RE: [vhdl-200x] VHDL enhancements wish list

David,

This sounds scary and error-prone to me. With this approach I have to
look all over the declarative regions inside and outside the entity (if
the attribute is specified where the entity is instantiated) to find out
what my clock is!

I much prefer a clocked process construct. The definition of this
construct in the LRM should include some way of communicating the clock,
reset, enable, etc. to operators within the process.

- Ryan

-----Original Message-----
From: owner-vhdl-200x@eda.org [mailto:owner-vhdl-200x@eda.org] On Behalf
Of David Bishop
Sent: Thursday, February 24, 2011 8:03 AM
To: vhdl-200x@eda.org
Subject: Re: [vhdl-200x] VHDL enhancements wish list

On 2/19/2011 1:23 AM, ben cohen wrote:

> The *always_ff * emphasizes the Flip-Flop implementation. I think
that
> this is nice. I gave you the details of meaning and checks if a FF is
> not implied.

This idea is nice, but may I propose an alternate?

First, lets add some predefined attributes:

   attribute default_clock : STD_ULOGIC;
   attribute default_reset : STD_ULOGIC;
   attribute default_enable : STD_ULOGIC;
   attribute default_reset_value : STD_ULOGIC;
   attribute default_clock_edge : STD_ULOGIC;

Attributes can be set on an entity, a process, or a group, or a signal
so I could write a template:

attribute default_clock of ent1 : entity is clk;
attribute default_reset of ent1 : entity is rst;

Where "clk" is the default clock that I want to use on entity ent1.

Then we would have to have a built in operator which could read these
attributes, and create the ff. I could see something like:

r2 <= ZT(r1, 2); or more simply:
r2 <= r1 @ 2;

Which would generate a FF, using the default_clock and default_reset
with a delay of 2 clocks. Since "default_enable" is not defined there
would be no clock enable, default_reset_value would default to '0', and
default_clock_edge would default to '1' or positive edge.

You could set the attribute on a signal, a process, a group, or an
entity, and search for them in that order. If "default_clock" is
undefined, then you get an error.

Yes, this would have to be a "special" operator because the language
currently can't support this.

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