0
\$\begingroup\$

In my testbench i set reset=1 and then reset=0 after 1 ns In the simulation, err_count <= std_logic_vector(err_count_int); does not execute, does anyone know why? reset_out <= '1'; is also executed


library IEEE;
use IEEE.Std_logic_1164.all;
use IEEE.Numeric_Std.all;

entity controller is
    port(
            enable : in std_logic:='0';
            reset: in std_logic:='0';
            clk : in std_logic;
            eds: in std_logic:='0';
            ecs: out std_logic:='0';
            reset_out: out std_logic:='0';
            en_pipeline: out std_logic:='0';
            err_count: out std_logic_vector(4 downto 0)
        );
end controller;

architecture beh of controller is

    signal err_count_int: unsigned(4 downto 0);
    signal increment: std_logic_vector(4 downto 0):= "00001";

begin
    process(reset) begin
        if reset = '1' then 
            err_count_int <= to_unsigned (0, err_count_int'length);
            err_count <= std_logic_vector(err_count_int);
            reset_out <= '1';
        elsif reset = '0' then
            reset_out <= '0';
        end if;
    end process;
    
    -- process(clk) begin
        -- if enable = '1' and rising_edge(clk) then
            -- en_pipeline <= '1';
            -- err_count_int <= err_count_int + unsigned(increment);
            -- err_count <= std_logic_vector(err_count_int);
        -- elsif rising_edge(clk) AND eds = '1' then
            -- err_count_int <= err_count_int + 1;
            -- err_count <= std_logic_vector(err_count_int);
            -- ecs <= '1';
        -- end if;
    -- end process;
    
end beh;




```
\$\endgroup\$
5
  • \$\begingroup\$ Note that there is an all keyword that should be supported by all newer simulators that would eliminate this problem. \$\endgroup\$
    – po.pe
    Jun 30, 2020 at 5:50
  • \$\begingroup\$ Does the sensitivity list mean that only operations on those signals in the sensivity list is possible upon any changes to the sensitivity list? \$\endgroup\$ Jun 30, 2020 at 12:22
  • \$\begingroup\$ Or does it simply mean when any signal in the sensitivity list changes, all statement in the process are evaluated and executed? \$\endgroup\$ Jun 30, 2020 at 12:23
  • 1
    \$\begingroup\$ The sensitivity list tells the simulator which signal changes trigger a new evaluation of the process. \$\endgroup\$
    – po.pe
    Jun 30, 2020 at 14:12
  • \$\begingroup\$ The thing to note here is that although this is how simulation works, it is not necessarily how a synthesized design will work. Using the "all" keyword is the safest option but potentially could slow down certain simulations. \$\endgroup\$
    – ks0ze
    Jun 30, 2020 at 18:52

1 Answer 1

0
\$\begingroup\$

You are missing err_count_int in your sensitivity list. Basically, a sensitivity list says "only perform calculations (in the simulation) when one of these signals changes".

Therefore, reset changes to 1 and you essentially simultaneously set err_count_int, err_count, and reset_out. But, the updated version of err_count_int isn't ready yet so I'm assuming you are getting undefined output for err_count. reset_out gets set correctly because the constant '1' is always available for assignment.

If you need further explanation, please also post the testbench and your simulation results.

\$\endgroup\$
2
  • \$\begingroup\$ But why when im using process(clk) begin with exact same assignment statements it works? \$\endgroup\$ Jun 30, 2020 at 12:21
  • \$\begingroup\$ @MarkHenderson, could you edit your question to include a simulation output and testbench? Making that change, you're telling the simulator to perform calculations based on a signal that is not part of the process. This will lead to mismatches between hardware and simulation and should not be done. The simulation calculation will be updated every rising/falling edge of clk, so I could see err_count being correct after a rising and falling edge. \$\endgroup\$
    – ks0ze
    Jun 30, 2020 at 13:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.