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Altera_Forum

Honored Contributor

11 years ago

VHDL : Problem with 4-Bit Adder and Subtractor !

I want to make a 4-Bit Adder and Subtractor with VHDL

I have created 1-Bit Full-Adder , XOR Gate ( for Subtract ) and a 4-Bit Adder as shown below :

Full-Adder :


LIBRARY ieee; USE ieee.std_logic_1164.all;  ENTITY FullAdder_1_Bit IS   PORT(     X, Y : IN STD_LOGIC;     CIn  : IN STD_LOGIC;     Sum  : OUT STD_LOGIC;     COut : OUT STD_LOGIC   ); END FullAdder_1_Bit;  ARCHITECTURE Behavier OF FullAdder_1_Bit IS BEGIN   Sum <= X XOR Y XOR CIn;   COut <= (X AND Y) OR (X AND CIn) OR (Y AND CIn); END Behavier;

XOR Gate :


LIBRARY ieee; USE ieee.std_logic_1164.all;  ENTITY XORGate IS   PORT(     X1, X2 : IN STD_LOGIC;     Y : OUT STD_LOGIC   );   END XORGate;  ARCHITECTURE Declare OF XORGate IS BEGIN    Y <= X1 XOR X2; END Declare;

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;     Sum  : 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;

and in my Main Codes , i have used those ( like Test-Bench ! ) :


LIBRARY ieee; USE ieee.std_logic_1164.all; USE ieee.std_logic_unsigned.ALL;  ENTITY Add_AND_Sub IS  END Add_AND_Sub;  ARCHITECTURE Declare OF Add_AND_Sub 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 A, B : STD_LOGIC_VECTOR(4 DOWNTO 0); SIGNAL Mode : STD_LOGIC; SIGNAL As, Bs, E, AVF : STD_LOGIC; SIGNAL XA, XB, Sum : STD_LOGIC_VECTOR(3 DOWNTO 0);  BEGIN    Add : Adder_4_Bit  PORT MAP(XA, XB, Mode, Sum, E);   PROCESS(A, B, Mode)  BEGIN   As <= A(4);   Bs <= B(4);    XA <= A(3 DOWNTO 0);   XB <= B(3 DOWNTO 0);      CASE Mode IS    WHEN '0' =>      IF ((As XOR Bs) = '1') THEN                           Mode <= '1';        XA <= Sum;               AVF <= '0';        IF (E = '1') THEN          IF (XA = "0000") THEN            As <= '0';          END IF;        ELSE                     XA <= (NOT XA) + "0001";          As <= NOT As;        END IF;       ELSE        XA <= Sum;      END IF;        WHEN '1' =>      IF ((As XOR Bs) = '1') THEN                      Mode <= '0';               XA <= Sum;               AVF <= E;              ELSE               AVF <= '0';        XA <= Sum;        IF (E = '1') THEN          IF (XA = "0000") THEN            As <= '0';          END IF;        ELSE          XA <= (NOT XA) + "0001";          As <= NOT As;        END IF;            END IF;    WHEN Others =>       --      END CASE;            END PROCESS;   END Declare;

The main scenario is to Model this algorithm :

http://i.stack.imgur.com/3H6hv.jpg

but now i want to have output in XA and As

I Should use registers shown in algorithm such as "E" and "AVF"

there is one question :

we know port maps are continuously connected , so when i change "Mode" Value , Result ( Sum ) must change , is it True ?!

I have tried this code but i cant get output in XA , and there is no True result for sum values , i know there is some problem in my main code ( Process ) , but i cant find problems

please check that codes and tell me what goes wrong !

thanks ...

11 Replies

Altera_Forum
Honored Contributor
11 years ago
any help ?!
Altera_Forum
Honored Contributor
11 years ago
Your question doesnt make a lot of sense, and your code has been posted without any carrige returns!

What exactly is the problem?

