std_logic_vector to unsigned conversion

[Al]

Hi guys,
I just had a disappointing problem, my code had something like:

vectorA <= unsigned (vectorB);

where ieee.std_logic_unsigned.all had been included in the library
declaration.

Unfortunately vectorA and vectorB where different in lenght, to be
precise vectorB was shorter than vectorA.
Synplify didn't complain anything, no errors, no warnings related to it
(that I noticed, at least), but what had been implemented was a vector
longer than required which turned out in a counter that will never reach
the vectorA value and will roll forever!
Is there any better way to convert std_logic_vector to unsigned with a
less clever function, but at least more reliable?

Thanks a lot

Al

 

[patrick.melet]

Hi

Try with ieee.numeric_std.all instead ieee.std_logic_unsigned.all...

 

[KJ]

Hi guys,
I just had a disappointing problem, my code had something like:

vectorA <= unsigned (vectorB);

where ieee.std_logic_unsigned.all had been included in the library
declaration.

Unfortunately vectorA and vectorB where different in lenght, to be precise
vectorB was shorter than vectorA.
Synplify didn't complain anything, no errors, no warnings related to it
(that I noticed, at least), but what had been implemented was a vector
longer than required which turned out in a counter that will never reach
the vectorA value and will roll forever!
Is there any better way to convert std_logic_vector to unsigned with a
less clever function, but at least more reliable?

First of all, use a simulator. My interpretation of your code from your
description is...
....
signal vectorA: unsigned(7 downto 0); -- Some size vector, by your
assignment statement it must've been unsigned
signal vectorB: std_logic_vector(6 downto 0); -- Some different sized
vector, by your assignment it must not have been unsigned
begin
vectorA <= unsigned (vectorB);
.....

Surprising that Synplify doesn't complain, but Modelsim gives the following
error (not warning) which is expected since the vectors are different sizes.

Error:
C:/Sim/ZPOC_Sim/HDL/ZFpga_Common/Avl_Track_Controller/Track_Controller/Track_Controller.vhd(265):
Length of expected is 8; length of actual is 7.

Assuming that the vectors are the size that you really do want them to be
then the proper assignment is...

vectorA <= ieee.numeric_std.resize(unsigned(vectorB), vectorA'length);
or
vectorA <= resize(unsigned(vectorB), vectorA'length);
if you've already got a "use ieee.numeric_std.all" statement in your code.

KJ

 

[Al]

First of all, use a simulator.

I was using it, but the testbench hided the error because wasn't
properly designed.

My interpretation of your code from your
description is...
...
signal vectorA: unsigned(7 downto 0); -- Some size vector, by your
assignment statement it must've been unsigned
signal vectorB: std_logic_vector(6 downto 0); -- Some different sized
vector, by your assignment it must not have been unsigned
begin
vectorA <= unsigned (vectorB);
....
correct.


Surprising that Synplify doesn't complain, but Modelsim gives the following
error (not warning) which is expected since the vectors are different sizes.

As I said, I'm not sure that Synplify didn't complain because I didn't
check carefully all the type of warnings (basically I have a lot of them
complaining because not all the global registers I declared in the
package were used by each entity, but this didn't really bother me).

Error:
C:/Sim/ZPOC_Sim/HDL/ZFpga_Common/Avl_Track_Controller/Track_Controller/Track_Controller.vhd(265):
Length of expected is 8; length of actual is 7.

Once synthesized I usually simulate a post-synthesis and a post-layout
file, which will not show the problem anymore

Assuming that the vectors are the size that you really do want them to be
then the proper assignment is...

unfortunately this is not true, the problem was just that I forgot to
change the length in vectorB declaration, so this led me to the problem.
Being just a warning (maybe), mixed up in several other meaningless
warning, didn't really catch my attention, that's why I was wondering if
it exists a conversion function which give you error when length are not
the same (as conv_unsigned in the std_logic_arith library) when
converting from a std_logic_vector to unsigned.
Maybe is an issue for the Synplify compiler...

vectorA <= ieee.numeric_std.resize(unsigned(vectorB), vectorA'length);
or
vectorA <= resize(unsigned(vectorB), vectorA'length);
if you've already got a "use ieee.numeric_std.all" statement in your code.

that's good to know, I've never used the function "resize". Thanks