RE: [IBIS] RE: [IBIS-Users] Query reg. USB1.1 IBIS model generation

Vivek,
 
Generally, legacy IBIS is not able to describe buffers which
have feedback.  You can, however, describe any behavior with
the *-AMS extensions in IBIS.  On the other hand, if you are
lucky, you may still be able to make a model using legacy
IBIS features if you know exactly what your buffer does and
you know how IBIS models work.  This may require some trial
and error, and I am not in a position to guide you through
this.
 
Regarding the 45 Ohm R_fixture question, you will get the best
correlation when R_fixture equals the T-line impedance.  The
model will still work well if the T-line has a different
impedance, but the farther you go away the less accuracy you
can expect.  Give it a try...
 
Arpad
----------------------------------------------------------------

________________________________

From: Vivek kumar Singh [mailto:Vivekkumar.Singh@arm.com] 
Sent: Monday, January 21, 2008 11:27 PM
To: Muranyi, Arpad; ibis-users@eda.org; ibis@eda.org
Subject: RE: [IBIS] RE: [IBIS-Users] Query reg. USB1.1 IBIS model
generation



Hi Arpad,

     For both low and high speed usb, I generated IBIS models with
R_fixture of 50 Ohms. 

 

Mine is USB1.1.  I then did correlation with spice model. Full speed was
matching, but Low speed is not matching with spice.

 

2 queries I have :

   1) In Low speed design, I have slew correcting feedback capacitors.
So, while generating IBIS should I take care of it separately or IBIS
will handle it? 

   

   2) For USB1.1, 45 Ohm R_fixture is used. So, while validating should
I use TLINE also of 45 Ohm? What changes should I make?

 

Regards,

Vivek

 

________________________________

From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of
Muranyi, Arpad
Sent: Monday, January 21, 2008 10:07 PM
To: ibis-users@eda.org; ibis@eda.org
Subject: [IBIS] RE: [IBIS-Users] Query reg. USB1.1 IBIS model generation

 

Vivek,

 

R_fixture has nothing to do with the 1.5k or 15k resistors.

Select a value for R_fixture that matches the transmission

line impedance.  In the USB case it would be 45 Ohms (single

ended) if my memory serves me right.

 

I would also suggest that you look at the IBIS Cookbook for

help.  There are a lot of good ideas in there.

 

http://www.vhdl.org/pub/ibis/cookbook/cookbook-v4.pdf

 

Arpad

=============================================================

 

________________________________

From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Vivek kumar Singh
Sent: Sunday, January 20, 2008 10:34 PM
To: ibis-users@server.eda.org; ibis@server.eda.org
Cc: Vijaya kumar Vinukonda
Subject: [IBIS-Users] Query reg. USB1.1 IBIS model generation

Hi All,

    I'm having problem with ibis model generation for USB1.1 . 

I have following 3 queries :

 

since USB operates in Upstream and Downstream applications with pullup
and pulldown resistors 1.5k and 15k Ohm respectively. Will it have any
effect on choosing R_fixture value for IBIS VT data generation ????

 

What actually we take the value of R_fixture for USB1.1 ?

 

Can someone elaborate on setup for Ibis model generation for USB1.1.

 

 

Regards,

Vivek

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