Re: [myhdl-list] A puzzling error

[garyr <ga...@fi...>]

----- Original Message ----- 
From: "Christopher Felton" <chr...@gm...>
To: <myh...@li...>
Sent: Saturday, February 16, 2013 12:31 PM
Subject: Re: [myhdl-list] A puzzling error


> On 2/16/13 8:57 AM, garyr wrote:
>>
>> ----- Original Message -----
>> From: "Christopher Felton" <chr...@gm...>
>> To: <myh...@li...>
>> Sent: Saturday, February 16, 2013 4:37 AM
>> Subject: Re: [myhdl-list] A puzzling error
>>
>>
>>> On 2/15/13 10:28 PM, garyr wrote:
>>>> Below is a portion of the code for a multiplier module that works fine in 
>>>> its
>>>> test bench. It also worked OK as part of a lowpass filter module; that is,
>>>> the
>>>> filter module test bench didn't reveal any problems. But when the filter
>>>> module
>>>> is converted to Verilog an error occurs. The -547 is the value assigned to
>>>> signal 'multiplier'. As you can see it wasn't negated properly; instead of
>>>> (--547) it should be (547). Is this a bug or have I screwed up somewhere?
>>>>
>>>>           elif start == ACTIVE_LOW:
>>>>               done.next = INACTIVE_HIGH
>>>>               result.next = 0
>>>>               if multiplier >= 0:
>>>>                   mpyVar.next = multiplier
>>>>                   negate.next = 0
>>>>               else:
>>>>                   mpyVar.next = -multiplier
>>>>                   negate.next = 1
>>>>
>>>>       else if ((FA_FO_startm == 0)) begin
>>>>           FA_FO_donem <= 1;
>>>>           FA_FO_result <= 0;
>>>>           if ((-547 >= 0)) begin
>>>>               FA_FO_MPY_mpyVar <= -547;
>>>>               FA_FO_MPY_negate <= 0;
>>>>           end
>>>>           else begin
>>>>               FA_FO_MPY_mpyVar <= (--547);
>>>>               FA_FO_MPY_negate <= 1;
>>>>           end
>>>>
>>>
>>>
>>> Why use "--"?  I did the following and it converted ok:
>>>
>>>
>>> ~~~~[MyHDL]~~~~
>>>      def m_neglit(clock, reset, ival, tval, oval, T=544):
>>>
>>>          @always_seq(clock.posedge, reset=reset)
>>>          def hdl():
>>>              if ival > (tval << 2):
>>>                  oval.next = -T
>>>              else:
>>>                  oval.next = --T
>>>
>>>          return hdl
>>>
>>> ~~~~[Verilog]~~~~
>>>      always @(posedge clock, negedge reset) begin: M_NEGLIT_HDL
>>>          if (reset == 0) begin
>>>              oval <= 0;
>>>          end
>>>          else begin
>>>              if ((ival > (tval << 2))) begin
>>>                  oval <= (-544);
>>>              end
>>>              else begin
>>>                  oval <= (-(-544));
>>>              end
>>>          end
>>>      end
>>> ~~~~~~~~~~~~~~~~~
>>>
>>> I tested it with using "T" and using the literal 544
>>> directly in the MyHDL source both methods convert fine.
>>>
>>> But I imagine your question is: why does it convert to
>>> (-(-544))?  The converter, typically, will not evaluate
>>> the code in the generators, it will convert it (translate
>>> line by line), essentially as is (only true for the code
>>> in the generators).  And the Python parsing does not simplify
>>> this expression either, so it is then converted to (-(-544)).
>>>
>>> The next question is: is this bad?  I don't think so, the
>>> converted Verilog behaves the same as the MyHDL and there
>>> should be no issues with synthesis.
>>>
>>> Regards,
>>> Chris
>>
>> Sorry, I didn't fully describe the problem.
>>
>> The signal 'multiplier' received its value from something like:
>>      multiplier.next = int(-0.26757*DATASF)
>> where DATASF had a value of 2**11. MyHDL generated the Verilog text shown 
>> above
>> with two consecutive minus signs. The problem arises when this Verilog is
>> loaded into Quartus:
>>
>> Error (10170): Verilog HDL syntax error at lowpass.v(334) near text "-";
>> expecting "(" File: C:/Documents and Settings/Owner/My
>> Documents/Python/MyHDL/AllpassLP/LP3MPY/lowpass.v Line: 334
>>
>> Line 334 is:       FA_FO_MPY_mpyVar <= (--547);
>>
>>
>
> If the converter generated /--547/ sounds like this could
> be a bug.  But I haven't been able to reproduce conversion
> that generates /--547/ versus /(-(-547))/.  What version of
> Python and MyHDL are you using?
>

I'm using Python 2.6.7 and MyHDL 0.7