Supplement of Lecture 5: VHDL Codes of some Combinational Devices.
VHDL model of comparator (8 bits):
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
-- 8 Bit Unsigned Comparator
entity cmp8 is
port (
A,B : in std_logic_vector(7 downto 0);
aeqb, altb, agtb : out std_logic
);
end cmp8;
architecture a of cmp8 is
begin
aeqb <= '1' when (a = b) else '0';
altb <= '1' when (a < b) else '0';
agtb <= '1' when (a > b) else '0';
end;
The operators =, /=, <=, <, >, >= are defined in the std_logic_arith package which is part of the IEEE
library.
-- vhdl model for the 3 to 8 decoder
--*************************************************************************
-- IO Interface Declaration
--*************************************************************************
library ieee;
use ieee.std_logic_1164.all;
entity dec3to8 is
port
(
-- inputs
signal sel: in std_logic_vector(2 downto 0); -- selector
signal en: in std_logic;
-- enable
-- outputs
1
signal y: out std_logic_vector(7 downto 0)
);
end dec3to8;
-- outputs are high true
--*************************************************************************
-- Architecture body
--*************************************************************************
architecture behavior of dec3to8 is
begin
process (sel,en)
begin
y <= "11111111";
if (en = '1') then
case sel is
when "000" => y(0) <= '1';
when "001" => y(1) <= '1';
when "010" => y(2) <= '1';
when "011" => y(3) <= '1';
when "100" => y(4) <= '1';
when "101" => y(5) <= '1';
when "110" => y(6) <= '1';
when "111" => y(7) <= '1';
when others => y(7) <= '1';
end case;
end if;
end process;
end behavior;
74LS138 (3 x 8 decoder)
VHDL Code:
Library ieee
Use ieee.std_logic_1164.all;
Entity V74x138 is
Port (G1, G2A_L, G2B_L: in std_logic; -- enable inputs
A: in std_logic_vector (2 downto 0); -- select inputs A= A(2), B= A(1), C = A(0)
Y_L: out std_logic_vector (0 to 7)); -- decoded outputs
End V74x138;
Architecture V74x138_arch of V74x138 is
Signal Y_int: std_logic_vector(0 to 7);
Begin
With A select Y_int <=
“01111111” when “000”,
“10111111” when “001”,
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“11011111” when “010”,
“11101111” when “011”,
“11110111” when “100”,
“11111011” when “101”,
“11111101” when “110”,
“11111110” when “111”,
“11111111” when others;
Y_L<=Y_L_i when (G1 and not G2A_L and not G2B_L) =’1’ else “11111111”;
End V74x138_arch;
-- vhdl model for 8 level priority circuit
--*************************************************************************
-- IO Interface Declaration
--*************************************************************************
library ieee;
use ieee.std_logic_1164.all;
entity priority_new is
port
(
-- inputs
signal y1: in std_logic;
signal y2: in std_logic;
signal y3: in std_logic;
signal y4: in std_logic;
signal y5: in std_logic;
signal y6: in std_logic;
signal y7: in std_logic;
-- outputs
signal vec: out std_logic_vector(2 downto 0)
);
end priority_new;
--*************************************************************************
-- Architecture body
--*************************************************************************
architecture behavior of priority_new is
begin
process (y1,y2,y3,y4,y5,y6,y7)
begin
vec <= "000";
if (y1 = '1') then vec <= "001";
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end if;
if (y2 = '1') then vec <= "010";
end if;
if (y3 = '1') then vec <= "011";
end if;
if (y4 = '1') then vec <= "100";
end if;
if (y5 = '1') then vec <= "101";
end if;
if (y6 = '1') then vec <= "110";
end if;
if (y7 = '1') then vec <= "111";
end if;
end process;
end behavior;
VHDL model of a 4 to 1 multiplexers
library ieee;
use ieee.std_logic_1164.all;
-- 4 to 1 mux, 8 bit inputs, using concurrent statements
entity mux4to1_8_conc is
port (
I0,I1,I2,I3 : in std_logic_vector(7 downto 0);
sel : in std_logic_vector(1 downto 0);
dout : out std_logic_vector(7 downto 0)
);
end mux4to1_8_conc;
architecture a of mux4to1_8_conc is
begin
WITH sel SELECT
dout <= I0 when "00",
I1 when "01",
4
I2 when "10",
I3 when others;
end a;
Arithmetic and Logic Unit
Consider a small 1-bit arithmetic logic unit (ALU) performs the following operations:
 A+1
 A + B + Cin
 A
 A-1
 Not A
 A and B
 A or B
 A xor B
Let us analyze and model the 1-bit ALU using VHDL. We will use the designed 1-bit ALU and the
structural-type modeling, to write the VHDL model of a 16-bit ALU. Assume that the arithmetic addition
generates the output carry Cout.
