Do synthesis tools now, or are they envisioned to in the
future, handle user defined primitives as input? As you know, one can
use '?' in UDP input specifications to indicate a don't care
conditions. One can also use 'x' as input, but presumably such a table
entry is not expected to be used for synthesis.
The only other place x is used as don't care in Verilog (that
comes to mind) is in the PLA system tasks. Presumably these are
ignored by synthesis tools, although one might imagine a tool offering
to reduce these for the user. Different domain, no doubt.
>
> Here is an UPDATED proposal, to add to section 5, on how x and z
> values are to be interpreted for synthesis.
>
> --------------------------
>
> 5.5 Support for meta-logical values, x and z
>
> The value x may be used only as a primary on the RHS of an
> assignment to indicate a dont care value for synthesis.
>
> The value x may be used in case item expressions in a casex
> statement to imply a dont care value for synthesis.
>
> The value x may be used in a case item expression in a case
> statement; however such a branch is never expected to occur for
> synthesis.
>
> The value x shall not be used with any operators or mixed with
> other expressions.
>
> The value z may be used only as a primary on the RHS of an
> assignment to infer a three-state driver as described in section
> 5.4.
>
> The value z (or ?) may be used in case item expressions for casex
> and casez statements to imply a dont care value for synthesis.
>
> The value z may be used in a case item expression of a case
> statement; however such a branch is never expected to occur for
> synthesis.
>
> The value z shall not be used with any operators or mixed with
> other expresions.
>
> -------------------------
>
> Please provide your comments on this UPDATED proposal by March 15.
>
> - bhasker
>
>
> ----- Begin Included Message -----
>
> >From owner-vlog-synth@eda.org Mon Mar 8 16:16 EST 1999
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> Date: Mon, 8 Mar 1999 16:12:11 -0500 (EST)
> From: Jayaram Bhasker <jbhasker>
> To: vlog-synth@eda.org
> Subject: Support for values x and z
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> Sender: owner-vlog-synth@eda.org
> Content-Type: text
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>
> Here is a proposal, to add to section 5, on how x and z values are to be
> interpreted for synthesis.
>
> --------------------------
>
> 5.5 Support for non-logical values, x and z
>
> The value x may be used only as a primary on the RHS of an
> assignment to indicate a dont care value for synthesis.
>
> The value x shall not be used with any operators or mixed with
> other expressions.
>
> The value z may be used only as a primary on the RHS of an
> assignment to infer a three-state driver as described in section
> 5.4.
>
> The value z shall not be used with any operators or mixed with
> other expresions.
>
> -------------------------
>
> Please provide your comments on this proposal by March 15.
>
> - bhasker
>
>
> ----- End Included Message -----