Decade Counter VHDL Code

Library IEEE;
USE IEEE std_logic_1164.all;
Entity Decade is
port (clk,rst : IN std_logic;
          q : OUT std_logic_vector (3 downto 0);

End Decade;

Architecture Behavioral of Decade is
Signal count : std_logic_vector  (3 downto 0);
begin
Process (clk,rst)
begin
if (rst = '0') then count <= "0000" ;
elsif (clk' event and clk = '1') then
count = count+1;
end if;
if (count = "1010") then count <= "0000" ;
end if;
end process;
q <= count ;
End behavioral;

4 Bit Counter VHDL Code

Library IEEE;
USE IEEE std_logic_1164.all;
Entity Counter is
port (clk : IN std_logic;
         rst : IN std_logic;
            q : OUT std_logic_vector(3 downto 0);
End Counter;
Architecture Behavioral of Counter is
Signal count : std_logic_vector(3 downto 0);
begin
Process (clk,rst)
begin
if (rst = '1') then count <= "0000";
elsif  (clk' event and clk = '1') then
count <= count+1;
end if;
if (count = "1111") then count <= "0000";
end if;
end process;
q <= count ;
end behavioral;

VHDL Code for D Flip Flop

Library IEEE;
USE IEEE std_logic_1164.all;
Entity D_Flip_Flop is
port(D,clr,pre,clk : IN std_logic ;
         nq : Out Std_logic;
           q : inout std_logic);
End D_Flip_Flop;

Architecture behavioral of D_Flip_Flop is
begin
process (clk,pre,clr)
begin
if ( pre = '0' and clr = '1' ) then q <= '1' ;
elsif ( pre = '1' and clr = '0') then q <= '0' ;
elsif ( clk' event and clk = '1') then q <= d ;
end if ;
end process ;
nq <= not q;
end behavioral ;

The respective Block diagram TTL Schematic and RTL Schematic are given below

4 Bit Comparator VHDL Code

Library IEEE;
USE IEEE.std_logic_1164.all;
USE IEEE.std_logic_arith.all;
Entity Comparator is
port(a,b : in std_logic_vector(3 downto 0);
         eq,lr,gr : Out std_logic);
End Comparator;