By analyzing the specification of the problem, we can establish the function table of a 1-bit ALU as
follows:
FS2
0
0
0
0
1
1
1
1
FS1
0
0
1
1
0
0
1
1
FS0
0
1
0
1
0
1
0
1
Function Implemented
A +1 (increment)
A + B + Cin (addition)
A (identity)
A – 1 (decrement)
Not A ( complement)
A and B (and logic)
A or B ( or logic )
A exor B (exclusive or
logic)
We have to implement two arithmetic operations (+ and - ) plus other logic functions (and, not, identity, or,
xor) using basic logic gates. Data involved are of type BIT. The truth Tables for a full adder and full
subtractor are given as follows:
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Subtraction Truth Table:
Bin
0
0
0
0
1
1
1
1
A
0
0
1
1
0
0
1
1
B
0
1
0
1
0
1
0
1
Diff. = A - B
0
1
1
0
1
0
0
1
Bout
0
1
0
0
1
1
0
1
Bin = Borrow in; Bout = Borrow out. Using K-map for minimization, we have :
 Diff  A exor B exor Bin

 Bout  [ Bin and (( not A) or B)] or [( not A) and B]
Full Adder Truth Table:
Cin
0
0
0
0
1
1
1
1
A
0
0
1
1
0
0
1
1
B
0
1
0
1
0
1
0
1
Sum = A + B
0
1
1
0
1
0
0
1
Cout
0
0
0
1
0
1
1
1
Cin: carry in; Cout: carry out.
Sum  A exor B exor Cin

Cout  [Cin and ( A or B)] or [ A and B]
In our design we will use Cin for Borrow in and Carry in; Cout for Carry out and
Borrow out depending of the type of operation requested (function selection). This is
done at the level of the VHDL code.
The VHDL model of a 1-bit ALU is given as:
library IEEE;
-- File alu_1.vhdl
use IEEE.std_logic_1164.all;
entity alu_1 is
port (
a: in BIT;
b: in BIT;
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Cin: in BIT; -- Use for Borrow in or Carry in
FS: in BIT_VECTOR(2 downto 0);
c: out BIT;
Cout: out BIT
-- Use for carry out or borrow out
);
end alu_1;
architecture alu_1_arch of alu_1 is
begin
-- <<enter your statements here>>
process(a,b,FS,Cin)
variable v1, v2: BIT;
begin
case FS is
when "000" => -- Increment a by 1 = add 1 to a ( b =1, Cin = 0)
v1 := '1'; -- v1 is used for b
v2 := '0'; -- is used for Cin
c<= a xor v1 xor v2;--addition
Cout <= (a and v1) or (v2 and (a or v1));
when "001" =>
c<= a xor b xor cin;--addition
Cout <= (a and b) or (Cin and (a or b));
when "010" =>
-- Identity
c<= a ;
Cout <= '0';
when "011" =>
-- Decrement a by 1
v1 := '1'; -- v1 is used for b = 1
v2 := '0';
-- v2 is used for Bin = 0
c<= a xor v1 xor v2;--addition
Cout <= ( (not a) and v1) or (v2 and ((not a) or v1) );
when "100" =>
-- Not a
c<= not a;
Cout <= '0';
when "101" =>
c<= a and b;
Cout <='0';
when "110" =>
c<= a or b;
Cout <= '0';
when "111" =>
c<= a xor b;
Cout <= '0';
when others =>
c<='0';
Cout <='0';
end case;
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end process;
end alu_1_arch;
The library xslib that contains alu_1 is built using a package alu_1pckg defined within
the file alu_1pck.vhd as follows:
library IEEE;
use IEEE.std_logic_1164.all;
PACKAGE
alu1_1pckg is
COMPONENT alu_1
port (
a: in BIT;
b: in BIT;
Cin: in BIT;
FS: in BIT_VECTOR(2 downto 0);
c: out BIT;
Cout: out BIT
);
end COMPONENT;
