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

[Norbo <Nor...@gm...>]

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);




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.
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!!


greetings
Norbert