[SI-LIST] Re: The stimulus rise/fall time in hspice
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, <zanglinyuan@xxxxxxxxxx>
- Date: Wed, 13 Nov 2002 11:12:55 -0500
> when making simulation in Hspice simulator,we need add a stimulus
> source to drive the circuit.
> what's the right way to determine the rise/fall time of stimulus?
> And are there any difference between IBIS model and spice model ?
Yes it differs between IBIS models and Spice models.
The stimulus to an IBIS model in Hspice is treated by the model as a
binary signal. Once it crosses the binary threshold, it doesn't matter
how fast it is moving. Different edge rates only cause the signal to
reach the binary threshold at slightly different times, which you should
compensate for. Zero rise/fall times would remove any uncertainty about
the exact time when that happens; but extremely fast rise/fall times can
be problematic in Spice, even if they happen in a totally different
section of the circuit being simulated. So use something reasonably
fast.
Structural SPICE models, on the other hand, CAN be affected by input
rise/fall times, depending on how many stages there are in the SPICE
model. Try some test simulations over a wide range of rise/fall times
to see what effect it has.
The more effect it has, the more you want to replicate the waveforms
that would have been present at the input to the buffer, if it were
still being driven by the rest of the circuit. Check with whoever made
the SPICE model for their recommendation. If you are designing an ASIC,
see if the ASIC design tools (for timing verification) also tell you the
estimated rise/fall times at that point in the circuit, which could
differ for each buffer.
Regards,
Andy
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