Vhdl code simulation

Question

I'm trying to simulate the following code :

entity schal is port ( SW  : in bit_vector(7 downto 0);
       LED : out bit_vector(7 downto 0)); 
end schal;


architecture BEHAVIOUR of schal is 
begin 

INOUT_PROS : process (SW)
begin 
    LED <= SW; 

end process INOUT_PROS;
end BEHAVIOUR;

I wrote this do file

vsim work.schal
restart
view wave
radix hex
add wave -height 25 -radix default sim:/schal/*

force SW 01000001
run 20ns
force SW 01000000
run 20ns

here is what I get :

as you can see the simulation affect only the first bit but not the whole vector ? any idea how should I adjust the do file to get the right simulation ?

You should probably have a look at the `force` command in the Modelsim manual. Maybe it expects a value in hexadecimal by default and ignores the extra characters... — Renaud Pacalet, Oct 02 '15 at 11:42

score 4 · Accepted Answer · answered Oct 02 '15 at 11:43

I think your force command is not using the correct syntax. You are trying to force a binary value, but the correct way to do this would be force SW 2#01000001, with the 2# specifying a binary value.

In ModelSim, go to Help > Documentation > PDF Bookcase, then open the 'Command Reference Manual'. This contains documentation on all commands, including force.

score 4 · Answer 2 · answered Oct 02 '15 at 23:09

This is a Read-The-Fine-Manual moment. See the <value> argument description under the force command Arguments section in the Command Reference Manual.

A one-dimensional array of character enumeration can be forced as a sequence of character literals or as a based number with a radix of 2, 8, 10 or 16. For example, the following values are equivalent for a signal of type bit_vector (0 to 3):

You could note that IEEE Std 1076-2008 15.5.3 Based literals tells us:

A based literal is an abstract literal expressed in a form that specifies the base explicitly. The base shall be at least two and at most sixteen.

There are VHDL standard compliant based literals that can't be expressed with the force command.

Also notice the question's use of

force SW 01000001

is compatible with the sequence of character literals example in the Command Reference Manual. See the following NOTE:

For based numbers in VHDL, ModelSim translates each 1 or 0 to the appropriate value for the number’s enumerated type. The translation is controlled by the translation table in the pref.tcl file. If ModelSim cannot find a translation for 0 or 1, it uses the left bound of the signal type (type’left) for that value.

Also note from the question's value and waveform that the right most position of SWs enumeration translates properly. This suggests that the behavior doesn't match the force command &LT;value> description.

In VHDL terms you're being force to use a bit string literal as demonstrated in scary_jeff's answer and not a string literal for providing the value. (Both sans quotation marks).

The character literal sequence is correct according to the example but does not translate correctly. You could wonder if quotation marks would help - how would otherwise force a string type object's value containing a leading space?

As an early MTI Modelsim user the simulator originally only supported VHDL. The problem is either the simulator or the example.

And of course there's the use of a VHDL test bench instead of embedding simulation sequence in a do file.

This method would be portable between VHDL simulators:

entity schal_tb is
end entity;

architecture foo of schal_tb is
    signal SW:  bit_vector(7 downto 0);
    signal LED: bit_vector(7 downto 0);
begin
DUT:
    entity work.schal
        port map (
            SW => SW,
            LED => LED
        );
STIMULUS:
    process 
    begin
        SW <= "01000001";
        wait for 20 ns;
        SW <= "01000000";
        wait for 20 ns;
        wait for 60 ns;   -- to show 100 ns on waveform
        wait;             -- suspends process permenently
    end process;
end architecture;

And gives:

There are also online VHDL Testbench outline generators such as the Doulos VHDL Testbench generator or this Online VHDL Testbench Template Generator.

Vhdl code simulation

2 Answers2