Unit 4

Design using VHDL

4.1 Designing basic gates

Figure 1. Logic gates

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

entity logic_gate is

     Port ( A,B : in std_logic;

               y_and,y_or,y_nand,y_nor,y_not,y_xor,y_xnor : out std_logic);

end logic_gate;

architecture all_gates of logic_gate is

begin

          y_and <= a and b;

          y_or <= a or b;

          y_nand <= a nand b;

          y_nor <= a nor b;

          y_not <= not a ;

          y_xor <= a xor b;

          y_xnor <= a xnor b;

end all_gates;

Key takeaways:

VHDL program to build all NAND, NOR, XOR, and XNOR gates using AND-OR-NOT gates

4.2 Combinational circuit

Half Adder

Library ieee;

use ieee.std_logic_1164.all;

entity half_adder is

   port(a,b:in bit; sum,carry:out bit);

end half_adder;

architecture data of half_adder is

begin

   sum<= a xor b; 

   carry <= a and b; 

end data;

Figure2. Half adder waveforms

Library ieee;

use ieee.std_logic_1164.all;

 Half subtractor

Library ieee;

use ieee.std_logic_1164.all;

entity half_sub is

   port(a,c:in bit; d,b:out bit);

end half_sub; 

architecture data of half_sub is

begin

   d<= a xor c;

   b<= (a and (not c));

end data;

Figure 3. Half subtractor

Full Subtractor

Library ieee;

use ieee.std_logic_1164.all;

entity full_sub is

   port(a,b,c:in bit; sub,borrow:out bit);

end full_sub;

architecture data of full_sub is

begin

   sub<= a xor b xor c;

   borrow <= ((b xor c) and (not a)) or (b and c);

end data;

Figure 4. Full subtractor

Key takeaways:

In combinational behavior can be specified as concurrent signal assignments

4.3 Designing general purpose processor

Figure 5. General purpose processor

Key takeaways

A microprocessor is a computer processor which incorporates the functions of a computer's central processing unit (processor) on a single integrated chip.

4.4 Datapath

Consider the example of an adder that can add 4-values. Assume, we want to add, a, b, c, d. If the output is s, the code can be written as,

s <= ( ( a + b ) + c ) + d ;

However, this results in a large circuit because the circuit will have 3 adders as shown in the figure,

Figure6. Adders

Use one adder and use it sequentially to add up all the four values. This is when we will need FSM/a control path. Lets take a look how the circuit might look like,

Figure7. Control path circuit

To define the data path it works on 2-bit input sel and 1-bit inputs load and clear from the control path.
-- datapath

library ieee ;

use ieee.std_logic_1164.all ;

use ieee.std_logic_arith.all ;

use work.averager_types.all ;

entity datapath is

port (

    a, b, c, d : in num ;

    sum : out num ;

    sel : in std_logic_vector (1 downto 0) ;

    load, clear, clk : in std_logic

) ;

end datapath ;

architecture rtl of datapath is

    signal mux_out, sum_reg, next_sum_reg :num ;

    constant sum_zero :num :=

    conv_unsigned(0,next_sum_reg’length) ;

begin

    -- mux to select input to add

    with sel select mux_out<=

    a when "00",

    b when "01",

    c when "10",

    d when others ;

    -- mux to select register input

    next_sum_reg<=

    sum_reg + mux_out when load = ’1’ else

    sum_zero when clear = ’1’ else

    sum_reg ;

    -- register sum

    process(clk)

    begin

        if clk’event and clk = ’1’ then

        sum_reg<= next_sum_reg ;

        end if ;

    end process ;

    -- entity output is register output

    sum <= sum_reg ;

end rtl ;

The next state is the controller which controls the datapath.

-- controller

library ieee ;

use ieee.std_logic_1164.all ;

use work.averager_types.all ;

entity controller is

port (

    update : in std_logic ;

    sel : out std_logic_vector (1 downto 0) ;

    load, clear : out std_logic ;

    clk : in std_logic

) ;

end controller ;

architecture rtl of controller is

    signal s, holdns, ns : states ;

    signal tmp :std_logic_vector (3 downto 0) ;

begin

    -- select next state

    with s select ns <=

    add_a when clr,

    add_b when add_a,

    add_c when add_b,

    add_d when add_c,

    hold when add_d,

    holdns when others ; -- hold

    -- next state if in hold state

    holdns<=

    clr when update = ’1’ else

    hold ;

    -- state register

    process(clk)

    begin

        if clk’event and clk = ’1’ then

        s <= ns ;

        end if ;

    end process ;

     -- controller outputs

    with s select sel<=

        "00" when add_a,

        "01" when add_b,

        "10" when add_c,

        "11" when others ;

    load <= ’0’ when s = clr or s = hold else ’1’ ;

    clear <= ’1’ when s = clr else ’0’ ;

end rtl ;

4.5 ALU

ALU’s comprise the combinational logic that implements logic operations such as AND, OR, NOT gate and arithmetic operations, such as Adder, Subtractor.

