VHDL Issue Number:      2008

Language_Version:       VHDL-93
Classification:         Language Definition Problem
Summary:                Source value of undriven, non-sourced INOUT, OUT or BUFFER port
Relevant_LRM_Sections:  4.3.1.2, 12.6.2
Related_Issues:
Key_Words_and_Phrases:  source, port, INOUT, OUT, BUFFER
Authors_Name:           Stephen Bailey
Authors_Phone_Number:   303-588-2001
Authors_Fax_Number:     303-652-1578
Authors_Email_Address:  stephen@srbailey.com
Authors_Affiliation:    Synopsys Inc.
Authors_Address1:       6664 Cherokee Ct.
Authors_Address2:       Niwot, CO 80503
Authors_Address3:

Current Status:         Analyzed

Superseded By:

------------------------
Date Submitted:         1 November 2000
Date Analyzed:          29-Oct-04
Author of Analysis:     Peter Ashenden
Revision Number:        2
Date Last Revised:      24-Nov-04

Description of Problem
----------------------
Take the following example:

    entity foo is
      port ( IO : inout std_logic;
              O : out   std_logic );
    end entity foo;

    architecture A of foo is
    begin
      O <= not IO;
    end architecture A;

    entity top is
    end top;
    architecture A of top is
      signal IO_actual, O_actual : std_logic;
    begin
      UUT : work.foo(A) port map (IO_actual, O_actual);
      IO_actual <= '1', '0' after 100 ns;
    end architecture A;

The LRM states in section 4.3.1.2 that the INOUT port IO creates a
source for IO_actual.  But, there is no driver defined for IO within
FOO(A).

The LRM also states in 12.6.2 that:

    "If S is a resolved signal and has one or more sources, then the
driving values of the sources of S are examined. ...  [T]he driving
value of S is obtained by executing the resolution function associated
with S, where that function is called with an input parameter
consisting of the concatenation of the driving values of the sources of
S...."

    My question is, where does the LRM define what the *driving value*
of O is and, therefore, what value IO contributes as a source for the
resolution and computation of the effective value of IO_actual?

    All implementations that I'm aware of, except for 1, applies the
default initial value ('U' in this case) as the source value of IO.
The exception implements this situation as IO contributing no value to
the resolution of IO_actual.  Therefore, in all but the exceptional
case, IO_actual remains 'U' during simulation.  In the exceptional
case, IO_actual toggles as though it has only one driver/one source.


Proposed Resolution
-------------------

I believe the intent of the LRM is that the source value of a port in
this case is the initial value of the port.  What is missing from the
LRM is a statement in section 12.6.2 that the source value of an
INOUT, OUT or BUFFER mode port that has no drivers and no sources is
the initial value of the port.

Alternatively, a note covering this situation might be sufficient and
probably belongs in section 12.6.2

VASG-ISAC Analysis & Rationale
------------------------------

The submitter poses the question, "where does the LRM define what the
*driving value* of O is, ..." and then proceeds to ask about the value
contributed by IO.  This analysis assumes that the submitter intended
to ask about the driving value of IO, not of O, since the O is driven
by the concurrent signal assignment within architecture A.

12.6.2, on page 173, specifies how the driving value is determined for
basic signals.  The first list item is:

  -- If S has no source, then the driving value of S is given by the
     default value associated with S (see 4.3.1.2).

Since the port IO in the submitter's example is a basic signal with no
source, its driving value is determined in this manner.

The default value for a port is specified in 4.3.2, page 64:

  In an interface signal declaration appearing in a port list, the
  default expression defines the default value(s) associated with the
  interface signal or its subelements. In the absence of a default
  expression, an implicit default value is assumed for the signal or
  for each scalar subelement, as defined for signal declarations (see
  4.3.1.2). The value, whether implicitly or explicitly provided, is
  used to determine the initial contents of drivers, if any, of the
  interface signal as specified for signal declarations.

Thus, in the submitter's example, the default value for IO is 'U', and
this is the driving value of IO.



VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------

No change.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------

Add a Note to the Note list in 4.3.2, page 65:

  5----The driving value of a port that has no source is the default
  value of the port (see 12.6.2).

Add a Note to the Note list in 12.6.2, page 175:

  9--The driving value of a port that has no source is the default
  value of the port (see 4.3.2).


-------------END OF IR----------------