Altera_Forum

Honored Contributor

11 years ago

I made some changes

I made a ALU unit as :


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;
    A, B : IN STD_LOGIC_VECTOR(4 DOWNTO 0);
    Sel  : IN STD_LOGIC;
    AOut : OUT STD_LOGIC_VECTOR(4 DOWNTO 0);
    AsO  : 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 As, Bs, E, AVF : STD_LOGIC;
SIGNAL XA, XB, Sum : STD_LOGIC_VECTOR(3 DOWNTO 0);
SIGNAL Mode : STD_LOGIC;
BEGIN   
  
 Add : Adder_4_Bit
 PORT MAP(XA, XB, Mode, Sum, E);   
 PROCESS
 BEGIN
 
 As <= A(4);
 Bs <= B(4);
 
 XA <= A(3 DOWNTO 0);
 XB <= B(3 DOWNTO 0);  
  
  CASE Sel IS
   WHEN '0' =>
     IF ((As XOR Bs) = '1') THEN                   
       Mode <= '1';              
       AVF <= '0';
       WAIT ON Sum;
       IF (E = '1') THEN
         IF (Sum = "0000") THEN
           As <= '0';
         END IF;
       ELSE           
         Sum <= (NOT Sum) + "0001";
         As <= NOT As;
       END IF; 
     ELSE
       Mode <= '0';
       WAIT ON Sum;
     END IF;
     
     AOut <= Sum;
     AsO  <= As;  
        
   WHEN '1' =>
     IF ((As XOR Bs) = '1') THEN              
       Mode <= '0';
       WAIT ON Sum;                     
       AVF <= E;        
     ELSE       
       Mode <= '1';
       WAIT ON Sum;
       AVF <= '0';
       IF (E = '1') THEN
         IF (Sum = "0000") THEN
           As <= '0';
         END IF;
       ELSE
         Sum <= (NOT Sum) + "0001";
         As <= NOT As;
       END IF;      
     END IF;
     AOut <= Sum;
     AsO  <= As;
   WHEN Others =>  
    --   
  END CASE;
            
 END PROCESS; 
  
END Declare;

and A Test Bench like this :


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;
    A, B : IN STD_LOGIC_VECTOR(4 DOWNTO 0);
    Sel  : IN STD_LOGIC;
    AOut : OUT STD_LOGIC_VECTOR(4 DOWNTO 0);
    AsO  : OUT STD_LOGIC
  );  
END COMPONENT;
SIGNAL Xs, S : STD_LOGIC;
SIGNAL X, Y, O : STD_LOGIC_VECTOR(4 DOWNTO 0);
BEGIN
  
  ALU_PM : ALU PORT MAP(X, Y, S, O, Xs);
 
  Main_Process : PROCESS
  BEGIN       
   WAIT FOR 100 ns;  
   X <= "00010";
   Y <= "11011";
             
   S <= '0';
   WAIT FOR 30 ns;
   S <= '1';
   WAIT FOR 30 ns;
   WAIT FOR 100 ns;  
   X <= "01110";
   Y <= "10011";
             
   S <= '0';
   WAIT FOR 30 ns;
   S <= '1';
   WAIT FOR 30 ns; 
   
   WAIT FOR 100 ns;  
   X <= "10011";
   Y <= "11111";
             
   S <= '0';
   WAIT FOR 30 ns;
   S <= '1';
   WAIT FOR 30 ns; 
      
  END PROCESS;
  
END Declare;

As i say , i want to model the algorithm i posted in first post

there is some problem ...

for example when i simulate and run test bench , there is no output value in O and Xs !

I know the problem is in ALU and Test Bench

I changed ALU many times and tested many ways but all times some things goes wrong !

If you want to code that algorithm , which units you will create or at all what will you create ?! and how will you code that ?!

thanks for your help ...

Altera_Forum
Honored Contributor
11 years ago
Well you have a test bench, so you can easy follow your signals backwards and debug it yourself.
But I will note that your first code is not synthesisable. You cannot have wait statements like you have inside a process. What do you expect the circuit to look like? Have you drawn the circuit diagram before you wrote the code?
Altera_Forum
Honored Contributor
11 years ago
No , I have no diagram !
I think the main problem is : "How to change Sum Value ( or calc Sum again ) after changing Mode Value ?"
this rule that the Port maps are continuously connected, is an important thing that i should attent to. I think I cant make this rule in my code !
Altera_Forum
Honored Contributor
11 years ago
You cant make the rule in the code when you treat the code as if it were a programming language. But this is not a programming language, it is a hardware description language - therefore you need to know what the hardware will be before you write the code!

