Full adder module and testbench#

Unlike Heartbeat, the target hardware design in this example is written using the synthesisable subset of VHDL. It is a full adder described in a file named adder.vhdl:

entity adder is
  -- `i0`, `i1`, and the carry-in `ci` are inputs of the adder.
  -- `s` is the sum output, `co` is the carry-out.
  port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
end adder;

architecture rtl of adder is
  --  This full-adder architecture contains two concurrent assignments.
  --  Compute the sum.
  s <= i0 xor i1 xor ci;
  --  Compute the carry.
  co <= (i0 and i1) or (i0 and ci) or (i1 and ci);
end rtl;

You can analyse this design file, ghdl -a adder.vhdl, and try to execute the adder design. But this is useless, since nothing externally visible will happen. In order to check this full adder, a testbench has to be run. The testbench is a description of how to generate inputs and how to check the outputs of the Unit Under Test (UUT). This one is very simple, since the adder is also simple: it checks exhaustively all inputs. Note that only the behaviour is tested, timing constraints are not checked. A file named adder_tb.vhdl contains the testbench for the adder:

--  A testbench has no ports.
entity adder_tb is
end adder_tb;

architecture behav of adder_tb is
  --  Declaration of the component that will be instantiated.
  component adder
    port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
  end component;

  --  Specifies which entity is bound with the component.
  for adder_0: adder use entity work.adder;
  signal i0, i1, ci, s, co : bit;
  --  Component instantiation.
  adder_0: adder port map (i0 => i0, i1 => i1, ci => ci, s => s, co => co);

  --  This process does the real job.
    type pattern_type is record
      --  The inputs of the adder.
      i0, i1, ci : bit;
      --  The expected outputs of the adder.
      s, co : bit;
    end record;
    --  The patterns to apply.
    type pattern_array is array (natural range <>) of pattern_type;
    constant patterns : pattern_array :=
      (('0', '0', '0', '0', '0'),
       ('0', '0', '1', '1', '0'),
       ('0', '1', '0', '1', '0'),
       ('0', '1', '1', '0', '1'),
       ('1', '0', '0', '1', '0'),
       ('1', '0', '1', '0', '1'),
       ('1', '1', '0', '0', '1'),
       ('1', '1', '1', '1', '1'));
    --  Check each pattern.
    for i in patterns'range loop
      --  Set the inputs.
      i0 <= patterns(i).i0;
      i1 <= patterns(i).i1;
      ci <= patterns(i).ci;
      --  Wait for the results.
      wait for 1 ns;
      --  Check the outputs.
      assert s = patterns(i).s
        report "bad sum value" severity error;
      assert co = patterns(i).co
        report "bad carry out value" severity error;
    end loop;
    assert false report "end of test" severity note;
    --  Wait forever; this will finish the simulation.
  end process;

end behav;

As usual, you should analyze the file, ghdl -a adder_tb.vhdl.


Then, if required, elaborate the testbench: ghdl -e adder_tb. You do not need to specify which object files are required, since GHDL knows them and automatically adds them.

Now, it is time to run the testbench, ghdl -r adder_tb, and check the result on screen:

adder_tb.vhdl:52:7:(assertion note): end of test

If your design is rather complex, you’d like to inspect signals as explained in Heartbeat.

See section Simulation options, for more details on other runtime options.