VHDL Coding .. conversion from integer to bit_vector

Question

I'm facing this problem , i'm asked to implement a function in VHDL that takes an integer and returns a bit_vector , assumed that this integer is represented by 4 bits.

i don't want to use already built in function, i have to code the function.

I have made a function to convert from bit_vector to integer which was kinda of easy, but im stuck here :S

Any ideas how can i do it ?

Answer 1

The VHDL standard packages is good inspiration for home brewed functions, and the numeric_bit package defines the to_unsigned function for conversion of natural type to unsigned type, which is the function VHDL actually uses for conversion to bit_vector. The function is implemented as:

function TO_UNSIGNED (ARG, SIZE: NATURAL) return UNSIGNED is
  variable RESULT: UNSIGNED(SIZE-1 downto 0);
  variable I_VAL: NATURAL := ARG;
begin
  if (SIZE < 1) then return NAU;
  end if;
  for I in 0 to RESULT'LEFT loop
    if (I_VAL mod 2) = 0 then
      RESULT(I) := '0';
    else 
      RESULT(I) := '1';
    end if;
    I_VAL := I_VAL/2;
  end loop;
  if not(I_VAL =0) then
    assert NO_WARNING
        report "NUMERIC_BIT.TO_UNSIGNED: vector truncated"
        severity WARNING;
  end if;
  return RESULT;
end TO_UNSIGNED;

The initial if (SIZE < 1) and final if not(I_VAL =0) checks may be removed, if it is known that the function is never used with values that makes the checks relevant.

This leaves the for I in 0 to RESULT'LEFT loop that creates one result bit per iteration.

Based on Brian's answer, the constant LUT can be initialized using the TO_UNSIGNED function, to avoid the hand written literals:

function to_bv(n, size : natural) return bit_vector is

  type bv_arr_t is array (0 to 2 ** size - 1) of bit_vector(size - 1 downto 0);

  function bv_arr_init(size : natural) return bv_arr_t is
    variable res_v : bv_arr_t;
  begin
    for i in 0 to 2 ** size - 1 loop
      res_v(i) := bit_vector(TO_UNSIGNED(i, size));
    end loop;
    return res_v;
  end function;

  constant LUT : bv_arr_t := bv_arr_init(size);

begin
  return LUT(n);
end to_bv;