Re: IBIS 101: RREF & VREF Keywords

Michael:

We will put this topic on the agenda.

Bob Ross
Mentor Graphics

> From: micohen@us.ibm.com
> To: ibis@eda.org
> Message-ID: <8525673D.0001776A.00@d54mta04.raleigh.ibm.com>
> Date: Mon, 22 Mar 1999 19:14:20 -0500
> Subject: IBIS 101: RREF & VREF Keywords

> 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: micohen@us.ibm.com

> IBM Internal Addresses:
> From Lotus Notes (#1): Michael Cohen/Raleigh/IBM
> From Lotus Notes (#2): micohen@ibmus
> From VM: micohen@ibmusm21
Received on Mon Mar 22 17:02:43 1999