Subject: Re: [vhdl-200x] Transmission gate mechanism and new parameter mechanism wanted
From: Jonas Nilsson (jonas@hardi.se)
Date: Fri Mar 21 2003 - 06:57:17 PST
"Paul J. Menchini" wrote:
> Erich is right. IIRC, one of the major problems is that causal loops
> can be created. Even if such loops are not created, the evaluation of
> generics becomes NP in the general case.
>
> Of course, such concerns are not reasons to reject this request out of
> hand....
Restrictions could be placed on how the "upward" parameters are used,
and how their values are assigned. (For instance: parameter values have
to be locally static in the lower level module or even restricted to be
declared in the entity before the generics, so they can't use generic
values or any static value calculated in the rest of the entity/architecture)
The drawback of restrictions like this is quite severe, since
it would prevents propagation of these values through modules,
both laterally and through hierarchy. But better ways could
perhaps be devised.
Figuring out the dependency graph during elaboration would be better
from a user standpoint. Even if this is an NP problem, real life
designs would probably contain a limited number of parameters that
needed this analysis.
The biggest problem I see is that the dependency graph can become
so complex that it's easy for the user to inadvertently create
loops but difficult to figure out how to break them when the
elaborator reports an error.
Regards,
Jonas
-- Jonas Nilsson HARDI Electronics AB Dalbyvagen 22 SE-224 60 LUND SWEDENPhone : +46-46-16 29 00 Fax : +46-46-16 29 01
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