-- The following example is an up-counter with asynchronous reset,
-- parallel load and configurable width. It demonstrates the use
-- of the 'unsigned' type, type conversions between 'unsigned' and
-- 'std_logic_vector' and VHDL generics. The generics are very
-- close to arguments or templates in other traditional programming
-- languages like C++.

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;    -- for the unsigned type

entity COUNTER is
  generic (
    WIDTH : in natural := 32);
  port (
    RST   : in std_logic;
    CLK   : in std_logic;
    LOAD  : in std_logic;
    DATA  : in std_logic_vector(WIDTH-1 downto 0);
    Q     : out std_logic_vector(WIDTH-1 downto 0));
end entity COUNTER;

architecture RTL of COUNTER is
  signal CNT : unsigned(WIDTH-1 downto 0);
begin
  process(RST, CLK) is
  begin
    if RST = '1' then
      CNT <= (others => '0');
    elsif rising_edge(CLK) then
      if LOAD = '1' then
        CNT <= unsigned(DATA); -- type is converted to unsigned
      else
        CNT <= CNT + 1;
      end if;
    end if;
  end process;

  Q <= std_logic_vector(CNT); -- type is converted back to std_logic_vector
end architecture RTL;