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-- 2:4 Decoder (ESD figure 2.5)
-- by Weijun Zhang, 04/2001
--
-- decoder is a kind of inverse process
-- of multiplexor
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library ieee;
use ieee.std_logic_1164.all;

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entity DECODER is
port(	I:	in std_logic_vector(1 downto 0);
	O:	out std_logic_vector(3 downto 0)
);
end DECODER;

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architecture behv of DECODER is
begin

    -- process statement

    process (I)
    begin
    
        -- use case statement 

        case I is
	    when "00" => O <= "0001";
	    when "01" => O <= "0010";
	    when "10" => O <= "0100";
	    when "11" => O <= "1000";
	    when others => O <= "XXXX";
	end case;

    end process;
	
end behv;

architecture when_else of DECODER is
begin
	
    -- use when..else statement

    O <= 	"0001" when I = "00" else
		"0010" when I = "01" else
		"0100" when I = "10" else
		"1000" when I = "11" else
		"XXXX";

end when_else;

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