VHDL : 'X' value in result of Adder - vhdl

Question

I have created a 4-Bit Adder , now I want to add and sub 2 registers as sign-magnitude values
so , there is two register named A and B , two bits named As and Bs have sign bits of values in A and B , one XOR Gate for making 2-complement of B in subtraction and at the end result should store in A and As ( value and Sign ) and overflow bit in a register named AVF
this is a simple diagram :
Mode = 1 => Sub; Mod = 0 => Add
I have written this codes :
4-Bit Adder :
LIBRARY ieee;
USE ieee.std_logic_1164.all;
ENTITY Adder_4_Bit IS
PORT(
A, B : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
Mode : IN STD_LOGIC;
Sum : OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
COut : OUT STD_LOGIC
);
END Adder_4_Bit;
ARCHITECTURE Structure OF Adder_4_Bit IS
COMPONENT FullAdder_1_Bit IS
PORT(
X, Y : IN STD_LOGIC;
CIn : IN STD_LOGIC;
FSum : OUT STD_LOGIC;
COut : OUT STD_LOGIC
);
END COMPONENT;
COMPONENT XORGate IS
PORT(
X1, X2 : IN STD_LOGIC;
Y : OUT STD_LOGIC
);
END COMPONENT;
SIGNAL COut_Temp : STD_LOGIC_VECTOR(2 DOWNTO 0);
SIGNAL XB : STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN
B_0 : XORGate PORT MAP(Mode, B(0), XB(0));
B_1 : XORGate PORT MAP(Mode, B(1), XB(1));
B_2 : XORGate PORT MAP(Mode, B(2), XB(2));
B_3 : XORGate PORT MAP(Mode, B(3), XB(3));
SUM_0 : FullAdder_1_Bit
PORT MAP (A(0), XB(0), Mode, Sum(0), COut_Temp(0));
SUM_1 : FullAdder_1_Bit
PORT MAP (A(1), XB(1), COut_Temp(0), Sum(1), COut_Temp(1));
SUM_2 : FullAdder_1_Bit
PORT MAP (A(2), XB(2), COut_Temp(1), Sum(2), COut_Temp(2));
SUM_3 : FullAdder_1_Bit
PORT MAP (A(3), XB(3), COut_Temp(2), Sum(3), COut);
END;
ALU :
LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.numeric_std.all;
USE ieee.std_logic_unsigned.ALL;
ENTITY ALU IS
PORT(
--Clk : IN STD_LOGIC;
C : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
D : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
Cs : IN STD_LOGIC;
Ds : IN STD_LOGIC;
Mode_ALU : IN STD_LOGIC;
Sum_ALU : OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
AVF : OUT STD_LOGIC
);
END ALU;
ARCHITECTURE Declare OF ALU IS
COMPONENT Adder_4_Bit IS
PORT(
A, B : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
Mode : IN STD_LOGIC;
Sum : OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
COut : OUT STD_LOGIC
);
END COMPONENT;
SIGNAL E, Temp_Cs, Temp_Ds : STD_LOGIC;
SIGNAL Temp_S : STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN
Add : Adder_4_Bit PORT MAP(C, D, Mode_ALU, Temp_S, E);
-- Sum_ALU <= Temp_S;
-- Temp_Cs <= Cs;
-- Temp_Ds <= Ds;
PROCESS
BEGIN
WAIT FOR 30 ns;
Sum_ALU <= Temp_S;
Temp_Cs <= Cs;
Temp_Ds <= Ds;
END PROCESS;
PROCESS(C, D, Cs, Ds, Mode_ALU)
BEGIN
CASE Mode_ALU IS
WHEN '0' =>
IF ((Cs XOR Ds) = '1') THEN
AVF <= '0';
IF (E = '1') THEN
IF (Temp_S = "0000") THEN
Temp_Cs <= '0';
END IF;
ELSE
Sum_ALU <= (NOT Temp_S) + "0001";
Temp_Cs <= NOT Cs;
END IF;
ELSE
AVF <= E;
END IF;
WHEN '1' =>
IF ((Cs XOR Ds) = '1') THEN
AVF <= E;
ELSE
AVF <= '0';
IF (E = '1') THEN
IF (Temp_S = "0000") THEN
Temp_Cs <= '0';
END IF;
ELSE
Sum_ALU <= (NOT Temp_S) + "0001";
Temp_Cs <= NOT Cs;
END IF;
END IF;
WHEN Others =>
--
END CASE;
END PROCESS;
END Declare;
Test Bench :
LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.numeric_std.all;
USE ieee.std_logic_unsigned.ALL;
ENTITY ALU_Test_Bench IS
END ALU_Test_Bench;
ARCHITECTURE Declare OF ALU_Test_Bench IS
COMPONENT ALU IS
PORT(
--Clk : IN STD_LOGIC;
C : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
D : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
Cs : IN STD_LOGIC;
Ds : IN STD_LOGIC;
Mode_ALU : IN STD_LOGIC;
Sum_ALU : OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
AVF : OUT STD_LOGIC
);
END COMPONENT;
SIGNAL Xs, Ys, M, Av : STD_LOGIC;
SIGNAL X, Y, O : STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN
ALU_PM : ALU PORT MAP(X, Y, Xs, Ys, M, O, Av);
Mode_Process : PROCESS
BEGIN
M <= '1';
WAIT FOR 10 ns;
M <= '0';
WAIT FOR 10 ns;
END PROCESS;
Calc_Process : PROCESS
BEGIN
X <= "0010";
Y <= "1011";
Xs <= '0';
Ys <= '1';
WAIT FOR 20 ns;
X <= "0110";
Y <= "0011";
Xs <= '1';
Ys <= '1';
WAIT FOR 20 ns;
X <= "0010";
Y <= "1011";
Xs <= '0';
Ys <= '1';
WAIT FOR 20 ns;
END PROCESS;
END Declare;
when I run test bench , the result value filled with 'X' :
I know the problem is in ALU , but I can`t find the problem.
There is no problem in 4-Bit Adder , I have tested.
Another problem is calc sign bit of the result , Is the PROCESSes I have written correct ?
At all what I should do to Code the diagram above ?
thanks ...

