[vhdl-200x-ft] Comments on FT11

Folks,

Jim asked me to think about FT11, association of expressions involving
signals with in-mode ports. I'd like to fly an idea past committee
members...

The desire in FT11 is to do something like:

  signal A, B : bit;
  ...

  UUT1 : UUT
    port map ( X => A and B, ... ); -- where X is an in-mode port

In our previous discussions on this, we considered expressing the semantics
in terms of an equivalent concurrent signl assignment to an anonymous signal
that is then associated with the port. This would introduce a delta delay,
which we decided to tolerate.

However, this introduces some anomalies, such as one mentioned in the FT11
proposal:

  UUT2 : UUT
    port map ( X => "not"(A), ... );

In this case, there is no delta delay, since the actual part is a conversion
function. Whereas:

  UUT3 : UUT
    port map ( X => not A, ... );

Here, there would be a delta cycle, since the actual part is an expression.

It occurs to me that the effect we really want is more akin to update of
guard signals. Consider the following model:

  B1: block ( A and B ) is
  begin
    UUT4 : UUT
      port map ( X => GUARD, ... );
  end block B1;

Here, when either of A or B is active, the GUARD signal is updated, causing
the effective value of the formal port X to be updated. This all occurs in
the same simulation cycle in which A or B is active. We avoid the anomaly
shown above.

We could define the semantics of associating an expression with an in-mode
port in terms of update of the effective value of the port. We would say
that an in-mode port that is associated with an expression is active if any
signal in the expression is active. This would be a simple extension of the
existing rule that says such a port is active if the associated signal is
active.

Furthermore, we may be able to subsume the semantics of type conversions and
conversion functions in actual parts into associating expressions as
actuals, since both type conversions and conversion functions are specific
cases of expressions.

What do folks think? Is this worth persuing, or is there something obvous
I've overlooked?

Cheers,

PA

--
Dr. Peter J. Ashenden                        peter@ashenden.com.au
Ashenden Designs Pty. Ltd.                   www.ashenden.com.au
PO Box 640                                   Ph:  +61 8 8339 7532
Stirling, SA 5152                            Fax: +61 8 8339 2616
Australia                                    Mobile: +61 414 70 9106