Re: gEDA-dev: Icarus verilog EDIF question

["Chris H" <bitmechanic-at-NOSPAMhotmail_dot_com>]

Hello,

Would it be possible to have a (* *) synthesis attribute to prevent the 
synthesis of the inverter for a ~Rst associated with negedge Rst and instead 
attach a 'polarity Invert' port attribute ?

Chris


>From: Stephen Williams <steve@icarus.com>
>To: Chris H <bitmechanic@hotmail.com>
>CC: geda-dev <geda-dev@seul.org>
>Subject: Re: Icarus verilog EDIF question
>Date: Thu, 05 Jul 2007 07:54:59 -0700
>
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>It is added in the synthesizer in the core compiler. It's just
>blindly synthesizing the "~Rst" expression for you, so the edif
>code generator only sees astive high DFF inputs.
>
>Chris H wrote:
> > Hello,
> >
> > I'm using the EDIF target, i.e. -tedif
> >
> > I present the two cases:
> >
> >    // Case 1: DFF with async clear active high
> >    always @ (posedge Clk or posedge Rst) begin
> >        if (Rst)
> >            Q <= 1'b0;
> >        else
> >            Q <= D;
> >    end
> >
> >    // Case 2: DFF with async clear active low
> >    always @ (posedge Clk or negedge Rst) begin
> >        if (~Rst)
> >            Q <= 1'b0;
> >        else
> >            Q <= D;
> >    end
> >
> > For Case 1 the EDIF will show a DFF with active high Aclr. For Case 2
> > EDIF will also show a DFF with active high Aclr, and an inverter will be
> > added an attached to Aclr.  It would be preferable if EDIF used the
> > 'polarity Invert' attribute on the Aclr port instead of adding the
> > inverter. This may be even more 'correct' in terms of the LPM spec but
> > I'm not sure.
> >
> > In my library I only have a DFF with *active low* Aclr. Having the
> > polarity attribute makes it possible for back-end tools to handle this
> > correctly.  Otherwise I need to find a way to remove the added inverters
> > every time I generate EDIF which is yucky.
> >
> > I've looked at the source code but it's not readily apparent to me where
> > the inverter gets synthesized. I'd be happy to add a EDIF option to add
> > the polarity attribute instead if someone could tell me which source
> > code files to look at.
> >
> > Thanks!
> > Chris
> >
> > _________________________________________________________________
> > http://im.live.com/messenger/im/home/?source=hmtextlinkjuly07
> >
>
>
>- --
>Steve Williams                "The woods are lovely, dark and deep.
>steve at icarus.com           But I have promises to keep,
>http://www.icarus.com         and lines to code before I sleep,
>http://www.picturel.com       And lines to code before I sleep."
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