Answer 1

You have multiple drivers on signals Sum_ALU, Temp_Cs and Temp_Ds in file alu.vhd.
PROCESS
BEGIN
WAIT FOR 30 ns;
Sum_ALU <= Temp_S;
Temp_Cs <= Cs;
Temp_Ds <= Ds;
END PROCESS;
PROCESS(C, D, Cs, Ds, Mode_ALU)
BEGIN
CASE Mode_ALU IS
WHEN '0' =>
IF ((Cs XOR Ds) = '1') THEN
AVF <= '0';
IF (E = '1') THEN
IF (Temp_S = "0000") THEN
Temp_Cs <= '0';
END IF;
ELSE
Sum_ALU <= (NOT Temp_S) + "0001";
Temp_Cs <= NOT Cs;
END IF;
ELSE
AVF <= E;
END IF;
WHEN '1' =>
IF ((Cs XOR Ds) = '1') THEN
AVF <= E;
ELSE
AVF <= '0';
IF (E = '1') THEN
IF (Temp_S = "0000") THEN
Temp_Cs <= '0';
END IF;
ELSE
Sum_ALU <= (NOT Temp_S) + "0001";
Temp_Cs <= NOT Cs;
END IF;
END IF;
WHEN Others =>
--
END CASE;
END PROCESS;
Whenever you assign a signal in multiple process, as you did here, it yields multiple drivers. If the drivers don't agree on the value (one drives '1' and the other '0' for example), the result is undefined ('X'). You will have to solve the issue yourself, as I'm not sure what is the correct behaviour. However, if you remove the first process, no undefined signal appears in the simulation.
Furthermore, you should be aware that the statement wait for 30 ns; is not synthesizable. The synthesizer may either fail or simply ignore the wait statement. If your goal was to simulate routing delay, then your usage is fine, otherwise you should change the logic if your goal is synthesis.
Finally, your second process would generate latches if synthesized. Latches are memory element which are known to break circuits when used improperly. They are the main reason why circuit behaviour do not match simulations, and should be removed. Latches appears whenever a signal you assign in a combinational process is not assign in every path of the process. That means Temp_Cs and Sum_ALU needs an assignment every time the process is evaluated (AVF is fine as is); every if must have an else, and all signals must be assigned. One simple way to deal with this is to give default values at the beginning of the process, so that every signal has an assignments. If a signal is assigned multiple times in the evaluation of the process, then only the last assignation will be effective. For example:
PROCESS(C, D, Cs, Ds, Mode_ALU)
BEGIN
Temp_Cs <= Cs;
Sum_ALU <= Temp_S;
CASE Mode_ALU IS
While making assignations in the others branch of the case is not necessary, I would recommend it nevertheless. You can assign all signals to 'X' for example.