Functions within VHDL

Question

I am trying to read the output from multiple components and display them within an integer form. The outputs are all 4-bit vectors. As I do not want to have to repeat the code numerous times, I am trying the use of functions native to VHDL. I have not used these before and also cannot see much well-explained documentation online.

function FourBitsToNumber (X : std_logic_vector(3 downto 0))
return integer is
begin
if (X = "0000") then return 0;
  elsif (X = "0001") then return integer(1);
  elsif (X = "0010") then return integer(2);
  elsif (X = "0011") then return integer(3);
  elsif (X = "0100") then return integer(4);
  elsif (X = "0101") then return integer(5);
  elsif (X = "0110") then return integer(6);
  elsif (X = "0111") then return integer(7);
  elsif (X = "1000") then return integer(8);
  elsif (X = "1001") then return integer(9);
 end if;
end FourBitsToNumber;

I am calling the function on the clock pulse with the following code:

variable1 <= FourBitsToNumber(A);

...With 'variable1' being of type integer declaration. 'A' is a 4-bit vector.

However, from my test bench, I can see that the line of code is being reached but am just receiving a value of 0 throughout (even when I know it should be more).

My guess is maybe the syntax or another stupid error on my behalf.

Answer 1

Try this...

function my_int(X : std_logic_vector(3 downto 0)) return integer is
    begin
        return to_integer(unsigned(X));
end function my_int;

I have a feeling that this could also work with an undefined length slv... e.g.

function my_int_of_any_len(X : std_logic_vector) return integer is
    begin
        return to_integer(unsigned(X));
end function my_int_of_any_len;

Matthew's suggestion: (see his comment below)

function my_int(X : std_logic_vector(3 downto 0)) return integer is
use ieee.numeric_std.all;
    begin
        return to_integer(unsigned(X));
end function my_int;

I should also point out that you dont actually have to build your own function for this. Where you have said that you want an integer, just put variable1 <= to_integer(unsigned(X));