[sv-ec] input skew 0 clocking block behavior

I know this is a bit late for the discussion today, but I wanted
to bring this up on the reflector so that people that aren't going
to attend are aware of the issue.

The 890 proposal says:
    Upon processing its specified clocking event, a clocking block shall
    update its sampled values before triggering the event associated with
    the clocking block name.

Question - does this apply to #0 input skews?  If not, I think that
this should be explicitly clarified.  If so, then I don't see how
one can claim that  @(cb) is equivalent to waiting on the clocking
event expression.  #0 input skews aren't updated until Observed;
if the update is *required* to occur before the @(cb) is triggered
then @(cb) can't occur until some point during or after observed.
But that is much later (in general) than the actual event.

So, I can't align these requirements.

A few options that I see:
   1) explicitly exclude #0 inputs from the "update before event" rule.
   2) remove the event equivalence claim and explicitly require
      the cb event to lag until after observed
   3) disallow use of a #0 skewed input outside of a program.

I think that (2) is likely quite undesirable; the event lag would
have surprising effects since the event would not occur until
after the nba region, etc.

(1) makes sense to me in terms of "active" region behavior.  The
upshot of (1) would be that use of a #0 input might not have
well defined behavior (we could explicitly state that) when
the event and the use of the input were both in the active set.
If the event is in the active region and the input use is in
reactive, then all is well.  Since that seems to be the most
useful scenario in any case, I don't mind accepting (1).
Alternative (3) would just tighten up (1) and disallow active
region use of a #0 input.

We could always go with other suggestions to just kill #0 inputs....

Gord.
-- 
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Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com


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