VHDL: use the length of an integer generic to determine number of select lines

Question

I\'m trying to create a reusable barrel shifter; it takes an input array of bits and shifts them a certain number of positions (determined by another input). I want to parameter

Answer 1

You can create a log2-function in a library, like this:

   function f_log2 (x : positive) return natural is
      variable i : natural;
   begin
      i := 0;  
      while (2**i < x) and i < 31 loop
         i := i + 1;
      end loop;
      return i;
   end function;

If the library is imported you may then specify the port like this:

shift_cnt : in std_logic_vector(f_log2(NUMBITS)-1 downto 0)

It is a somewhat ugly solution, but it doesn't use any resources (since the function is pure and all the inputs are known at compile time).

I usually do this, but you may prefer specifying the log value as the generic like you're mentioning.