VHDL: Bidirectional Bus

bidir.vhd (Tri-state bus implementation)

LIBRARY ieee;

USE ieee.std_logic_1164.ALL;

ENTITY bidir IS

    PORT(

        bidir   : INOUT STD_LOGIC_VECTOR (7 DOWNTO 0);

        oe, clk : IN STD_LOGIC;

        inp     : IN STD_LOGIC_VECTOR (7 DOWNTO 0);

        outp    : OUT STD_LOGIC_VECTOR (7 DOWNTO 0));

END bidir;

ARCHITECTURE cpld OF bidir IS

SIGNAL  a  : STD_LOGIC_VECTOR (7 DOWNTO 0);  -- DFF that stores

                                             -- value from input.

SIGNAL  b  : STD_LOGIC_VECTOR (7 DOWNTO 0);  -- DFF that stores

BEGIN                                        -- feedback value.

    PROCESS(clk)

    BEGIN

    IF clk = '1' AND clk'EVENT THEN  -- Creates the flipflops

        a <= inp;                   

        outp <= b;                 

        END IF;

    END PROCESS;   

    PROCESS (oe, bidir)          -- Behavioral representation

        BEGIN                    -- of tri-states.

        IF( oe = '0') THEN

            bidir <= "ZZZZZZZZ";

            b <= bidir;

        ELSE

            bidir <= a;

            b <= bidir;

        END IF;

    END PROCESS;

END cpld;