( ESNUG 243 Item 4 ) ---------------------------------------------- [7/17/96]
Subject: (ESNUG 240 #7 242 #6 ) Simulators Giving Frustrating X's
> If I have a 2-input multiplexer with zero's on both inputs and an 'X' on
> the select line, should I get a zero out? The answer is probably yes, but
> most models of the multiplexer would put out an 'X' if the select line is
> unknown regardless of the inputs instead of handling the special case of
> all inputs being zero (or one, for that matter). ... In some cases the
> 'X' would be the better answer; if the select line was metastable and was
> not at a legal logic level, then the output would be unknown.
From: peer@iis.fhg.de (Dieter Peer)
John,
This is right from a purely logic standpoint. But are you aware, that in
real world an 2-input multiplexer MAY definitely produce a spike if you
leave both inputs on the same level and switch the select line?
Although this spike is not X it can cause lots of trouble in the logic
that is fed from the mux's output.
- Dieter Peer
Fraunhofer-Gesellschaft
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From: Jeff Nuckols <jnuckols@aircom.com>
John:
I understand the frustration with getting X's from a simulator because one
of the logic inputs is unknown even though I know real gates will give me a
predictable output. At the same time, though, this feature of simulators has
exposed for me an *unexpected* unknown on one of my inputs that would have
been masked if the simulator had behaved more like real gates. So, for me,
this simulation behavior is useful.
- Jeff Nuckols
AirNet Communications Corporation
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From: victor_duvanenko@truevision.com (Victor Duvanenko)
John,
On the subject of X's in your simulation... About 4 years ago several
of us discovered this, and it took us a while to convince Synopsys that
this was a problem with "synthesis and then gate-level simulation
methodology". Synopsys wrote up a very nice App note that they should be
handing out to every customer. However, the conclusion was that there is
nothing wrong with the circuit and it will work perfectly fine, but that
the problem is with simulation (and as was said before maybe the simulation
vendors will take note and improve the simulators).
- Victor J. Duvanenko
Truevision
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