LRM guru question

[Jonathan Bromley]

Hi all,

Normally I reckon that I know my way around VHDL pretty well.
This one, however, has me stumped. I expect it's in the
LRM - most things are - but I can't find it.
Clarification welcome...

Suppose I have an entity and (to keep things simple) a
matching component declaration, and it has an unconstrained
array output port...

entity E is port (P: out bit_vector); end;

Now, if I instance it in an architecture, the unconstrained
array port is of course constrained to have the same subtype
as the signal connected to it...

....
signal V: bit_vector(2 downto 0);
....
inst: E port map (P => V);
-- port P is now constrained (2 downto 0) like V

However, suppose I now apply a conversion function to that
output port...

....
signal W: widget;
function F(BV: in bit_vector) return widget is...
....
inst: E port map (F(P) => W);

Since the unconstrained port is an *input* to the
conversion function, how does it get its subtype constraint?
Or is this illegal? Illegal sounds likely, but where is this
defined in the LRM?

Thanks in advance
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
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[Edwin Naroska]

Hi,

I'm no guru, but perhaps this is what you are looking for
(cited from LRM Section 3.2.1.1 "Index constraints and discrete
ranges"):

"If the index ranges for an interface object or member of
an interface object are obtained from the corresponding
association element (when associating in whole) or elements
(when associating individually), then they are determined
either by the actual part(s) or by the formal part(s) of
the association element(s), depending upon the mode of the
interface object, as follows:

...

- For an interface object or member of an interface
object whose mode is out, buffer, inout, or
linkage, if the formal part includes a conversion
function or a type conversion, then the parameter
subtype of that function or the type mark of the
type conversion must be a constrained array subtype,
and the index ranges are obtained from this constrained
subtype; otherwise, the index ranges are obtained from
the object denoted by the actual designator(s)."

 

[Jonathan Bromley]

(cited from LRM Section 3.2.1.1 "Index constraints and discrete
ranges"): [...]
if the formal part includes a conversion
function or a type conversion, then the parameter
subtype of that function or the type mark of the
type conversion must be a constrained array subtype,
and the index ranges are obtained from this constrained
subtype

Bulls-eye! Thanks very much.
--

Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * Perl * Tcl/Tk * Verification * Project Services

Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, Hampshire, BH24 1AW, UK
Tel: +44 (0)1425 471223 mail: (e-mail address removed)
Fax: +44 (0)1425 471573 Web: http://www.doulos.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Deb P]

Since the unconstrained port is an *input* to the
conversion function, how does it get its subtype constraint?
Or is this illegal? Illegal sounds likely, but where is this
defined in the LRM?

This is legal VHDL. Not sure how many simulators support this correctly though