Re: [vhdl-200x] FSM safe design

All,
Adapting from Brent's, let's say we have the following typical FSM
description:
< type my_fsm is (a, b, c);
> ...
>     case my_fsm_signal is
>        when a => if (start = true) then my_fsm_signal <= b; end if;
>        when b => my_fsm_signal <= c;
>        when c => my_fsm_signal <= a;
>        when others => my_fsm_signal <= a;
>     end case;

Perhaps it's a good idea to coerce synthesis tools to implicitly implement
safe statemachines whenever the designer explicitly writes the "when
others" clause.

I bumped into these issues as well, and my solutions at that time were to
explicitly write extra logic that implements the catch-all condition
(either using if-else statements, or using synthesis attributes), but I
admit, these solutions are ugly as they tend to clutter the code and make
the design less readable.

I would very much prefer if synthesis tools, by default, are intelligent
enough to implement safe statemachines on its own.

Best regards, Daniel


On 20 February 2014 20:15, Brian Drummond <brian@shapes.demon.co.uk> wrote:

> On Thu, 2014-02-20 at 00:42 +0000, Brent Hayhoe wrote:
> > State machine design in VHDL can be performed in quite an elegant manner
> IMHO.
> > That is using the provision of enumerated types.
> >
> > However, there is a problem which some people are still unaware of.
> >
> > I remember questioning various vendors about safe FSM design in the late
> > nineties and only one had any sort of provision for it at that time. The
> > situation has since improved.
> >
> > The classic VHDL problem is this:
> >
> >     type my_fsm is (a, b, c);
> > ...
> >     case my_fsm_signal is
> >        when a => if (start = true) then my_fsm_signal <= b; end if;
> >        when b => my_fsm_signal <= c;
> >        when c => my_fsm_signal <= a;
> >     end case;
>
> > Although there are methods of adding synthesis attributes to handle
> this, it
> > would be nice to incorporate it within the language semantics, e.g;
> >
> >      when others safe(my_fsm_signal <= a);
> >
> > Has anyone else had any thoughts regarding this issue?
>
> I like the intent, but I wouldn't want to add reserved words or syntax
> to the language to handle it. Adding an attribute "safe" or
> "full_case_coverage" to the enumerated type and using a plain "when
> others" case seems more appropriate.
>
> Define a standard attribute for the purpose, establish its intent,
> (namely, to suppress "redundant clause" optimisations and implement full
> coverage. A warning describing the excess resources over "optimal"
> solution might be appropriate) Then work with (at least some) tool
> vendors to support it, and let peer pressure bring the rest on board.
>
> If there already is a named standard attribute for the purpose, ... then
> all I can say is, there isn't enough noise about it yet!
>
> - Brian
>
>
>
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