Re: gEDA-dev: Icarus verilog EDIF question

[Stephen Williams <steve-at-NOSPAMicarus_dot_com>]

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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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