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

Gord,

Ouch.

> For the "normal" cases of having drives appear before the clocking
> event, this is fine.

I don't think that *is* the "normal" case.  Recall that we 
should not attempt to read input clockvars until after @cb,
because otherwise we may have a read/update race.  But I
probably want to *use* the input clockvar values in 
constructing my output stimulus.  So the typical flow
of execution of a reactive testbench (reactive in the sense 
that it reacts to the DUT's behaviour) - for example, one
doing handshaking on a DUT output - would be...

  @cb;
  local_variable = cb.invar;
  cb.outvar <= some_function(local_variable);

Using this idiom, I get testbench-controlled feedback from 
DUT outputs back to DUT inputs corresponding to a logic 
path with a register in it.  (Note, in passing, that I 
can't directly use a clocking block to mimic combinational 
feedback from DUT output to DUT input, unless I use 
the dreaded input #0.)


>     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.

I agree with your analysis, but I think there's a way out.  
As you say, this is a *very* forced example.  The key point, 
though, is that we're making two separate passes through 
Re-NBA.  The guarantee of last-write-wins can only reasonably
be enforced for a single pass through Re-NBA.  For 
non-pathological code, this (I think) is OK.  So all we need
to do is to find a form of words that expresses this....

I think your last paragraph is close.  Maybe we could just
avoid making any explicit claims about last-write-wins, and
simply say that a clocking drive schedules an assignment to
its target clocking signal in the Re-NBA region of the 
appropriate timeslot, and that assignment shall supersede 
any other pending assignment to the same variable in 
that Re-NBA region?

It just emphasises once again that the only really hygienic
usage of clocking blocks is to have their clock event happen
in the active region set, and all manipulation of their 
clockvars happen in the reactive region set.
-- 
Jonathan Bromley, Consultant

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