VHDL Issue Number:      0010
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                The disconnection time of a guarded signal is unclear
                        when the signal is associated through a port map.
Related Issues:         0063
Relevant LRM Sections:  5.3, 9.5, 12.3.2.3
Key Words and Phrases:  Disconnection specification, interface signals,
                        guarded signals, signal association
Current Status:         ISAC-Approved
1076-1993 Disposition:  closed
Disposition Rationale:  Issue has been addressed
Superseded By:          N/A
-----------------------
Date Submitted:         1988/11/09
Author of Submission:   Daniel Barclay
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  15245 Shady Grove Avenue, Suite 310
                        Rockville, MD  20850-3344
Author's Phone Number:  (919) 361-1913
Author's Fax Number:    
Author's Net Address:   daniel@clsi.com
-----------------------
Date Analyzed:          1991/01/11
Author of Analysis:     Paul Menchini (mench@clsi.com)
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/07/26 18:23:17 $


Description of Problem
----------------------
It appears that more than one disconnection specification is applicable to a
signal when the signal is associated through a port map.  Consider the
following example:

  B1:   block
          function ResFn (V: Bit_Vector) return Bit is ... end ResFn;
          signal S1: ResFn Bit bus;
          disconnect S1: Bit after 10 nS;
        begin
    B2:   block (Ena = '1')
            port (S2: out ResFn Bit bus);
            port map (S2 => B1.S1);
         -- disconnect S2: Bit after 0 nS;      -- implicit disconnect. spec.
          begin
            S2 <= guarded '1' after 42 nS;
         end block B2;
        end block B1;

In the guarded signal assignment statement's equivalent process statement,
what turn-off time is used?  Is the explicitly specified time for B1.S1 (10 nS)
used, or is the implicitly specified time for B2.S2 used (0 nS)?

(If an explicit disconnection specification for S2 was given, and the time was
other than 10 nS, the situation would be even more confusing.)

LRM Section 12.3.2.3, item 3 states:

        "The disconnection time is associated with each affected signal
         for later use in constructing disconnection statements in the
         equivalent processes for guarded assignments to the affected
         signals."

In Section 9.5, paragraph 14:

        "... [the] time expression [of the waveform of the sequential
         signal assignment forming the disconnection statement] is
         given by the applicable disconnection specification."

Section 5.3, paragraph 8 states:

        "It is an error if more than one disconnection specification
         applies to drivers of the same signal."

However, "applicable disconnection specification" is not defined.  Moreover,
nowhere in the LRM is the effect of signal association on disconnection
times of the associated signals discussed.


Proposed Resolution
-------------------
Use as the turn-off time the value supplied by the disconnection specification
applicable to the local signal.  In the above example, the applicable turn-off
time is 0 nS.  State this explicitly in Section 5.3.


VASG-ISAC Analysis & Rationale
------------------------------
The ISAC believes that the LRM is clear on this point; furthermore, there are
no inconsistencies in the LRM's approach.  Disconnection specifications
corresponding to a given signal are used only to construct the equivalent
processes for guarded signal assignment statements that drive that signal;
hence, any disconnection specifications applicable to any signals associated
with the signal via association lists are not germane.

To illustrate, here is the process equivalent to the guarded signal assignment
statement in the above example:

        process
        begin
          if GUARD then
            S2 <= '1' after 42 nS;
          else
            S2 <= null after 0 nS;
          end if;
          wait on GUARD;
        end process;

Note that S2, and not B1.S1, is driven by the guarded signal assignment
statement; therefore, it is S2's disconnection specification that applies.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
No changes need be made to the LRM or any interpretations with respect to this
issue.  However, see issue 0063 regarding "applicable disconnection
specifications."


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
No changes need be made to the LRM or any interpretations with respect to this
issue.  However, see issue 0063 regarding "applicable disconnection
specifications."