VHDL: Is there a convenient way to assign ascii values to std_logic_vector? - ascii

Question

In verilog, I can assign a string to a vector like:
wire [39:0] hello;
assign hello = "hello";
In VHDL, I'm having difficulty finding a method like this:
SIGNAL hello : OUT std_logic_vector (39 DOWNTO 0);
...
hello <= "hello";
I've been using:
hello <= X"65_68_6c_6c_6f";
which is unclear and time consuming for large strings.
I've looked at the textio package and thetxt_util package, but neither seem to be very clear on how to interpret a string and convert it to std_logic.
Is there a simple method of assigning ascii codes to std_logic in VHDL?
Here's a minimal example:
LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
ENTITY test IS
PORT(
ctrl : IN std_logic;
stdout : OUT std_logic_vector (39 DOWNTO 0)
);
END ENTITY;
ARCHITECTURE rtl OF test IS
SIGNAL temp : std_logic_vector (39 DOWNTO 0);
BEGIN
stdout <= temp;
PROCESS(ctrl)
BEGIN
IF (ctrl = '0') THEN
temp <= "hello"; -- X"68_65_6C_6C_6F";
ELSE
temp <= "world";
END IF;
END PROCESS;
END rtl;

Answer 1

This one varies little for Morten's answer - it only uses one multiply, it copies the string instead of creating an alias, it uses an additional variable and it returns a standard logic vector with an ascending index range.
From a package called string_utils:
library ieee;
use ieee.numeric_std.all;
-- ...
function to_slv(s: string) return std_logic_vector is
constant ss: string(1 to s'length) := s;
variable answer: std_logic_vector(1 to 8 * s'length);
variable p: integer;
variable c: integer;
begin
for i in ss'range loop
p := 8 * i;
c := character'pos(ss(i));
answer(p - 7 to p) := std_logic_vector(to_unsigned(c,8));
end loop;
return answer;
end function;
You could add an argument with a default specifying ascending/descending index range for the return value. You'd only need to provided the argument for the non default.

Answer 2

A small general function is one way to do it, with a suggestion below:
library ieee;
use ieee.numeric_std.all;
...
-- String to std_logic_vector convert in 8-bit format using character'pos(c)
--
-- Argument(s):
-- - str: String to convert
--
-- Result: std_logic_vector(8 * str'length - 1 downto 0) with left-most
-- character at MSBs.
function to_slv(str : string) return std_logic_vector is
alias str_norm : string(str'length downto 1) is str;
variable res_v : std_logic_vector(8 * str'length - 1 downto 0);
begin
for idx in str_norm'range loop
res_v(8 * idx - 1 downto 8 * idx - 8) :=
std_logic_vector(to_unsigned(character'pos(str_norm(idx)), 8));
end loop;
return res_v;
end function;

Answer 3

To return an ascii value of a character, use this code:
some_variable <= character'pos('a'); --returns the 'a' ascii value

Answer 4

In your example you are trying to assign a string type to a std_logic_vector type.
That is simply not allowed. VHDL is strongly typed.
SIGNAL hello : OUT std_logic_vector (39 DOWNTO 0);
...
hello <= "hello";
If your goal is to convert from hexa to ascii for printing simulation result
you can simply do that:
character'val(to_integer(unsigned(my_std_logic_vector)))