end alu1_1pckg;
library IEEE;
-- File alu_1.vhdl
use IEEE.std_logic_1164.all;
entity alu_1 is
port (
a: in BIT;
b: in BIT;
Cin: in BIT; -- Use for Borrow in or Carry in
FS: in BIT_VECTOR(2 downto 0);
c: out BIT;
Cout: out BIT
-- Use for carry out or borrow out
);
end alu_1;
architecture alu_1_arch of alu_1 is
begin
-- <<enter your statements here>>
process(a,b,FS,Cin)
variable v1, v2: BIT;
begin
case FS is
when "000" => -- Increment a by 1 = add 1 to a ( b =1, Cin = 0)
v1 := '1'; -- v1 is used for b
v2 := '0'; -- is used for Cin
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c<= a xor v1 xor v2;--addition
Cout <= (a and v1) or (v2 and (a or v1));
when "001" =>
c<= a xor b xor cin;--addition
Cout <= (a and b) or (Cin and (a or b));
when "010" =>
-- Identity
c<= a ;
Cout <= '0';
when "011" =>
-- Decrement a by 1
v1 := '1'; -- v1 is used for b = 1
v2 := '0';
-- v2 is used for Bin = 0
c<= a xor v1 xor v2;--addition
Cout <= ( (not a) and v1) or (v2 and ((not a) or v1) );
when "100" =>
-- Not a
c<= not a;
Cout <= '0';
when "101" =>
c<= a and b;
Cout <='0';
when "110" =>
c<= a or b;
Cout <= '0';
when "111" =>
c<= a xor b;
Cout <= '0';
when others =>
c<='0';
Cout <='0';
end case;
end process;
end alu_1_arch;
Finally the structural model of a 16-bit ALU using 16 1-bit ALUs is given:
library IEEE, XSLIB; -- File alu_16b.vhd
use IEEE.std_logic_1164.all;
use XSLIB.alu1_1pckg.all;
entity alu_16b is
port (
A: in BIT_VECTOR (15 downto 0);
B: in BIT_VECTOR (15 downto 0);
Cin: in BIT;
Sel: in BIT_VECTOR(3 downto 0);
9
C: out BIT_VECTOR (15 downto 0);
Cout: out BIT
);
end alu_16b;
architecture alu_16b_arch of alu_16b is
signal C_inout: BIT_VECTOR(14 downto 0);
component alu_1 is
port (
a: in bit;
b: in bit;
Cin: in bit;
FS: in BIT_VECTOR (3 downto 0);
c: out bit;
Cout: out bit);
end component;
begin
-- <<enter your statements here>>
alu0: alu_1 port map(A(0), B(0),Cin,Sel,C(0), C_inout(0));
alu1:alu_1 port map(A(1), B(1),C_inout(0),Sel,C(1), C_inout(1));
alu2:alu_1 port map(A(2),B(2),C_inout(1),Sel,C(2), C_inout(2));
alu3:alu_1 port map(A(3), B(3),C_inout(2),Sel,C(3), C_inout(3));
alu4:alu_1 port map(A(4), B(4),C_inout(3),Sel,C(4), C_inout(4));
alu5:alu_1 port map(A(5), B(5),C_inout(4),Sel,C(5), C_inout(5));
alu6:alu_1 port map(A(6), B(6),C_inout(5),Sel,C(6), C_inout(6));
alu7:alu_1 port map(A(7), B(7),C_inout(6),Sel,C(7), C_inout(7));
alu8:alu_1 port map(A(8), B(8),C_inout(7),Sel,C(8), C_inout(8));
alu9:alu_1 port map(A(9), B(9),C_inout(8),Sel,C(9), C_inout(9));
alu10:alu_1 port map(A(10), B(10),C_inout(9),Sel,C(10), C_inout(10));
alu11:alu_1 port map(A(11), B(11),C_inout(10),Sel,C(11), C_inout(11));
alu12:alu_1 port map(A(12), B(12),C_inout(11),Sel,C(12), C_inout(12));
alu13:alu_1 port map(A(13), B(13),C_inout(12),Sel,C(13), C_inout(13));
alu14:alu_1 port map(A(14), B(14),C_inout(13),Sel,C(14), C_inout(14));
alu15:alu_1 port map(A(15), B(15),C_inout(14),Sel,C(15), Cout);
end alu_16b_arch;
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