[sv-ec] When do clocking output skew 0 values show up on a var output?

The 890 proposal claims the following:

    When more than one synchronous drive on the same clocking block
    output (or inout) is scheduled to mature at the same simulation time,
    the last value is the only value driven onto the output signal.

and

    When multiple drives are applied to a given clocking output at
    different times within the same clock cycle (i.e., between clocking
    events) then the last value written is the only value driven onto
    the output signal.

For the "normal" cases of having drives appear before the clocking
event, this is fine.  But consider the following:

     bit a = 1;
     default clocking cb @(posedge clk);
         output a;
     endclocking

    initial begin
      ## 1;
      cb.a <= 0;
      @(x);
      cb.a <= 1;
    end

Assume that the above is all active region set code but that
"x" is an event that happens as a result of re-active
stimulus in the same time step.

This is a bit contrived ("a bit"?!?), but something along this
line could happen with multiple processes in the system.

So - does "a" glitch?

The 890 text I quoted claims that it does not and that only
the *last* value is driven.  So when does this drive occur?
It certainly cannot occur in re-nba since then the "0" would
occur before the "cb.a <= 1" and cause a glitch.

In general the "last write is the only write" can't possibly
be guaranteed since the simulator can't possibly know which
write is the last one (halting problem anyone?).


We need to be really careful here.  I would like to say
"only the writes prior to the cb event are considered in
this cycle" but that means that an intuitive write such
as "##1 cb.a <= 1" would not occur when the user expects.

Another alternative is to say that at the cb event the
most recent value is written (as a re-nba write) and that
subsequent writes to cb.a while cb.triggered is true are
ALSO scheduled as re-nba writes.  This means that "a" can
glitch, but only in edge case scenarios.  It does imply
that the current wording of 890 needs to be weakened to not
guarantee that only the last write shows up.

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


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