Re: Differential pins in IBIS

Christian Klein wrote:
>
> DC, Stephen,
>
> What if you created additional "virtual" Vref pins, and referenced each
> differential input to a different pin? I don't think this would be a problem, as
> long as they share the same POWER and GND busses.

I talked about this with Bob Ross (Please chime in, Bob) and it seems
that this is one of those cases where Mr. Nolan's IBIS files are
within the letter of the specification but violate tool requirements.

Arguably, we don't need to make any changes in IBIS to accomodate this
application, except maybe some comments to the effect that this is a
supported use. The tool companies just need a nudge to get them to
support this.

In the meantime, your proposed workaround might trick them into doing
what they Should Have In the First Place.

> "D. C. Sessions" wrote:
>
> > Weston Beal wrote:
> > >
> > > Stephen,
> > >
> > > Although this receiver is implemented as a differential receiver, from the
> > > outside it looks more like a single ended receiver because the system really
> > > only feeds in one data signal. Vref is held constant at 1.25V. You can
> > > simply use the parameters Vinl and Vinh to specify the thresholds Vref -
> > > 350mV (0.9V) and Vref + 350mv (1.6V).
> > >
> > > Now, since Vref should follow Vcc, it changes for min and max cases. For
> > > this we use Vinl and Vinh under the [Model_Spec] keyword. They work the
> > > same but now have typ, min, and max values.
> >
> > Not good enough. Vref and Vtt are related to Vddq, but there are both
> > tolerances and noise terms involved which don't track the simplistic
> > typ/min/max axis. In particular, for the devices Mr. Nolan is discussing,
> > the common-mode rejection of the receivers is an issue. Differential is
> > a must.
> >
> > For instance, right now there is a JC-42.3 task group dealing with the
> > impact of SSI noise on Vref. Crosstalk analysis where is can make or
> > break standard technology.
> >
> > > The IBIS committee is still working on the AC thresholds vs. DC thresholds
> > > problem.
> >
> > One of the reasons that Mr. Nolan now represents TI on the IBIS committee.
> >
> > --
> > D. C. Sessions
> > dc.sessions@vlsi.com

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