How to ensure proper simulation in ModelSim

Question

LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.numeric_std.all;
ENTITY satUpCount ISPORT(clock, en : IN STD_LOGIC;
flag8  : OUT STD_LOGIC);
END satUpCount;
ARCHITECTURE RTL OF satUpCount IS
SIGNAL sCntVal : INTEGER RANGE 0 TO 15 := 0;
CONSTANT maxVal : NATURAL := 15;
BEGINPROCESS(clock, en)
BEGIN
IF en = '0' THEN
sCntVal &lt;= 0;
ELSIF FALLING_EDGE(clock) AND sCntVal &lt; maxVal THEN
sCntVal &lt;= sCntVal + 1;
ELSE 
sCntVal &lt;= sCntVal;
END IF;
IF sCntVal &gt;= 8 THEN
 flag8 &lt;= '1';
ELSE
 flag8 &lt;= '0';
END IF;
 END PROCESS;
END RTL;
  After synthesizing this code in Quartus, we call the simulation waveform editor and perform a functional simulation as shown https://alteraforum.com/forum/attachment.php?attachmentid=14539&amp;stc=1  The desired behaviour is obtained, i.e. flag8 is set on the 8'th falling edge of the clock... All is good... However, when simulating the same VHDL code in an instance of ModelSim that is not called from within Quartus, then a different behaviour will occur: https://alteraforum.com/forum/attachment.php?attachmentid=14540&amp;stc=1  That is, the flag is set 0.5 clock cycle to late... This seemed really strange at first, because both simulations are carried out in the same software, i.e. ModelSim-Altera. However, the Quartus' simulation waveform viewer (swv) generates a .vho file, containing VHDL code that I guess represents hardware more closely. If I simulate the .vho file in ModelSim, then I get the same behaviour.   My issue then, is that now I cannot be certain that my ModelSim testbenches actually perform the functional behaviour of the synthesized circuit. I could generate testbenches via the Quartus' swv, but for larger circuits the compile time is really large.  Are there some ModelSim settings that would generate the same output ? Any advices or comments ?

altera_forum · Answer

your flag logic is outside clk edge. your sensitivity list does not cover it. your enable may be starts differently.

altera_forum · Answer

--- Quote Start ---

your flag logic is outside clk edge. your sensitivity list does not cover it. your enable may be starts differently.

--- Quote End ---

Yes I know. But why is there discrepancy between simulating VHO and VHDL ? Is there some settings/scripts that will ensure the same behaviour ?

altera_forum · Answer

There are several issues here:

1. You have not really used the proper template for registered logic.

2. Synthesis ignores sensitivity lists, simulation does not. The synthesis is generating flag purely from async logic, whereas in your simulation flag can only change when clock or enable change. To match the two behaviours, you either need to add sCntVal to the sensitivity list (or if you use VHDL 2008 you can use process(all) ) or synchronise all of your code.

3. If you follow standard practice, and proper templates, Modelsim behaviour will always match real hardware.

altera_forum · Answer

--- Quote Start ---

There are several issues here:

1. You have not really used the proper template for registered logic.

2. Synthesis ignores sensitivity lists, simulation does not. The synthesis is generating flag purely from async logic, whereas in your simulation flag can only change when clock or enable change. To match the two behaviours, you either need to add sCntVal to the sensitivity list (or if you use VHDL 2008 you can use process(all) ) or synchronise all of your code.

3. If you follow standard practice, and proper templates, Modelsim behaviour will always match real hardware.

--- Quote End ---

Thank you. Good answer... By templates, you mean those that Altera provide from the quartus text editor ? Are there more templates than those available from the insert template ?

altera_forum · Answer

I mean just general good practice for VHDL logic, usually presented in any decent tutorial - I think altera provides them via the text editor. But stick to this:  
process(clk, async_rst)   -- no other signals. No async reset needed if you dont need one
begin
  if async_rst = '1' then
    -- async reset here
  elsif rising_edge(clk) then
    -- sync logic here, including enables
  end if;
  -- NOTHING goes here
end process;

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How to ensure proper simulation in ModelSim

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