VHDL II

Tom Kelliher, CS 220

Mar. 13, 2006

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Assignment

Read 6.1-2.

Assignment due Mar. 27.

From Last Time

Structural VHDL.

Outline

1. Dataflow VHDL.

2. Hierarchical VHDL.

3. Behavioral VHDL.

4. Class practice.

Coming Up

Introduction to sequential circuits. Latches.

Dataflow VHDL

Last time's EXOR3:

library ieee;
use ieee.std_logic_1164.all;

entity EXOR3 is
   port
   (
      i : in std_logic_vector (2 downto 0),
      o : out std_logic
   );
end EXOR3;

architecture dataflow of EXOR3 is

   signal i2_n, i1_n, i0_n : std_logic;
   signal m1, m2, m3, m4 : std_logic;     -- Minterms.

begin
   i2_n <= not i(2);
   i1_n <= not i(1);
   i0_n <= not i(0);

   m1 <= i2_n and i1_n and i(0);
   m2 <= i2_n and i(1) and i0_n;
   m3 <= i(2) and i1_n and i0_n;
   m4 <= i(2) and i(1) and i(0);

   o <= m1 or m2 or m3 or m4;
end dataflow;

architecture function_table of EXOR3 is
begin
   with i select
      o <= '0' when "000",
           '1' when "001",
           '1' when "010",
           '0' when "011",
           '1' when "100",
           '0' when "101",
           '0' when "110",
           '1' when "111",
           'X' when others;
end function_table;

new VHDL elements: vectors and downto (also ``to''). not, and, or. Concurrent assignment. With-Select, when, others.

Hierarchical VHDL

Use of components. Full binary adder constructed from two half-adders example:

library ieee;
use ieee.std_logic_1164.all;

entity ha is
   port
   (
      a, b : in std_logic;
      c, s : out std_logic
   );
end ha;

architecture dataflow of ha is
begin
   s <= a xor b;
   c <= a and b;
end dataflow;


library ieee;
use ieee.std_logic_1164.all;

entity fa is
   port
   (
      a, b c_i : in std_logic;
      c_o, s   : out std_logic
   );
end fa;

architecture mixed of fa is

   component ha
      port
      (
         a, b : in std_logic;
         c, s : out std_logic
      );
   end component;

   signal hs, hc, tc : std_logic;

begin
   ha1 : ha port map (a, b, hc, hs);
   ha2 : ha port map (hs, c_i, tc, s);

   c_o <= hc or tc;
end mixed;

Nothing really new -- we've seen components already.

Behavioral VHDL

High level VHDL. Four bit adder:

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;

entity adder4 is
   port
   (a, b : in std_logic_vector (3 downto 0);
    c0   : in std_logic;
    s    : out std_logic_vector (3 downto 0);
    c4   : out std_logic
   );
end adder4;

architecture behavioral of adder4 is

   signal sum : std_logic_vector (4 downto 0);

begin
   sum <= ('0' & a) + ('0' & b) + ("0000" & c0);
   c4 <= sum(4);
   s <= sum(3 downto 0);
end behavioral;

Note use of ``high level'' operators: +, &.

Class Practice

Write dataflow (includes function table) VHDL for a 4-1 mux.

Using the fa component, design a hierarchical four-bit adder.

Write dataflow VHDL for an eight-input priority encoder.

Tom Kelliher