Functionally, the operation of typical ALU is represented as shown in diagram below,

Figure 8. Functional Description of 4-bit Arithmetic Logic Unit

Controlled by the three function select inputs (sel 2 to 0), ALU can perform all the 8 possible logic operations

VHDL Code for 4-bit ALU

Testbench VHDL Code for 4-Bit ALU

Simulation Result for 4-bit ALU

Figure 9. ALU

4.6 Encoder and Decoder

In a 4:2 encoder, the circuit takes 4 bits of data as input. It then codes the data to give an output of two bits.

By behavior, we mean, the response or output of a circuit under the application of a certain set of inputs.

Truth table of a 4:2 encoder

The entity-architecture pair. The entity will have the port declaration statements of four input ports and two output ports. This is defined as  STD_LOGIC_VECTOR datatype.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity ENCODER_SOURCE is

Port ( I : in  STD_LOGIC_VECTOR (3 downto 0);

          Y : out STD_LOGIC_VECTOR (1 downto 0));

end ENCODER_SOURCE;

The behavioral code always has a process statement. The sensitivity list variable here would be the input vector I. We begin the architecture first, define the process, and then begin the process. This order is imperative.

architecture Behavioral of ENCODER_SOURCE is

begin

process (I)

begin

The if sequential statement that we use in VHDL is not to be confused with the if or else-if statement that we are accustomed to in the C programming language. The syntax is a bit different here.

if I = "0001" then Y <= "00";

elsif I = "0010" then Y <= "01";

elsif I = "0100" then Y <= "10";

else Y <= "11";

Towards the end, you need to close the begin statements, as well as the if statement.

end if;

end process;

end Behavioral;

Binary decoder

Binary decoder has n-bit input lines and 2 power n output lines. It can be 2-to-4, 3-to-8 and 4-to-16 line configurations. Binary decoder can be easily constructed using basic logic gates. VHDL Code of 2 to 4 decoder can be easily implemented with structural and behavioral modelling.

Figure 10. 2 to 4 Decoder design using logic gates

Figure 11. Truth Table for 2 to 4 Decoder

Similar to Encoder Design, VHDL Code for 2 to 4 decoder can be done in different methods like using case statement, using if else statement, using logic gates etc.

VHDL Code for 2 to 4 decoder using case statement

library IEEE;

use IEEE.STD_LOGIC_1164.all;

entity decoder is

port(

a : in STD_LOGIC_VECTOR(1 downto 0);

b : out STD_LOGIC_VECTOR(3 downto 0)

);

end decoder;

architecture bhv of decoder is

begin

process(a)

Begin

case a is

when "00" => b <= "0001"; when "01" => b <= "0010"; when "10" => b <= "0100"; when "11" => b <= "1000";

end case;

end process;

end bhv;

VHDL Code for 2 to 4 decoder using if else statement

library IEEE;

use IEEE.STD_LOGIC_1164.all;

entity decoder1 is

 port(

 a : in STD_LOGIC_VECTOR(1 downto 0);

 b : out STD_LOGIC_VECTOR(3 downto 0)

 );

end decoder1;

architecture bhv of decoder1 is

Begin

process(a)

Begin

 if (a="00") then

 b <= "0001";

 elsif (a="01") then

 b <= "0010";

 elsif (a="10") then

 b <= "0100";

 Else

 b <= "1000";

 end if;

end process;

end bhv;

VHDL Code for 2 to 4 decoder using logic gates

library IEEE;

use IEEE.STD_LOGIC_1164.all;

entity decoder2 is

 port(

 a : in STD_LOGIC_VECTOR(1 downto 0);

 b : out STD_LOGIC_VECTOR(3 downto 0)

 );

end decoder2;

architecture bhv of decoder2 is

Begin

b(0) <= not a(0) and not a(1);

b(1) <= not a(0) and a(1);

b(2) <= a(0) and not a(1);

b(3) <= a(0) and a(1);

end bhv;

