Why on Simulation the result is not what is expected?

[b.g.dariush]

Hey guys
I have a presentation for next couple days on VHDL and i need to show
some simple example project, I managed to write a <b>4 Bit Adder</b> as
follow:
Adder4.vhd:
--------------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;

ENTITY Adder4 IS
PORT
(
Cin : IN STD_LOGIC;
X, Y : IN STD_LOGIC_VECTOR (3 DOWNTO 0);
R : OUT STD_LOGIC_VECTOR (3 DOWNTO 0);
Cout : OUT STD_LOGIC
);
END Adder4;

ARCHITECTURE Adder4_Behav OF Adder4 IS
SIGNAL Carry : STD_LOGIC_VECTOR ( 2 DOWNTO 0);

COMPONENT FullAdder
PORT
(
Cin, A, B : IN STD_LOGIC;
Cout, S : OUT STD_LOGIC
);
END COMPONENT;
BEGIN
FA0: FullAdder PORT MAP ( Cin, X(0), Y(0), Carry(0), R(0));
FA1: FullAdder PORT MAP ( Carry(0), X(1), Y(1), Carry(1), R(1));
FA3: FullAdder PORT MAP ( Carry(1),X(2),Y(2),Carry(2),R(2));
FA4: FullAdder PORT MAP ( Carry(2),X(3),Y(3),Cout,R(3));
END;
--------------------------------------------------------------------------------

FullAdder.vhd

--------------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;

ENTITY FullAdder IS
PORT
(
Cin, A, B : IN STD_LOGIC;
Cout, S : OUT STD_LOGIC
);
END FullAdder;

ARCHITECTURE FullAdder_Behav OF FullAdder IS
BEGIN
S <= A XOR B XOR Cin;
Cout <= (A AND Cin) OR (B AND Cin) OR (A AND B);
END;
--------------------------------------------------------------------------------

but when i run simulation the output result is not synced at all!
I know i am not considering something here, but what is that?
I need HELP ASAP, please
What is it that i am doing wrong!?

I don't see any attachment option here
but i have uploaded the .vwf and the simulation result as attachments
in
http://embdev.net/attachment/160752/Vector_Wave_File.png

AND

http://embdev.net/attachment/160753/Simu_Result.jpg

Thanks in advace

 

[Nicolas Matringe]

Le 18/11/2012 20:36, (e-mail address removed) a écrit :

Hey guys [...]
but when i run simulation the output result is not synced at all!
I know i am not considering something here, but what is that?
I need HELP ASAP, please
What is it that i am doing wrong!?

Absolutely nothing.
You are simulating an implemented design and what you see is actual gate
delays. That's how the real world works. What did you expect ?

Nicolas

 

[rickman]

Hey guys
I have a presentation for next couple days on VHDL and i need to show
some simple example project, I managed to write a<b>4 Bit Adder</b> as
follow:
Adder4.vhd:
--------------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;

ENTITY Adder4 IS
PORT
(
Cin : IN STD_LOGIC;
X, Y : IN STD_LOGIC_VECTOR (3 DOWNTO 0);
R : OUT STD_LOGIC_VECTOR (3 DOWNTO 0);
Cout : OUT STD_LOGIC
);
END Adder4;

ARCHITECTURE Adder4_Behav OF Adder4 IS
SIGNAL Carry : STD_LOGIC_VECTOR ( 2 DOWNTO 0);

COMPONENT FullAdder
PORT
(
Cin, A, B : IN STD_LOGIC;
Cout, S : OUT STD_LOGIC
);
END COMPONENT;
BEGIN
FA0: FullAdder PORT MAP ( Cin, X(0), Y(0), Carry(0), R(0));
FA1: FullAdder PORT MAP ( Carry(0), X(1), Y(1), Carry(1), R(1));
FA3: FullAdder PORT MAP ( Carry(1),X(2),Y(2),Carry(2),R(2));
FA4: FullAdder PORT MAP ( Carry(2),X(3),Y(3),Cout,R(3));
END;
--------------------------------------------------------------------------------

FullAdder.vhd

--------------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;

ENTITY FullAdder IS
PORT
(
Cin, A, B : IN STD_LOGIC;
Cout, S : OUT STD_LOGIC
);
END FullAdder;

ARCHITECTURE FullAdder_Behav OF FullAdder IS
BEGIN
S<= A XOR B XOR Cin;
Cout<= (A AND Cin) OR (B AND Cin) OR (A AND B);
END;
--------------------------------------------------------------------------------

but when i run simulation the output result is not synced at all!
I know i am not considering something here, but what is that?
I need HELP ASAP, please
What is it that i am doing wrong!?

I don't see any attachment option here
but i have uploaded the .vwf and the simulation result as attachments
in
http://embdev.net/attachment/160752/Vector_Wave_File.png

AND

http://embdev.net/attachment/160753/Simu_Result.jpg

Thanks in advace

You didn't show the test bench file you are using. My guess is that in
your test bench you are generating the X, Y and Cin signals, then
running them through registers before they reach the full adder. It
looks like you are using a 10 nS (100 MHz) clock.

The changes that occur just after the inputs change at 20 and 30 ns in
the latter image are the result of logic delay times. You seem to be
running a timing mode simulation. These will go away if you run a logic
mode simulation which is just on the VHDL code and not on the placed and
routed chip.

Can we see your test bench file? Oh, I see the first image is a vector
file. Is that your stimulus? I don't think I have done that in a long,
long time. You might try writing a test bench instead.

Rick

 

[goouse99]

Am Sonntag, 18. November 2012 22:46:16 UTC+1 schrieb Nicolas Matringe:

Le 18/11/2012 20:36, b.g. a écrit :

Hey guys
[...]

but when i run simulation the output result is not synced at all!

I know i am not considering something here, but what is that?

I need HELP ASAP, please

What is it that i am doing wrong!?

Absolutely nothing.

You are simulating an implemented design and what you see is actual gate

delays. That's how the real world works. What did you expect ?



Nicolas

Hi,
I'm not sure about the behavior of the Quartus simulation tool,
But even if this simple combinatorical circuit has been simulated in timingsim mode, why is the first result there immediately (t=0).
Also the inputs seem to be applied to the actual circuit quite late (exceptfor t=0). This might irritate a beginner, because one would expect something different from a simple combinatorical circuit. It might be something caused by this vector file simulation.

As rickman suggested, a HDL Tesbench would be useful.

Have a nice simulation
Eilert