Error adding std_logic_vectors

Question

I wanna have a simple module that adds two std_logic_vectors. However, when using the code below with the + operator it does not synthesize.

library IEE         


        

Answer 1

Don't use std_logic_arith - I've written about this (at some length :).

Do use numeric_std - and do use the right type on your entity ports. If you are doing arithmetic, use numerical types (either integers, or (un)signed vectors, as appropriate). They'll synthesise perfectly well.

std_logic_vectors are good for

• when you don't care about numerical values (a set of control bits, some random data bits)
• when you don't know about the type of the input (say an adder which can operate on both signed and unsigned numbers based on a control flag).

Answer 2

The easy way to solve this error is:
Add library of unsign,
After that your code starts to work.

Use

ieee.std_logic_unsigned.all;
pr_out <= pr_in1 + pr_in2;

Answer 3

How do you want the compiler to know if your std_logic_vectors are signed or unsigned ? Adder implementation is not the same in these two cases, so you need to explicitly tell the compiler what you want it to do ;-)

Note: VHDL syntax highlighting in StackOverflow is crappy. Copy/paste this code in your preferred VHDL editor to read it more easily.

library IEEE; 
use IEEE.std_logic_1164.all;
-- use IEEE.std_logic_arith.all; -- don't use this
use IEEE.numeric_std.all; -- use that, it's a better coding guideline

-- Also, never ever use IEEE.std_unsigned.all or IEEE.std_signed.all, these
-- are the worst libraries ever. They automatically cast all your vectors
-- to signed or unsigned. Talk about maintainability and strong typed language...

entity add_module is
  port(
    pr_in1   : in std_logic_vector(31 downto 0);
    pr_in2   : in std_logic_vector(31 downto 0);
    pr_out   : out std_logic_vector(31 downto 0)  
  );
end add_module;

architecture Behavior of add_module is
begin

  -- Here, you first need to cast your input vectors to signed or unsigned 
  -- (according to your needs). Then, you will be allowed to add them.
  -- The result will be a signed or unsigned vector, so you won't be able
  -- to assign it directly to your output vector. You first need to cast
  -- the result to std_logic_vector.

  -- This is the safest and best way to do a computation in VHDL.

  pr_out <= std_logic_vector(unsigned(pr_in1) + unsigned(pr_in2));

end architecture Behavior;

Answer 4

Good advice from @Aurelien to use numeric_std.

Bear in mind that adding two 32 bit values can result in a 33 bit value and decide how you want to handle the overflow.