TestBench VHDL Code for 2 to 4 decoder

LIBRARY ieee;

USE ieee.std_logic_1164.ALL;

ENTITY tb_decoder IS

END tb_decoder;

ARCHITECTURE behavior OF tb_decoder IS

 – Component Declaration for the Unit Under Test (UUT)

 COMPONENT decoder

 PORT(

 a : IN std_logic_vector(1 downto 0);

 b : OUT std_logic_vector(3 downto 0)

 );

 END COMPONENT;

 – Inputs

 signal a : std_logic_vector(1 downto 0) := (others => '0');

 – Outputs

 signal b : std_logic_vector(3 downto 0);

 – appropriate port name

BEGIN

 – Instantiate the Unit Under Test (UUT)

 uut: decoder PORT MAP (

 a => a,

 b => b

 );

 – Stimulus process

 stim_proc: process

 Begin

 – hold reset state for 100 ns.

 wait for 100 ns;

 a <= "00";

 wait for 100 ns;

 a <= "01";

 wait for 100 ns;

 a <= "10";

 wait for 100 ns;

 a <= "11";

 wait;

 end process;

END;

Figure 11. Test Bench waveform

4.7 Adder and Subtractor

Library ieee;

use ieee.std_logic_1164.all;

entity full_adder is port(a,b,c:in bit; sum,carry:out bit);

end full_adder;

architecture data of full_adder is

begin

   sum<= a xor b xor c;

   carry <= ((a and b) or (b and c) or (a and c));

end data;

Figure 12. Full Adder

Full Subtractor

Library ieee;

use ieee.std_logic_1164.all;

entity full_sub is

   port(a,b,c:in bit; sub,borrow:out bit);

end full_sub;

architecture data of full_sub is

begin

   sub<= a xor b xor c;

   borrow <= ((b xor c) and (not a)) or (b and c);

end data;

Figure 13.  Full Subtractor

4.8 Multiplexer and Demultiplexer 

Library ieee;

use ieee.std_logic_1164.all;

entity mux is

   port(S1,S0,D0,D1,D2,D3:in bit; Y:out bit);

end mux;

architecture data of mux is

begin

   Y<= (not S0 and not S1 and D0) or

      (S0 and not S1 and D1) or

      (not S0 and S1 and D2) or

      (S0 and S1 and D3);

end data;

Waveforms

Figure14. Multiplexer

VHDL Code for a Demultiplexer

Library ieee;

use ieee.std_logic_1164.all;

entity demux is

port(S1,S0,D:in bit; Y0,Y1,Y2,Y3:out bit);

end demux;

architecture data of demux is

begin

   Y0<=  ((Not S0) and (Not S1) and D);

   Y1<=  ((Not S0) and S1 and D);

   Y2<=  (S0 and (Not S1) and D);

   Y3<=  (S0 and S1 and D);

end data;

Waveforms

Figure 15. Demultiplexer

4.9 Tristate drivers 

library IEEE;

use IEEE.std_logic_1164.all;

entity TS_DRVR is

  port ( IN_BITS  : in BIT_VECTOR ( 15 downto 0 );

  OE       : in BIT := '1';

         OUT_BITS : out STD_LOGIC_VECTOR (15 downto 0)  );

end TS_DRVR;

architecture SIMPLE of TS_DRVR is

begin

process ( OE, IN_BITS )   -- if OE or input data

      -- change, may change output

  begin

    if OE = '0' then    -- if OE asserted, then

      OUT_BITS <= To_StdLogicVector ( IN_BITS );

    else     -- otherwise

      OUT_BITS <= "ZZZZZZZZZZZZZZZZ";   -- tri-state output

    end if;

  end process;

end SIMPLE;

4.10 PIPO

Figure 16. 4 bit PIPO

library IEEE;
use IEEE.STD_LOGIC_1164.all;

entity pipo_behavior is
     port(
         clk : in STD_LOGIC;
         reset : in STD_LOGIC;
         din : in STD_LOGIC_VECTOR(3 downto 0);
         dout : out STD_LOGIC_VECTOR(3 downto 0)
         );
end pipo_behavior;


architecture pipo_behavior_arc of pipo_behavior is
begin

     pipo : process (clk,din,reset) is                 
    begin
        if (reset='1') then
            dout<= "0000";
        elsif (rising_edge (clk)) then      
            dout<= din;   
        end if;     
    end process pipo;

end pipo_behavior_arc;

References: