Subject: IBIS 101: RREF & VREF Keywords

From: [email-masked]
Date: Mon, 22 Mar 1999 19:14:20 -0500

Hello all.

As an IBIS user, and model librarian, I am seeing a growing problem with
basic IBIS understanding.  I do know that there are people currently
working on creating tutorials and other education for IBIS developers;
however, I think we need to clarify the IBIS spec as well for the subject
keywords.

Here is the problem I'm seeing:

Vendors are not sure when to use RREF and VREF.  VMEAS and CREF seem to be
understood.  My understanding is that for open-drain (open-sink) devices,
VREF should be set to VCC, and RREF should be set to a value that pulls the
voltage up near VCC for the rising time-to-Vmeas and will allow the device
to pull the voltage back down to near zero for the falling time-to-Vmeas.
VREF should be set to Ground for open-source devices, and RREF should be
set to a value that allows the device to pull the voltage up near VCC for
the rising time-to-Vmeas and pulls the value down towards ground for the
falling time-to-Vmeas.

HOWEVER, I am seeing vendors put VREF to obscure values. Sometimes they are
set to zero for open-drain devices (hence, the time-to-Vmeas cannot be
calculated since the device never gets to Vmeas).  Sometimes they put VREF
to equal VMEAS; this really causes the various tools to calculate
interesting time-to-Vmeas values.  Sometimes, I have no idea what the value
of VREF really means.

I am also seeing vendors use the RREF and VREF on normal
totem-pole/push-pull devices; this also causes interesting results in the
calculation of the time-to-Vmeas.  Should RREF and VREF really be used on
these devices?

I have reviewed the IBIS Spec (Version 3.2) and the IBIS Cookbook, and
there is no clear answer as to when these keywords should be used, and when
they should not be used.  Should we create a BIRD to clarify the use of
these keywords?  Also, if my understanding is correct, should we also
create a BIRD to add an additional keyword to the "[Model Spec]" keyword:
"Vref", such as the "Vmeas" that currently exists in the "[Model Spec]"?
If VREF should be set to VCC, then there are MIN, MAX, and TYP values for
VREF, not just one TYP value.

Bob Ross, can we discuss this issue at the next Open Forum this Friday?

Regards,
Michael Cohen

IBM Personal Systems Group
Design Tools Department
D-26D/B-201/R-D104H
3039 Cornwallis Road
Research Triangle Park, NC 27709

Phone:  919-543-4042 (T/L 441-4042)
FAX:  919-543-8221 (T/L 441-8221)
Internet Address:   [email-masked]

IBM Internal Addresses:
    From Lotus Notes (#1):   Michael Cohen/Raleigh/IBM
    From Lotus Notes (#2):   micohen@ibmus
    From VM:   micohen@ibmusm21

---

From: [email-masked] (Bob Ross)
Date: Mon, 22 Mar 99 16:56:41 PST

Michael:

We will put this topic on the agenda.

Bob Ross
Mentor Graphics

> From: [email-masked]
> To: [email-masked]
> Message-ID: <[email-masked]>
> Date: Mon, 22 Mar 1999 19:14:20 -0500
> Subject: IBIS 101:  RREF & VREF Keywords