -0700

Muranyi, Arpad wrote:

> I agree that IBIS can't do the detection/adjustment part of these self
> adjusting
> buffers, but you can still handle them in IBIS through the buffer selector
> mechanizm if you made a separate model for each one of the strengths the
> buffer
> can adjust itself to.
>
> This will work well as long as the buffer doesn't have those wonderful
> feedback
> circuits. It might be a little cumbersom for the user of the IBIS model to
> do
> the selection manually, but the buffer can still be modeled.

Agreed, with the proviso that the selection inevitably involves
quantization error. External resisitors are in principle infinitely
variable, so there is some inherent error (as well as serious
inconvenience) involved in selecting from a finite list of models.
In practice, of course, this isn't a biggie because the matching
isn't perfect anyway and so long as the error from selection isn't
large compared to the other variables it can be discounted.

The lack of an automatable mechanism for selecting drivers is IMHO
more of a problem in the design-tradeoff stage.

> > DC Sessions also wrote:
> > >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.
> >
> > 4. What is a flexible impedance driver, and how does it work?
>
> A flexible-impedance driver is one which has a drive impedance
> which can be adjusted to match its load. JEDEC has a standard
> in the works which addresses some of the issues involved. The
> obvious method is to have a reference pin on the IC which is
> tied to ground via a resistor with five times the desired
> impedance. This allows well-controlled reflected-wave switching
> without the component cost, board crowding, and SI compromises
> of external resistors.

-- 
D. C. Sessions
dc.sessions@tempe.vlsi.com