VHDL 1 bit two input multiplexer structural design code test in circuit and test bench

This video is part of a series which final design is a Controlled Datapath using a structural approach. A Structural approach consist in designing all components needed for the design such as gates to form subsystems and then joining them together to form a larger design like adders and Arithmetic logic units,etc.

The design in these labs was first developed in VHDL you can check the final VHDL version in the link below as well as intructions on how to set up the Waveshare development board to get started, the setup is the same for VHDL and Verilog:

Lab Sheets:
http://viahold.com/y37

Lab guide

http://cogismith.com/1OwP

This tutorial uses components to understand the basics of microcontroller design.

Intended design:

Datapath and controller internals:

     

Parts working on now:

Constructing the alu:

For constructing the ALU we need an adder, the adder is formed by two half adders and the half adder is formed with gates. Refer to the lab sheets:  

VHDL code:

 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 entity twoinputmultiplexer is  
   Port ( a : in STD_LOGIC;  
       b : in STD_LOGIC;  
       s : in STD_LOGIC;  
                 output : out STD_LOGIC);  
 end twoinputmultiplexer;  
 architecture Behavioral of twoinputmultiplexer is  
 begin  
 process(a,b,s)  
 begin  
           case s is  
            when '0' => output <= a;  
            when '1' => output <= b;  
            when others => output <= b;  
           end case;  
 end process;  
 end Behavioral;  
 --s a b output  
 --0 0 0  0  
 --0 0 1  0  
 --0 1 0  1   
 --0 1 1  1  
 --1 0 0  0  
 --1 0 1  1  
 --1 1 0  0  
 --1 1 1  1  

Testbench

 LIBRARY ieee;  
 USE ieee.std_logic_1164.ALL;  
 ENTITY MUXtest IS  
 END MUXtest;  
 ARCHITECTURE behavior OF MUXtest IS   
   -- Component Declaration for the Unit Under Test (UUT)  
   COMPONENT twoinputmultiplexer  
   PORT(  
      a : IN std_logic;  
      b : IN std_logic;  
      s : IN std_logic;  
      output : OUT std_logic  
     );  
   END COMPONENT;  
   --Inputs  
   signal a : std_logic := '0';  
   signal b : std_logic := '0';  
   signal s : std_logic := '0';  
       --Outputs  
   signal output : std_logic;  
 BEGIN  
      -- Instantiate the Unit Under Test (UUT)  
   uut: twoinputmultiplexer PORT MAP (  
      a => a,  
      b => b,  
      s => s,  
      output => output  
     );  
   -- Stimulus process -- test for all possible inputs of MUX - 8 total combinations  
   stim_proc: process  
   begin            
  wait for 100 ns; -- wait for global reset  
  a <= '0'; -- check 0 nand 0 = 1  
  b <= '0';   
  control <= '0';  
  wait for 100 ns;  
  a <= '0';   
  b <= '0';   
  control <= '1';  
  wait for 100 ns;  
  a <= '0';  
  b <= '1';   
  control <= '0';  
  wait for 100 ns;  
  a <= '0';  
  b <= '1';   
  control <= '1';  
  wait for 100 ns;   
  a <= '1';   
  b <= '0';   
  control <= '0';  
  wait for 100 ns;  
  a <= '1';   
  b <= '0';   
  s <= '1';  
  wait for 100 ns;  
  a <= '1';   
  b <= '1';   
  s <= '0';  
  wait for 100 ns;  
  a <= '1';   
  b <= '1';   
  s <= '1';  
  wait;   
   end process;  
 END;