Viewing intermediate variable value within a process in ModelSim

[rwdfan]

I am debugging a VHDL block which has a "for loop" inside a
procedure. Within each iteration of the for loop, a variable is
conditionally assigned. For troubleshooting the design, I am
interested in viewing the value of the variable at specific locations
of the process. Since the variable is re-assigned subsequently within
the process, the variable has changed state multiple times in zero
time. I realize this rules out any viewing in a wave window.

process(clk,reset)
variable a;
variable b;
begin
if rising_edge(clk) then
for i in 0 to 7 loop
if condition1 then
a := x(i);
end if;
b := a;
if condition2 then
a := x(i+1);
end if;
end for;
end if
end process;

Here is a simple example. Please do not recommend a different coding
style, as this is merely an attempt to illustrate the problem. Let's
say I want to lok at the value of a (or i, for that matter) at the end
of the first if statement. I do not want to see it after the process
completes. I would be happy if I could insert a line of code similar
to the report command, that could display the value of a variable at
the time of execution.

Any suggestions?
Brett

 

[Mike Treseler]

I am debugging a VHDL block which has a "for loop" inside a
procedure. Within each iteration of the for loop, a variable is
conditionally assigned. For troubleshooting the design, I am
interested in viewing the value of the variable at specific locations
of the process. Since the variable is re-assigned subsequently within
the process, the variable has changed state multiple times in zero
time. I realize this rules out any viewing in a wave window.

Not true.
If I name the process, I can add it by name with an "add wave" command.
However, only the final value is shown on the wave.

I would be happy if I could insert a line of code similar
to the report command, that could display the value of a variable at
the time of execution.

You can do exactly that in simulation.

Any suggestions?

With modelsim, I just say STEP at the command line,
and watch the variables as the code comes into scope.

-- Mike Treseler

 

[backhus]

I am debugging a VHDL block which has a "for loop" inside a
procedure.  Within each iteration of the for loop, a variable is
conditionally assigned.  For troubleshooting the design, I am
interested in viewing the value of the variable at specific locations
of the process.  Since the variable is re-assigned subsequently within
the process, the variable has changed state multiple times in zero
time.  I realize this rules out any viewing in a wave window.

process(clk,reset)
variable a;
variable b;
begin
    if rising_edge(clk) then
        for i in 0 to 7 loop
            if condition1  then
                a  :=  x(i);
            end if;
            b  :=  a;
            if condition2  then
                a  :=  x(i+1);
            end if;
        end for;
    end if
end process;

Here is a simple example.  Please do not recommend a different coding
style, as this is merely an attempt to illustrate the problem.  Let's
say I want to lok at the value of a (or i, for that matter) at the end
of the first if statement.  I do not want to see it after the process
completes.  I would be happy if I could insert a line of code similar
to the report command, that could display the value of a variable at
the time of execution.

Any suggestions?
Brett

Hi,
with Modelsim you also have the option to use the LIST view.
This can be expanded to show each delta cycle, and the changes of a
value in a loop should cause delta cycles.
So you should be able to see the behavior of your variable there.

Have a nice simulation
Eilert

 

[Jonathan Bromley]

with Modelsim you also have the option to use the LIST view.
This can be expanded to show each delta cycle, and the changes of a
value in a loop should cause delta cycles.

Really? If that's true, then VHDL is far too much like
Verilog for my taste

 

[HT-Lab]

Hi,
with Modelsim you also have the option to use the LIST view.
This can be expanded to show each delta cycle, and the changes of a
value in a loop should cause delta cycles.

Just a side note, the waveform window is much easier for looking at Delta
cycles, this is supported in 6.5x and later.

Hans
www.ht-lab.com

 

[Brad Smallridge]

I am debugging a VHDL block which has a "for loop" inside a

procedure. Within each iteration of the for loop, a variable is
conditionally assigned. For troubleshooting the design, I am
interested in viewing the value of the variable at specific locations
of the process. Since the variable is re-assigned subsequently within
the process, the variable has changed state multiple times in zero
time. I realize this rules out any viewing in a wave window.

I assume this is a ModelSim issue.

Just set up a signal to mirror all variables.

Use two or more signals to take care of your
zero time issue. Perhaps put a flag signal
in each of your if statements. You should be
able to see anything reasonable.

Brad Smallridge
AiVision

 

[Andy]

This can be expanded to show each delta cycle, and the changes of a
value in a loop should cause delta cycles.
So you should be able to see the behavior of your variable there.

Jonathan eluded to this, but here's another hint:

Delta cycles occur at suspensions of processes. Thus the iterations of
a loop in a process (so long as the loop does not contain a wait
statement) do not generate delta cycles.

I suppose you could insert "wait for 0 ns;" inside the loop, and it
would generate delta cycles for each loop iteration.

Like Mike and others, I use breakpoints, single-stepping, etc., or I
use a report/assert statement.

Andy