RE: [IBIS-Users] RE: Role of C_comp

From: Todd Westerhoff <twesterh@sisoft.com>
Date: Thu Sep 05 2013 - 05:58:00 PDT
Ling Li,

 

The V-T curves are generated with c_comp present, so, in theory, the
output waveforms should remain the same even if c_comp is changed.

 

Reality is not always the same as theory, however.  In some cases, you may
see differences that can be overcome by changing the simulator setup, as
Arpad suggests.

 

But - and this is a big but - there are, and will always be, limits. The
V-T waveforms tell the simulator what the output behavior should be into a
specified loading condition, with c_comp included.  The output driver has
finite resistance, and a capacitance of c_comp, which forms a circuit with
an RC time constant. If you crank c_comp way up, say to 50pf, you will
describe a condition that is physically impossible.  The RC time constant
of the output driver and c_comp will be large, while the switching time
described by the V-T curves will be small.  There is no way for a
simulator to resolve the conflict - one number or the other will win out.

 

Here's my point - an IBIS [model] contains matched sets of data that
describe a buffer in multiple ways, with overlap and redundancy. If all of
the data tells the same story [a well-constructed IBIS model], the correct
simulation result is well-defined. If, however, different parts of the
model tell different stories, you get what you get. This is usually the
result of a model having been modified or created by hand . when automated
tools are used to generate models, all the data is consistent. When models
are edited manually, the result tends to reflect the user's understanding
of the spec, with that can produce different results in different tools.

 

. a long way of saying, people should edit IBIS files with caution backed
by understanding of the spec. Just because a model works and produces a
good result in one tool doesn't mean it will work in another.

 

Todd.  

 

Todd Westerhoff

VP, Software Products

Signal Integrity Software Inc. . www.sisoft.com

6 Clock Tower Place . Suite 250 . Maynard, MA 01754

(978) 461-0449 x24  .  twesterh@sisoft.com

 

"I want to live like that"

                                             -Sidewalk Prophets

 

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Ling Li Ong
Sent: Thursday, September 05, 2013 1:09 AM
To: Muranyi, Arpad; ibis-users@eda.org
Cc: Siong Hee Lim
Subject: [IBIS-Users] RE: Role of C_comp

 

Hi Muranyi,

 

Do you mean for an output buffer, if purely changing the c_comp value
between 0.1pF and 50pF and maintain other parameters as identical, it
should not generate any different waveforms?

 

I tried out the a simple output buffer simulation ran on HSPICE
2011.09-SP1.  No transmission line or termination impedance but mainly the
ibis model.   C_comp swept at 3.5pF (purple) and 5pf (green).

I observed the waveforms are not overlapped.  Is this the correct result
from HSPICE?  If it is not, what will be the expected result? 

 

 



 

Regards,

Ling Li

 

 

From: Muranyi, Arpad [mailto:Arpad_Muranyi@mentor.com] 
Sent: Thursday, September 05, 2013 11:48 AM
To: Ling Li Ong; ibis-users@eda.org
Cc: Siong Hee Lim
Subject: RE: Role of C_comp

 

Ling Li,

 

Regarding: "it has no difference between the simulation on C_comp 0.1pF
and 50pF".

That's correct (for the case when the buffer model is driving), and

that's the way HyperLynx 9.0 should also behave.

 

Thanks,

 

Arpad

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

 

From: Ling Li Ong [mailto:llong@altera.com] 
Sent: Wednesday, September 04, 2013 2:19 AM
To: Muranyi, Arpad; ibis-users@eda.org
Cc: Siong Hee Lim
Subject: RE: Role of C_comp

 

Hi Arpad,

 

Thanks for your fast response.  I was referring to the mentor's website
which commented with using the version 8.2.1, it has no difference between
the simulation on C_comp 0.1pF and 50pF.

This version may be outdated compare to latest as I noticed the posting
date was on last year.  However, I assume it should perform the same
simulation result after this version.

http://communities.mentor.com/mgcx/thread/6935

 

May I know is this comment align to your understanding?  

 

Thanks.

 

Regards,

Ling Li

 

From: Muranyi, Arpad [mailto:Arpad_Muranyi@mentor.com] 
Sent: Wednesday, September 04, 2013 2:46 PM
To: Ling Li Ong; ibis-users@eda.org
Cc: Siong Hee Lim
Subject: RE: Role of C_comp

 

Ling Li,

 

It seems that your conclusion from reading Todd Westerhoff's email

in the link you provided is not quite correct.  Todd seems to say

(correctly) that the output waveform of a driver will not change

when C_comp is changed in the IBIS [Model].  You should only see

differences in the waveforms when a reflection hits the driver

and that reflection bounces back off of the driver.  This secondary

reflection from the driver should depend on the C_comp value.

 

Similarly, if an I/O buffer model is in the receive mode (i.e. input

state), you should see different waveforms when you change the C_comp

value.

 

As far as I can tell from my days when I worked at Intel and used

HSPICE in my work, HSPICE does what I described above correctly.

 

Now that I work for Mentor, I can also tell you that the results

of HyperLynx are correct the same way.  I am actually surprised to

hear that you see different results.  Are you sure your test case

is designed well to show and observe these effects correctly?

 

By the way, the latest HyperLynx version is now 9.0 (not 8.2.1).

 

I hope this helps answering your question.

 

Thanks,

 

Arpad

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

 

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Ling Li Ong
Sent: Wednesday, September 04, 2013 12:47 AM
To: ibis-users@eda.org
Cc: Siong Hee Lim
Subject: [IBIS-Users] Role of C_comp

 

Hi,

 

I saw some discussions on c_comp issue at user archive at link:

http://www.eda.org/ibis/users_archive/0487.html

 

I am confused by the c_comp whether it is double-counted and the different
result obtained when using different tools. 

According to the comment in the link above, varying the c_comp may see the
changes in rise/fall curve.  This is definitely true when using the HSPICE
simulator.  But, if you using the latest Hyperlynx  version 8.2.1,
changing the c_comp will not reflect the changes in rise/fall curve.  

 

Can you please advise which tools are showing the correct response on
c_comp when sweeping at different value?

Thanks.

 

Regards,

Ling Li

 

 

 

 

 

 

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