I'm trying to implement this formula :

$$ output = \sum_{i=0}^{N-1} (input[i] ~~ \text{xor} ~~ input[N - i - 1]) ~~ \times ~~ 2^i $$

The input has \$ N \$ bits and the output has \$ S = N/2 \$ bits.

This is what I have:

library IEEE;
use ieee.numeric_std.all;
use IEEE.STD_LOGIC_1164.all;
use ieee.std_logic_unsigned.all;

entity eq is
    generic (
        N : positive := 4;
        S : positive := 2
    port (
        I : in std_logic_vector(N-1 downto 0);
        O : out std_logic_vector(S-1 downto 0)
end eq;

architecture arch of eq is 
    process (I)
        variable total : integer;
        variable temp : std_logic_vector(S-1 downto 0);
        variable zeros : std_logic_vector(0 to S-2) := (others => '0');
        for k in 0 to N-1 loop
            -- do the xor operation first and store the bit at the end of an std_logic_vector of total length S
            temp := zeros & (I(k) xor I(N-k-1));
            -- multiply the above std_logic vector by 2^i, convert to integer and add to the total
            total := total + to_integer(unsigned(shift_left(unsigned(temp),k)));
            end loop;
    O <= std_logic_vector(to_unsigned(total,S));
    end process;
end arch;

I keep getting type conversion messages. It throws this on the line with total := total + ...:

Error: Near std_logic_vector ; type conversion expression type cannot be determined uniquely

When I try to fix a particular type conversion, another one pops up.

  • 3
    \$\begingroup\$ You're importing arithmetic operators from multiple libraries including a non-standard one. So the compiler can't tell which of several + operators you really want. Stick to std_logic_1164 and numeric_std. \$\endgroup\$
    – user16324
    Feb 6, 2018 at 22:54
  • \$\begingroup\$ Who's tool? The fourth use clause (std_logic_unsigned) is in the immediate scope of the first (numeric_std) and makes no type declarations directly visible anyway. The only directly visible type unsigned in found in numeric_std (IEEE Std 1076-2008 12. Scope and visibility, 1.2 4 Use clauses paras 7 (scope) & 8 (visibility). You appear to have discovered a bug. As Brian comments, eliminate the use clause with std_logic_unsigned, it's not used. \$\endgroup\$
    – user8352
    Mar 5, 2018 at 2:16

1 Answer 1


Variables are used like constants in c, not for actual signals. Stick with integer, signed, unsigned and std_logic_vector.

If you do need to translate between them then here are the functions to do that:

enter image description here

Source: http://www.lothar-miller.de/s9y/categories/16-Numeric_Std

  • \$\begingroup\$ For the English speakers, what do the German (?) words in the image mean? Otherwise very nice diagram. \$\endgroup\$
    – stanri
    Feb 13, 2018 at 12:31

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