Subject: IBIS differential model questions

From: John Martin <[email-masked]>
Date: Mon, 23 Nov 1998 14:26:19 -0800

I am trying to build a differential IBIS model.  I am not aware of a way to
create a single IBIS model from a differential buffer which has two output
pins and two independent voltage sources.  It does not seem logical to
create two IBIS models for one differential buffer, one for each output pin.
Is there a way to create a single IBIS model for a differential I/O buffer
using s2ibis2?  I have read the section in the s2ibis2 manual that describes
the differential pin list and it states that s2ibis2 does not process any of
the information in that list, it simply copies it to the output file.  I use
s2ibis2 for Solaris to create IBIS models.

Thanks,
John Martin
QLogic Corporation

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From: Syed Huq <[email-masked]>
Date: Tue, 24 Nov 1998 13:25:40 -0800 (PST)

John,

Here are my thoughts on this. s2ibis2 or IBIS looks at a differential buffer
as if it was a single ended buffer. So only one leg of the buffer output is
modeled. Even the [Ramp] numbers are derived out of one single-ended output.

It uses the [Diff pin] keyword to indicate the other leg of the Diff buffer.
The simulators take info of the modeled single ended buffer and re-creates the
other leg of the Diff buffer(using [Diff pin]).

One problem here is the R_load gets tied to either Vdd or GND(based on test).
There is no true differential load. You can get by by using the V/T table data
and specifying V_fixture, R_fixture.

So, in summary, for a Diff buffer, you create ONE IBIS model(single-ended)and
use the [Diff pin] keyword to mention the other pin of the output.

Regards,
Syed
Cisco Systems, Inc

> From [email-masked] Mon Nov 23 14:30:37 1998
> X-SMAP-Received-From: outside
> From: John Martin <[email-masked]>
> To: "[email-masked]'" <[email-masked]>
> Subject: IBIS differential model questions
> Date: Mon, 23 Nov 1998 14:26:19 -0800
>

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From: Weber Chuang <[email-masked]>
Date: Wed, 25 Nov 1998 10:50:47 +0800

Hi Syed and all,

  Though I don't quite get  the meaning but I still want to express my
question and concern here, it sounds to me that the tools will use the
single ended characteristic to generate another leg, but we have some
drivers which I believe to be the differential drivers, for example, the USB
driver which has p0+ and p0-. And due to some pad pitch and layout
optimization problems, we are not able to make the driving and edge
rate...etc of the two legs exactly the same, so it is not accurate to derive
one leg  from the other, then how should we do? Thanks for any input and
hope that I am not off the topic.. 

  Best Regards

ChingFu Chuang

> -----Original Message-----
> From:	Syed Huq [SMTP:[email-masked]]
> Sent:	Wednesday, November 25, 1998 5:26 AM
> To:	[email-masked]; [email-masked]
> Subject:	Re: IBIS differential model questions
> 
> John,
>