Re: [myhdl-list] Problems converting an integer assignment into VHDL

[Angel E. <ang...@gm...>]

Thank you Norbo.

My comments below.

On Thu, Oct 11, 2012 at 11:24 AM, Norbo <Nor...@gm...> wrote:
> Am 11.10.2012, 10:31 Uhr, schrieb Angel Ezquerra
> <ang...@gm...>:
>
>> Hi,
>>
>> I got an issue with conversion to VHDL, where MyHDL seems to do the
>> wrong thing (although most likely it's me who is doing it wrong).
>>
>> The problem is that MyHDL maps an integer assignment into an intbv
>> signal as a direct integer assignment in VHDL, instead of using a
>> to_unsigned conversion.
>>
>> That is, I got a signal declared as follows:
>>
>>     signal s_my_signal: unsigned(29 downto 0);
>>
>> Then I do:
>>
>>     s_my_signal.next = s_my_signal.max - 1
>>
>> which in VHDL becomes:
>>
>>     s_my_signal.next <= 1073741824 - 1;
>>
>> Which is wrong. Instead I expected it to be:
>>
>>     s_my_signal.next <= to_unsigned(1073741824 - 1, 30);
>>
>
> Yup that causes problems. which is funny because if you write in the myhdl
> code:
>
> s_my_signal.next <= 1073741824 - 1;
>
> the result is:
>
> s_my_signal <= to_unsigned(1073741824 - 1, 30);
>
> or if you write:
>
> CONST_MAX=s_my_signal.max
> @always(clk.posedge)
> def ttt():
>     s_my_signal.next <= CONST_MAX - 1;
>
>
> which converts to:
>
> s_my_signal <= to_unsigned(1073741824 - 1, 30);
>
>
> if you write:
>
> s_my_signal.next = int(s_my_signal.max) - 1
>
> it converts to:
>
> s_my_signal <= to_unsigned(1073741824 - 1, 30);
>
>
>
> or if you write:
>
> s_my_signal.next = intbv(s_my_signal.max) - 1
>
> it converts to:
>
> s_my_signal <= to_unsigned(1073741824 - 1, 30);


The problem is even easier to run into, since the convertor fails if
you do not add anything.

This:

    s_2pps_counter.next = int(s_2pps_counter.min)

Converts into:

    s_2pps_counter <= 0;

Which is wrong, while this:

    s_2pps_counter.next = int(s_2pps_counter.min) + 0

Converts into:

    s_2pps_counter <= to_unsigned(0 + 0, 30);

Which is correct.

Another failure is that this:

    s_2pps_counter.next = int(s_2pps_counter.min) + 0

Converts into:

    s_2pps_counter <= to_integer(0 + 0);

Which is also wrong. So you cannot just use "int" to make sure that
.min and .max are properly interpreted.


> so it turns out that the converter doesn't recognise the "s_my_signal.max"
> as an const integer, but instead as a signal.
> This seems to be also the case for the simulator.

That seems to be the problem indeed.

> e.g:  In a combinatorical process this leads to a -> combinatorical loop
> error, when there is actually none.
>
>
>      sigTest=Signal(intbv(0)[4:])
>      @always_comb
>      def comb_setConst():
>        sigTest.next=int(sigTest.max)-1
>
>
> leads to the error message:
>
> --myhdl.AlwaysCombError: signal (sigTest) used as inout in always_comb
> function argument
>
>
>
> but it could be written like this:
>
>      sigTest=Signal(intbv(0)[4:])
>      maxvalue=sigTest.max
>      @always_comb
>      def comb_setConst():
>        sigTest.next=maxvalue-1
>
> this would make the simulator core happy and would be fine if the
> sensitivity list is not empty!!

Yes, it seems that using an intermediate variable to get the value of
s_my_signal.max works fine and is an easy work around.

That being said, is this technique to set a signal to all '1' the one
you guys would use? Or is there a more idiomatic way?

Cheers,

Angel