[SOLVED] If-else statement in process is not working with multiple inputs in sensitive list

[JulianCas]

Hi,

I was trying to implement a finite state machine, and I started using If else statement, then I realized that no matter the condition, all were met which was not expected, I was expecting only one. Then I tried using the case control and it worked. The following example is a simplified version of my program that reflects this condition. The commented section works but the if-else control statement does not, both conditions make the LED lit.

A couple of points:

- When I initialized the IN ports it is to simulate that there is another entity that initializes those inputs with std_logic '0', that entity provides 3 outputs with 0 state and are connected to the 3 inputs in this entity. In this example I just initialized those inputs with '0'.

- If I use local signals initialize to '0' instead of the input ports, the else-if control works which I tried to understand this behavior but so far no success.

I can move on with the case statement but if any of you can help I really appreciate it.

entity selector is

port(
	toLed: out std_logic_vector (3 downto 0);

	sel0: in std_logic:='0';
	sel1: in std_logic:='0';
	sel2: in std_logic:='0';
	clk: in std_logic

);


end selector;

architecture main of selector is

--signal sel00:std_logic:='0';  local signals case
--signal sel11:std_logic:='0';
--signal sel22:std_logic:='0';

begin


proc: process (sel0,sel1,sel2) 

	variable current:std_logic_vector(2 downto 0);

	begin

		current:=sel0 & sel1 & sel2;
		
		if (current = "000")then
			toLed(2)<='1';

			else toLed(3)<='1';

		end if;

--	case current is                        --this case works
--		when "000" => toLed(2) <= '1';
--		when others => toLed(3) <='1';
--	end case;


end process;

end main;

 

[TrickyDicky]

This is so oversimplified, it tells us nothing, as they are the same code, that sets toLed to '1' and nothing else, so may get synthesised to just logic one.
You would be better off posting the real code.

 

[FvM]

To convert the "simplified" example into something meaningful, there should be a first process line like

toLed<= "0000";

Reason: A combinational process should set the state of any affected signal in any conditional path and not generate latches. Signals working as state memory should be only set in clock edge sensitive processes.

 

[JulianCas]

Hi all,

Thanks for you reply and I got your point. I was trying to check each state machine by lighting one LED per state which does not mean anything. It worked when I used toLed <= "0000" but I needed to re-design the process understanding the way combinational and state memory processes work.