Subject: Re: FSM Enhancement Goals and Thoughts
From: Stuart Sutherland (stuart@sutherland-hdl.com)
Date: Thu Dec 13 2001 - 09:33:39 PST
IMHO, yes, at the language level we need to ensure that what we add to
SystemVerilog will be well supported by software tools. I think what the
whole discussion has revolved around coming up with a state machine style
that allows BOTH a higher level of abstraction, and the ability to tweak
bits, if desired--without having to totally rewrite the code if there is a
need to migrate from abstract to more detail. I think the combination of a
generic enumerated state type with attributes, coupled with the ability to
specify bits/bit-selects of that type, would give us both levels of
abstraction. The other criteria that has been discussed, is that the FSM
style needs support abstract and more detailed specifications of
reset--again without having to totally rewrite the code to add reset detail.
Stu
At 08:47 AM 12/13/2001, Adam wrote:
> >But a design with enumerated onehot state assignments (such as
> enum >{S0=4'b0001, S1=4'b0010, S2=4'b0100, S3=4'b1000} state;) will not
> >synthesize nearly as efficient as adding (* synthesis fsm_style =
> >onehot *), and per my experience, telling the synthesis tool
> to >synthesize the onehot FSM rarely yields the same efficient results
> >that I achieve when use indexed parameters and case (1'b1).
>
>This is a critique of synthesis tools.
>
>
>I do understand that in the real world, the synthesis tools are not
>going to change by the time that you need your design to meet its
>physical metrics and so you will recode to meet the requirements.
>But at the language level, must we also concede this point?
>
>Should it be possible for a tool to receive an attribute (a hint)
>that the encoding should be synthesized as a decoded (one hot) style,
>and the result be the same as if the code explicitly checked a single
>bit?
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Stuart Sutherland Sutherland HDL Inc.
stuart@sutherland-hdl.com 22805 SW 92nd Place
phone: 503-692-0898 Tualatin, OR 97062
www.sutherland-hdl.com
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