vlsi project centre in chennai : Sample VHDL code for passcode generation

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

entity encrypt is
port(clk,clr: in std_logic;
data_out: out std_logic_vector(63 downto 0);
sel_out: out std_logic_vector(3 downto 0);
original_data: in std_logic_vector(31 downto 0));
end encrypt;

architecture behave of encrypt is

signal pass_code_data: std_logic_vector(11 downto 0);
signal prio_data: std_logic_vector(3 downto 0);
signal din: std_logic_vector(31 downto 0);
signal  ad: std_logic_vector(15 downto 0);
signal sel: std_logic_vector(3 downto 0);

begin

din<= original_data;

Counter_as_Selection:process(clk,clr)
begin
    if clr='1' then
        sel<="0000";
        elsif rising_edge(clk) then
        sel<= sel+1;
   end if;
end process Counter_as_Selection;          

sel_out   <=   sel;

-- Encryption Process
ad(0)   <= din(0) xor din(1);
ad(1)   <= din(2) xor din(3);
ad(2)   <= din(4) xor din(5);
ad(3)   <= din(6) xor din(7);
ad(4)   <= din(8) xor din(9);
ad(5)   <= din(10) xor din(11);
ad(6)   <= din(12) xor din(13);
ad(7)   <= din(14) xor din(15);
ad(8)   <= din(16) xor din(17);
ad(9)  <= din(18) xor din(19);
ad(10)  <= din(20) xor din(21);
ad(11)  <= din(22) xor din(23);
ad(12)  <= din(24) xor din(25);
ad(13)  <= din(26) xor din(27);
ad(14)  <= din(28) xor din(29);
ad(15)  <= din(30) xor din(31);

-- Priority informations
priority_selector:process(sel,clr)
begin
    if clr='1' then
        prio_data<="0000";
        else
        case sel is
            when    "0001"    =>    prio_data    <=    "0100";
            when    "0010"    =>    prio_data    <=    "1000";
            when    "0011"    =>    prio_data    <=    "0110";
            when    "0100"    =>    prio_data    <=    "0011";
            when    "0101"    =>    prio_data    <=    "1001";
            when    "0110"    =>    prio_data    <=    "1010";
            when    "0111"    =>    prio_data    <=    "0010";
            when    "1000"    =>    prio_data    <=    "0001";
            when    "1001"    =>    prio_data    <=    "1100";
            when    "1010"    =>    prio_data    <=    "1111";
            when    "1011"    =>    prio_data    <=    "1110";
            when    "1100"    =>    prio_data    <=    "1011";
            when    "1101"    =>    prio_data    <=    "0101";
            when    "1110"    =>    prio_data    <=    "0111";
            when    others    =>    prio_data    <=    "1101";
         end case;
end if;                     
end process priority_selector;

-- PASSCODE_GENERATOR
PASSCODE_GENERATOR:process(sel)
begin
    if clr='1' then
        pass_code_data<="000000000000";
        else
        case sel is
            when    "0001"    =>    pass_code_data    <=    ("0100")&("1111")&("0100" XOR "1111");
            when    "0010"    =>    pass_code_data    <=    "1000"&"1111"& ("0100" XOR "1111");
            when    "0011"    =>    pass_code_data    <=    "0110"&"1111"& ("0100" XOR "1111");
            when    "0100"    =>    pass_code_data    <=    "0011"&"1111"& ("0100" XOR "1111");
            when    "0101"    =>    pass_code_data    <=    "1001"&"1111"& ("0100" XOR "1111");
            when    "0110"    =>    pass_code_data    <=    "1010"&"1111"& ("0100" XOR "1111");
            when    "0111"    =>    pass_code_data    <=    "0010"&"1111"& ("0100" XOR "1111");
            when    "1000"    =>    pass_code_data    <=    "0001"&"1111"& ("0100" XOR "1111");
            when    "1001"    =>    pass_code_data    <=    "1100"&"1111"& ("0100" XOR "1111");
            when    "1010"    =>    pass_code_data    <=    "1111"&"1111"& ("0100" XOR "1111");
            when    "1011"    =>    pass_code_data    <=    "1110"&"1111"& ("0100" XOR "1111");
            when    "1100"    =>    pass_code_data    <=    "1011"&"1111"& ("0100" XOR "1111");
            when    "1101"    =>    pass_code_data    <=    "0101"&"1111"& ("0100" XOR "1111");
            when    "1110"    =>    pass_code_data    <=    "0111"&"1111"& ("0100" XOR "1111");
            when    others    =>    pass_code_data    <=    "1101"&"1111"& ("0100" XOR "1111");
         end case;
end if;                     
end process PASSCODE_GENERATOR;
   
data_out<= ad & prio_data & pass_code_data & original_data;
end behave;