Subject: Re: Related Question [Re: your question about digital variable access]
From: Martin O'Leary (oleary@cadence.com)
Date: Mon Mar 19 2001 - 18:46:10 PST
Ian,
my concern with requiring an explicit conversion element for every
cross-domain variable access is that it is somewhat cumbersome.
It will make mixed-signal modules more verbose and difficult
to read than they would otherwise be.
--Martin
On Mar 19, 4:15pm, Ian Wilson wrote:
> Subject: RE: Related Question [Re: your question about digital variable ac
> I'm reassured that everyone sees the problems inherent in the
> example (repeated below) - especially since this is by no
> means a pathological case, but was taken from a 'simple'
> tutorial example in a connect module.
>
> The correct solution, in my view, is to take the position that
> if you replace each occurrence of a 'wrong domain' entity with
> an explicit conversion element, then the two representations of
> the problem should return the same result. There are several
> alternative ways to think about this - the simplest is to recognize
> that when you need to access a variable of one domain from another,
> that both domains must agree about what time it is. As Kevin says,
> there are several ways to implement this, but we are not concerned
> with that here.
>
> Applying this to the original example:
>
> module mixed1(clk,vout);
> wire clk;
> electrical vout;
>
> integer np; initial np =0;
>
> always @(posedge clk)
> np = np + 1;
>
> analog V(a) <+ np;
>
> endmodule
>
> this can be formally rewritten as:
>
> module mixed1(clk,vout);
> wire clk;
> electrical vout;
>
> integer np; initial np =0;
>
> always @(posedge clk)
> np = np + 1;
>
> integer anp; // analog representation of np
> digital_to_analog(anp, np);
>
> analog V(a) <+ anp;
>
> endmodule
>
> The digital_to_analog operation provides the mixed-signal
> synchronization that ensures that changes to np result in a
> synchronization event between the analog and digital world,
> and for a transfer of data between np and anp.
>
> The synchronization necessary in all the cross-domain cases
> can be deciphered by rewriting similarly. The problem is to
> find words that express the required semantics accurately.
>
> --ian
>
>-- End of excerpt from Ian Wilson
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