RE: [IBIS-Users] C_comp issue?

Subject: RE: [IBIS-Users] C_comp issue?
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Mon Nov 11 2002 - 15:46:03 PST

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Todd,

Your description is absolutely correct, and Stephen Peters and Bill Panos
are only partially correct. Thank you for the excruciating and unambiguous
details in your description.

All,

To reiterate what Todd said, C_comp is the capacitance you see when you look
into the die. Package is not included. The waveforms in the model (Vt curves)
are not effected by C_comp, i.e. the model should reproduce the same waveforms
if simulated using the same R_fixture and V_fixture combinations that are
in the Vt curves REGARDLESS of what C_comp is.

Waveforms will only change due to C_comp if the signal comes from the outside
world, either due to reflections while the driver is driving, or when it is
in the receiver mode.

If a particular tool does not give these results it is time to talk to the
tool vendor. Most tools I know of do it this way.

Arpad Muranyi
Intel Corporation
===========================================================================

-----Original Message-----
From: Todd Westerhoff [mailto:twesterh@cisco.com]
Sent: Monday, November 11, 2002 1:34 PM
To: ibis-users@eda.org
Subject: RE: [IBIS-Users] C_comp issue?

There are a few points here worth mentioning - if only to verify that I
understand how this stuff is *supposed* to work.

If you vary the C_comp value of the *receiver* on a net - then yes, I expect
to see the waveforms change, as you're effectively changing the loading the
receiver places on the net.

However, if you vary the value of C_comp for the *driving* device, then the
fact that you've changed C_comp will *only* affect any reflections on the
net that get back to the driver. In other words, changing the driving
device's C_comp value does *NOT* affect its actual behavior as a driver;
only its behavior as a load is affected. This is true whether the driver is
acting as a receiver (i.e. an I/O in the "input" state), or whether the
driver is absorbing reflections (i.e. the driver is "quiet" because it has
stopped sweeping the V-T curve).

... all of which is a long-winded way of saying that IBIS simulators place
C_comp in parallel with the driver's output and then compensate its presence
out; thereby reproducing the V-T behavior from the IBIS model. So, changing
the value of C_comp doesn't actually change the output's *driving* behavior.

Am I understanding this correctly, or have I gotten myself confused again?

Todd ;-}

Todd Westerhoff
Signal Integrity Engineer
Cisco Systems
250 Apollo Drive - Chelmsford, MA - 01824
email:twesterh@cisco.com
ph: 978-497-0272
============================================

"When did the choices get so hard, with so much more at stake?
Life gets mighty precious when there's less of it to waste"

- Bonnie Raitt, "Nick of Time"

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
Behalf Of Peters, Stephen
Sent: Monday, November 11, 2002 3:47 PM
To: 'Eric Hsu'; ibis-users@eda.org
Subject: RE: [IBIS-Users] C_comp issue?

Hello Eric:

C_comp represents the I/O (input or ouptut) capacitance of the driver or
reciever itself. It is the capacitace seen when standing on the pad of the
die and looking back into the I/O cell.

Changing C_comp will effect a number of things, but yes -- C_comp has a
direct effect on a drivers rise or fall time and waveforms.

  Regards,
  Stephen Peters
  Intel Corp.

-----Original Message-----
From: Eric Hsu [mailto:ehsu@netlogicmicro.com]
Sent: Monday, November 11, 2002 12:00 PM
To: ibis-users@eda.org
Cc: ehsu@netlogicmicro.com
Subject: [IBIS-Users] C_comp issue?

Hi All,

What is the [c_comp] role in ibis? If changing [c_comp] value, will I
get different results in rising/falling waveform?

Best Regards,

-eric

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