Re: [SI-LIST] : Looking Inside IBIS Model

Ken Mohamed (kmohamed@viewlogic.com)
Wed, 19 May 1999 12:36:06 -0700

Abe, Mark,

Point of Information...

Abe mentions that ibis2xtk judges the usefulness of the
IBIS rising and falling waveforms by evaluating the test
fixture (R_fixture, V_fixture, C_fixture, etc.) to "certain
criteria acceptable by IBIS2XTK." The criteria can be
found in the man page for ibis2xtk. I've inserted the
relavant section below...

Regards,
Ken Mohamed

----------- from ibis2xtk man page -----------------------
RECOMMENDED WAVEFORM DATA

To trigger enhanced model generation, the following waveform data must
be included in the IBIS file.

* Standard Devices: One test jig (1 rising, 1 falling):
50 ohms to a terminating voltage midway between the no-load-high-state
output voltage and the no-load- low-state output voltage.

An enhanced model will not be built for this particular case; it is not
necessary.

* Pull-Up Devices: One test jig (1 rising, 1 falling):
If the driver requires a pull-up or pull-down device to function, the
termination resistor and supply used for the design should be used to
terminate the driver for this test.

* Enhanced CMOS: Two test jigs (2 rising, 2 falling):
50 ohms to a terminating voltage equal to the drivers negative supply
voltage.
50 ohms to a terminating voltage equal to the drivers positive supply
voltage.

* Enhanced TTL: Two test jigs (2 rising, 2 falling):
500 ohms to a terminating voltage equal to the drivers negative supply
voltage.
7 ohms to a terminating voltage equal to the drivers positive supply
voltage.

** Many model providers are supplying waveforms, however the test jigs
used do not follow the above guidelines resulting in unusable waveforms.
Specifically, ibis2xtk will use waveforms that meet the following
criteria:

- C_Fixture = 0.0pF
- L_Fixture = 0.0nH
- V_Fixture - (VCC or GND) <= 0.7V
- 3.0 < R_Fixture < 30.0 (TTL to VCC)
- 200.0 < R_Fixture < 1000.0 (TTL to GND)
- 25.0 < R_Fixture < 200.0 (CMOS)

Of all waveforms meeting the above criteria, the two (VCC and GND)
waveforms with R_fixture values closest to the recommended R_fixture
values are chosen. For open-ended devices, only one (VCC or GND)
waveform is needed.

--------- end man page exerpt -----------

Abe Riazi wrote:
>
> Mark Nass Wrote:
>
> >> One thing I have always hated about IBIS models, at least the ones I have
> >>received, that nobody has mentioned is that they usually do not contain
> >>enough
> >>of the Waveforms to generate a good QUAD model. I always get the message,
> >>REVERTING
> >>TO RAMPS. This gives a terrible QUAD model, and of no use.
> >
> >>Mark
> >
> >
> Hi Mark:
>
> I have converted some IBIS models using IBIS2XTK and have seen the
> message:
>
> REVERTING TO RAMPS, as you had mentioned.
>
> There have been also some IBIS models that have yielded XTK models with
> DEF_RISE and DEF_FALL curves. It depends on the type of test fixture
> used by the model developer. Some test jigs are rejected by IBIS2XTK,
> and hence the rise and fall waveforms of the IBIS model are considered
> useless. When the test fixture (R_fixture, V_fixture, C_fixture, etc.)
> parameters meet certain criteria acceptable by IBIS2XTK , then it
> generates superior models with DEF_RISE and DEF_FALL sections.
>
> Regards,
>
> >Abe Riazi
>
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