RE: [IBIS-Users] C_comp issue?

Subject: RE: [IBIS-Users] C_comp issue?
From: Todd Westerhoff (twesterh@cisco.com)
Date: Mon Nov 11 2002 - 13:34:23 PST

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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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