Re: [myhdl-list] Problems using signal slices and variables

[Christopher F. <chr...@gm...>]

On 10/11/2012 11:17 AM, Angel Ezquerra wrote:
> On Thu, Oct 11, 2012 at 5:45 PM, Christopher Felton
> <chr...@gm...> wrote:
>> On 10/11/2012 6:10 AM, Angel Ezquerra wrote:
>>> Hi,
>>>
>>> I'm playing around with concatenating and slicing signals in MyHDL.
>>> I'm having some trouble with slicing in particular.
>>>
>>> I am having a hard time predicting what MyHDL will do when the code is
>>> converted to VHDL, and sometimes what I get seems wrong (or at least
>>> it is not what I expected).
>>>
>>> In particular, I've got the following code (note that the code has no
>>> purpose other than testing how MyHDL works):
>>>
>>>       s_input = Signal(intbv(0)[59:])
>>>       s_output = Signal(intbv(0)[59:])
>>>       s_slice = s_input(15, 0) # Use a shadow signal
>>>
>>>       @always(clk.posedge, rst.posedge)
>>>       def p_process():
>>>           if rst == 1:
>>>               max_value = s_input.max - 1
>>>               s_input.next = max_value
>>>               var_test = s_input[15:0] # I expect var_test to be a
>>> variable of range 14 downto 0, i.e. size 15
>>>               s_output.next = concat(s_input[15:0], s_input[30:15],
>>> s_input) # Total size: 15 + 15 + 30 = 60
>>>               s_output.next = concat(s_slice, s_input[30:15], s_input)
>>> # Total size: 15 + 15 + 30 = 60
>>>               s_output.next = concat(var_test, s_input[30:15], s_input)
>>> # Total size: 15 + 15 + 30 = 60
>>>           else:
>>>               pass
>>>
>>
>> Not following your math for the last concat
>>
>>   >  s_output.next = concat(var_test, s_input[30:15], s_input)
>>
>>      var_test = 15 (14 downto 0)
>>      s_input[30:15] = 15 (29 downto 15 )
>>      s_input = 59 (58 downto 0)
>>
>> trying to concat 89.
>>
>> In [63]: s_input = Signal(intbv(0)[59:])
>>       ...: s_output = Signal(intbv(0)[59:])
>>       ...: var_test = s_input[15:0]
>>       ...: len(concat(var_test, s_input[30:15], s_input))
>>       ...:
>> Out[63]: 89
>>
>> .chris
>
> Sorry Chris,
>
> I made a typo on my example. s_input is 30 bits long, not 60, i.e.
>
> s_input = Signal(intbv(0)[30:])
>
> The problem is that var_test is 30 bits long rather than 15 as I expected.
>
> Cheers,
>
> Angel
>

These seems to be a low-priority bug.  It is not functionally
broken but not optimal.  The attached is a slightly more
complete version, with a testbench.

The sourceforge bug tracker has been setup for bug reporting
but I don't think it has been used much.  Not sure the best
method to keep track of issues like this.

Regards,
Chris