Subject: Questions re: [Rising Waveform], [Falling Waveform]

From: [email-masked]
Date: 03 May 1997 16:00:49 -0700

Hello, I'm seeking some advice about the subject keywords.  I have included
them in a model I've developed for a customer (since the drivers I'm modeling
do have slew rate control), but I'm not entirely sure how simulators use
the V/T tables versus using the [Ramp] data.  In particular, I have the
following questions:

1) What xxx_fixture values should I specify for my SPICE simulations (and
hence place into the IBIS model)?

2) Should the xxx_fixture values vary for each customer based on their
application?

3) What would happen (in other words, what would a simulator do?) if the
V/T tables use a different loading assumption than the assumptions used
to produce the dV/dt_r and dV/dt_f values? (a response that this is not
appropriate for an IBIS model would be acceptable to me).

4) Would the simulations be any less accurate if I simply didn't include
the V/T tables?

Any light shed on this topic (or a reference to an explanation) would
be most appreciated.

Thank you,
Jerry Isaac

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From: [email-masked] ( Bob Ross)
Date: Mon, 5 May 97 11:59 PDT

Jerry:

I am providing you some thoughts related to how Interconnectix
processes the waveform data to add to the responses already
provided.

I suggest that you actually provide prototypes of your IBIS
models to various IBIS CAE vendors found on the IBIS roster
document to help validate the performance of your models.

Best Regards,
Bob Ross
Interconnectix

 Date: 03 May 1997 16:00:49 -0700
> Subject: Questions re: [Rising Waveform], [Falling Waveform]
> To: [email-masked]

> hence place into the IBIS model)?

For CMOS, we also support typical 50 ohm loads to GND and to Vcc.  While
permissible, we will get BAD simulations in general if you include a
significant (such as 50 pF) C_fixture component.   The only cases where the
results will be reasonable is if the actual circuit contained that discrete
capactance.

> 2) Should the xxx_fixture values vary for each customer based on their

A major application of IBIS models is for simulation of high-speed
signals in transmission line networks that model printed circuit board
traces.  Such designs typically have transmission line impedances in the
30 to 100 ohm region, and 50 ohms seems like a good nominal value.

If you knew beforehand that the characteristic impendance environment
was at a different level, then you would get (probably) slighly more
accurate by tuning your waveform extractions to the actual characteristic
impendance.  However, I do not feel this is necessary for a model to 
be provided for multiple applications.

> appropriate for an IBIS model would be acceptable to me).

In our simulator, the V/T tables always override the [Ramp] data.  We
use the first two [Rising Waveform] and first two [Falling Waveform]
tables encountered.  If two tables cause a mathematical inconsistency,
(or if the driver is an "Open_***"), then we use just the first table.
If there is a mathematical problem (e.g., some really bad data values)
then we default to the [Ramp] data, and use its loading assumptions.

In all cases, simultors should give a reasonable response for the actual
loading conditions that are seen, regardless of whether the time response
is V/T or Ramp based.  The beginning and ending amplitudes should be
based on actual loading conditions in conjunction with the internal
model I/V tables.

> 4) Would the simulations be any less accurate if I simply didn't include

Our simulators are more accurate by including the V/T tables to capture some
some actual driver pulse shape details that would be seen in practice.

To the extent that much of the simulation detail is based on the network
topology and its set of distributed loads, a Ramp based driver with correct
dt_r and dt_f values also provide accurate simulations.