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-- implementation of the HDB1 decoder. 

entity hdb1_dec is

        port (

        clr_bar,

        clk, e0, e1 : in    bit; -- inputs.

        s           : out   bit  -- output.

        );

end hdb1_dec;

architecture behaviour of hdb1_dec is

     signal q0, q1: bit;  -- two flipflops.

begin

     process (clk, clr_bar) begin

                if clr_bar = '0' then

                        q0 <= '0';

                        q1 <= '0';

                        s  <= '0';

                elsif clk'event and clk = '1' then

                        s  <= ( q0 and (not e0) ) or ( q1 and (not e1) );

                        q0 <= (not q0) and e0;

                        q1 <= (not q1) and e1;

                end if;

     end process;

end behaviour;