[sv-ec] What is meant by "simple edge"?

Clause 14.3 refers to the situation where a clocking block clocking event
specifies a "simple edge".  This term is not defined, nor is it used
anywhere else in the LRM.  The situation is required before you can use
an alternate edge as a skew.  So what was it intended to mean?

My guess is that it refers to the situation where the clocking event is
simply an edge (posedge, negedge, edge or the implicit "any change" that
is indicated by the lack of an explicit edge) applied to an expression,
and nothing else.  It clearly must exclude any composite expression with
multiple events separated by event-OR.  I would assume that it also
excludes any event conditioned with "iff", but this is not as clear.

I assume this partly because it doesn't seem to match the description
"simple edge".  Also, if an event conditioned with "iff" were allowed,
it doesn't seem clear what the skew would be.  Would you keep the iff
condition with the new edge, or remove it?  Since this is unclear, it
seems that this situation is more complex than was intended to be allowed.

BTW, the wording of this sentence in the LRM is also confusing.  It says

"When the clocking event specifies a simple edge, instead of a number,
the skew can be specified as the specific edge of the signal."

It seems to be saying that the simple edge is instead of a number, which
makes no sense.  The clocking event cannot be a number.  Only on repeated
reading did I decide that it was saying that instead of the skew being
specified as a number, the skew can be specified as the specific edge of
the signal.  This would have been clearer if that phrase were moved to the
end of the sentence, where it is more clearly associated with the second
half:

"When the clocking event specifies a simple edge, the skew can be specified
as the specific edge of the signal, instead of a number."

This still seems to be slightly incorrect.  The BNF allows specifying an
edge AND a number.  I presume that this gives an additional skew from the
specific edge of the signal.  So it really ought to say

"When the clocking event specifies a simple edge, the skew can be specified
as the specific edge of the signal, in addition to a number."

Can somebody clarify what was intended by this text?

Steven Sharp
sharp@cadence.com


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