You need to take a step back, and draw the circuit you expect. If you dont know what to expect, then you need to go and get a text book on digital logic design. Trying to treat VHDL as if it were a programming language just leads to bad results, or just no results at all.

Altera_Forum

Honored Contributor

11 years ago

The simple Chart is like this :

http://www.alteraforum.com/forum/attachment.php?attachmentid=10757&stc=1

I made some changes

New ALU Code :


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;
    A : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
    B : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
    As : INOUT STD_LOGIC;
    Bs : IN STD_LOGIC;
    Mode : IN STD_LOGIC;
    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 : STD_LOGIC;
SIGNAL Sum : STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN   
  
 Add : Adder_4_Bit PORT MAP(A, B, Mode, Sum, E);   
 PROCESS(As, Bs, A, B, Mode)
 BEGIN   
  CASE Mode IS
   WHEN '0' =>
     IF ((As XOR Bs) = '1') THEN                                
       AVF <= '0';
       IF (E = '1') THEN
         IF (Sum = "0000") THEN
           As <= '0';
         END IF;
       ELSE           
         Sum <= (NOT Sum) + "0001";
         As <= NOT As;
       END IF;
     ELSE
       AVF <= E;
     END IF;
        
   WHEN '1' =>
     IF ((As XOR Bs) = '1') THEN                                  
       AVF <= E;        
     ELSE       
       AVF <= '0';
       IF (E = '1') THEN
         IF (Sum = "0000") THEN
           As <= '0';
         END IF;
       ELSE
         Sum <= (NOT Sum) + "0001";
         As <= NOT As;
       END IF;      
     END IF;
     
   WHEN Others =>  
    --   
  END CASE;
            
 END PROCESS; 
  
END Declare;

New Test Bench Code :


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;
    A : INOUT STD_LOGIC_VECTOR(3 DOWNTO 0);
    B : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
    As : INOUT STD_LOGIC;
    Bs : IN STD_LOGIC;
    Mode : IN STD_LOGIC;
    AVF : OUT STD_LOGIC
  );   
END COMPONENT;
SIGNAL Xs, Ys, M, Av : STD_LOGIC;
SIGNAL X, Y : STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN
  
  ALU_PM : ALU PORT MAP(X, Y, Xs, Ys, M, 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;

but when I tried to simulate test bench, got the result below !! :

http://www.alteraforum.com/forum/attachment.php?attachmentid=10758&stc=1

Why the X value is 'U' ?!?

what goes wrong with my Test-Bench ?

* Edit :

I tested ALU , Problem is in ALU that Set Value of Sum to 'U'

I can`t find the problem ! , help please ! ...

multiple-attachments.zip71 KB

Altera_Forum
Honored Contributor
11 years ago
are you sure you are running this testbench? it doesnt look like you're running the correct code
Altera_Forum
Honored Contributor
11 years ago
Yes, I`m sure , when Sum is INOUT this problem occurs !
I tested ALU Directly , problem is in ALU
may you test ALU and find the problem ?
...
I have a 4 bit Adder , A and B are 4-Bit Registers , As and Bs are sign bit of A and B ( A + As = 5 Bit ) , I want to Add or Sub A and B, and calc the sign bit of the result with comparing As and Bs , at the end , the result should store in A and As

this is a simple scenario but i can`t find the problem !
Altera_Forum
Honored Contributor
11 years ago
The problem is the inout - because you are not driving it internally the 'U' overrides the '1' driven from outside.

But you should not be using inout anyway - there are no internal tri-states in an FPGA. You should use in and out only.
Plus, your component declaration does not match the entity, as the entity uses an in, and the component uses inout.

Forum Discussion

VHDL : Problem with 4-Bit Adder and Subtractor !

11 Replies

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