when does a change in a variable in the sensitivity list trigger a process in vhdl?

Question

I'm having difficulty understanding the effects of this code: My component:

library IEEE;
use IEEE.std_logic_1164.all;

entity problem is
  port(
    clk : in std_logic;
    a : in std_logic);
end problem;

architecture impl of problem is
  signal a_sig : std_logic;

begin
  clk_proc : process(clk)
  begin
    if rising_edge(clk) then
      a_sig <= '0';
    end if;
  end process;

  a_proc : process(a)
  begin
    report "a received : " & std_logic'image(a);
    a_sig <= a;
  end process;

  a_sig_proc : process(a_sig)
  begin
    report "a_sig set : " & std_logic'image(a_sig);
  end process;
end impl;

and this is my testbench.vhd:

library IEEE;
use IEEE.std_logic_1164.all;

entity testbench is
end testbench;

architecture tb of testbench is
  component problem is
    port ( clk : in std_logic;
           a : in std_logic);
  end component;

  constant clk_period : time := 1 ms;
  signal clk_sig : std_logic;
  signal a_sig : std_logic;
begin
  dut : problem port map (clk_sig, a_sig);

  process
  begin
    clk_sig <= '1';
    wait for clk_period/2;
    clk_sig <= '0';
    wait for clk_period/2;
  end process;

  process
  begin
    wait for clk_period * 0.75;
    a_sig <= '1';
  end process;

end tb;

and the result of running the code is as follows:

$ ghdl -r testbench --vcd=testbench.vcd --stop-time=2ms
problem.vhd:23:5:@0ms:(report note): a received : 'U'
problem.vhd:29:5:@0ms:(report note): a_sig set : 'U'
problem.vhd:23:5:@750us:(report note): a received : '1'
problem.vhd:29:5:@1ms:(report note): a_sig set : 'X'
./testbench:info: simulation stopped by --stop-time

I can understand the 'U' signals being received at 0ms, and I can understand the '1' signal being received in problem.a_proc at 750microseconds. The first thing that confuses me is, why isn't the problem.a_sig_proc being triggered by a_sig being set in the same process? And then, when problem.a_sig_proc is triggered, a_sig has the value of 'X'. If someone could point me to a ressource to explain this, it would be great :)

Thanks in advance!

Answer 1

You are driving the a_sig signal from multiple processes (clk_proc and a_proc). You will need to remove the assignment to a_sig in one of the processes (as the simulator cannot resolve which assignment takes precedence), or drive 'Z' (high impedence) onto a_sig from the processes, while it is not "their turn". There are good explanations here and here