Re: [Iverilog-devel] What is a constant?

[Guy H. <ghu...@gm...>]

Hi Martin,

Variable bit indexes are not synthesizable, although I have not tried indexes using a generate.  If they are supported by synth that would be the most straightforward approach.  At present I don't have access to a synthesis tool; I'll have to try this next time one is available.

It's true that it is actually a priority encoder; since bit selects were already available this was easier than an encoded mux.

Guy 

On Mar 23, 2012, at 5:20 PM, Martin Whitaker <mai...@ma...> wrote:

> Guy Hutchison wrote:
>> Good catch by all replies, changing this to:
>>   generate
>>     for (g=0; g<inputs; g=g+1)
>>       begin : breakout
>>         wire [width-1:0] insel;
>>         if (g==0)
>>      assign insel = (rr_state[0]) ? c_data[width-1:0] : {width{1'b0}};
>>         else
>>           assign insel = (rr_state[g]) ? c_data>>  (g*width) :
>> breakout[g-1].insel;
>>       end
>>   endgenerate
>>   assign p_data = breakout[inputs-1].insel;
>> 
>> Fixes the access-zero problem, as zero is the special case.
>> 
>> As to why I am using this goofy structure, it is because building a
>> parameterized mux is a little painful if you have to have constant bit
>> selects, so I'm forced in to more odd constructions.
> 
> You've actually coded a priority encoder. If you really want a mux, it's a 
> fairly simple change:
> 
>   generate
>     for (g=0; g<inputs; g=g+1)
>       begin : breakout
>         wire [width-1:0] insel;
>         wire [width-1:0] out;
>    assign insel = {width{rr_state[g]}} & c_data[width*g +: width];
>         if (g==0)
>      assign out = insel;
>         else
>           assign out = insel | breakout[g-1].out;
>       end
>   endgenerate
>   assign p_data = breakout[inputs-1].out;
> 
> This should synthesise without any trouble.
> 
> If you don't need a one-hot coded select, you can always write:
> 
>   assign p_data = c_data[rr_index*width +: width];
> 
> although I've never tried synthesising a construct like this.
> 
> Martin
> 
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