Subject: RE: [IBIS-Users] FW: Modelling an open drain driver
From: Ingraham, Andrew (Andrew.Ingraham@hp.com)
Date: Thu Nov 21 2002 - 08:16:44 PST
> 1)
> I'm in doubt how to write the S2IBIS setup file (.s2i) for an OD
> driver.
> I know that I should use the 'Open_sink' modeltype, but how should I
> define the rising and falling waveforms.
> I would assume that R_fixture = 60 ohm and V_fixture = Vpullup
For R_fixture, using 60 ohms in parallel with Rpullup, would come closer
to the actual circuit. Unless you intended for Rpullup to be on the far
(receiver) end of the 60 ohm trace.
> Moreover I imagine that I should only do the falling waveform and not
> the rising, which would be controlled by the external pullup resistor.
No; I think you should have both rising and falling waveforms. While
the external circuit dominates the rising edge, it doesn't control it
100%. Driver capacitance affects it. If the driver transistors turn
off slowly, they may affect it too.
As a user, I would definitely want to simulate rising edges, so I'd want
the IBIS model to be accurate. Include rising waveforms.
> 2)
> Additionally it is possible to include both cases in the same model,
> i.e. Vpullup equals either 2.5V or 3.3V
Yes. IBIS models let you include up to 100 waveform tables per model.
> and would a simulator be able to select the right waveforms to use?
It has the means to, but this depends on the simulator. Some have
limitations. (HSpice, for example, can use at most two waveform
tables.)
Regards,
Andy
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