[SI-LIST] Re: IBIS Driver Models, Simulators and CComp
- From: "Haller, Robert" <rhaller@xxxxxxxxxx>
- To: "'twester@xxxxxxxxxxx'" <twester@xxxxxxxxxxx>,"'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 10 Jan 2002 09:33:23 -0500
Todd,
I have wrestled with this exact issue also.
I analyzed cases where I could not come up with one
value of Ccomp that works for an Input and an Output on a
BI-directional I/O cell (when comparing SPICE and IBIS).
Using Ideal drivers and ideal receivers in multiple simulations
I figured out the optimal value for each, then split the difference
(and insured the error induced was acceptable). I advocate using
behavioral tools (IBIS) to find problem areas in a design, then fix
them or spend more time analyzing with increased accuracy (SPICE).
I also saw cases where it would be desirable to distribute Ccomp
across elements (i.e. Resistor). As Arpad points what rail the Ccomp gets
tied to and return current are still a deficiency.
Bob Haller
Cereva Networks
Marlboro Ma.
rhaller@xxxxxxxxxx
508-787-5365
p.s.
(Kumar was right !)
-----Original Message-----
From: Todd Westerhoff [mailto:twester@xxxxxxxxxxx]
Sent: Wednesday, January 09, 2002 5:47 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] IBIS Driver Models, Simulators and CComp
Hi all,
We've been doing an investigation recently, comparing IBIS models and
simulation results to their HSpice counterparts. One of the things that has
emerged as a focal point is the concept of Ccomp (the die capacitance) and
how the IBIS simulator handles it for a driving output.
Ccomp (as I understand it) is meant to represent the intrinsic parasitics of
the I/O cell. As such, it isn't anything that can be precisely identified
in the Spice model of the I/O - rather, Ccomp models the cumulative effect
of a bunch of things. That being the case, the V-T curves created for the
IBIS model using Spice will already have the effect of Ccomp included. If
the IBIS simulator adds a Ccomp capacitor across the IBIS driver's output,
the effect of Ccomp may be "double-counted" when the device is driving.
On the other hand, if the capacitor is not added to the circuit - then the
reflections off the driver will be incorrect, because the driver's output
capacitance will not be modeled.
So the question(s) are -
a) Have other people run into this issue?
b) I suspect the handling of Ccomp for an IBIS driver is EDA tool dependent.
Does anyone know how the different tools handle this?
c) Are there applications > 150 MHz where people are using IBIS and HSpice
interchangably? If so, how are the correlation issues between the two
simulators managed?
One problem that seems especially insidious (we're still not sure of exactly
what's going on) is that we have a bidirectional I/O whose effective
capacitance seems to be different in the driving and tri-stated conditions.
Given that IBIS allows only one value for Ccomp, that makes correlation even
more challenging.
All responses, public and private - are both welcomed and appreciated.
Todd.
Todd Westerhoff
SI Engineer - Hammerhead Networks
5 Federal Street - Billerica, MA - 01821
email:twester@xxxxxxxxxxx - ph: 978-671-5084
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