From: Michael McNamara (mac@verisity.com)
Date: Wed Jul 10 2002 - 12:46:11 PDT
Precedence: bulk
My initial intuitive impression is that variable delays should not be
automatically included in @* sensitivity lists.
-mac
Steven Sharp writes:
> Precedence: bulk
>
> Reading the recent list of errata that didn't make it into the printed
> standard brought up an issue for me.
>
> There are two descriptions in section 9.7.5 of what goes into the
> implicit event_expression list for @*. The first is an "everything but"
> description, which includes all net and reg identifiers except for those
> in wait or event expressions, or that are lvalues.
>
> The second description lists nets and variables that are supposed to be
> included by the rules.
>
> Unfortunately, there are cases that are included by the first description
> but not included by the second one. I had assumed that the second
> description was intended as an incomplete set of examples of what might
> get included by the first description (which was the official rule).
> However, the errata modify the second description to add identifiers used
> in index expressions on the left hand side of assignments. The fact that
> such a correction was made implies that the second description was intended
> to be a complete one.
>
> One case that is unclear is identifiers used in delay controls. These are
> identifiers that are not excluded in the first description, but are not
> mentioned in the second description. Are they supposed to be included in
> the sensitivity list?
>
> Example:
>
> always @*
> a = #(delay) b;
>
> Should "delay" be included in the sensitivity list?
>
> Note that in the similar situation in a continuous assignment
>
> assign #(delay) a = f(b);
>
> changes in the value of delay do not cause the continuous assignment to
> be evaluated (at least not in Verilog-XL or NC-Verilog). This can be
> seen by putting a $display in the function f, or by testing the delay
> behavior. Based on this, I would expect that delay controls should not
> contribute to the implicit event_expression list.
>
> Other opinions? Should we add this as an erratum? What if we find other
> cases where these descriptions disagree?
>
> Steven Sharp
> sharp@cadence.com
>
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