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/*
 * RS-trigger with assynch. reset
 */

library ieee;
use ieee.std_logic_1164.all;

entity RS_trigger is
    generic (T: Time := 0ns);
    port ( R, S  : in  std_logic;
           Q, nQ : out std_logic;
           reset, clock : in  std_logic );
end RS_trigger;

architecture behaviour of RS_trigger is
    signal QT: std_logic; -- Q(t)
begin
    process(clock, reset) is
        subtype RS is std_logic_vector (1 downto 0);
    begin
        if reset = '0' then
            QT <= '0';
        else
            if rising_edge(C) then
                if not (R'stable(T) and S'stable(T)) then
                    QT <= 'X';
                else
                    case RS'(R&S) is
                        when "01" => QT <= '1';
                        when "10" => QT <= '0';
                        when "11" => QT <= 'X';
                        when others => null;
                    end case;
                end if;
            end if;
        end if;
    end process;

    Q  <= QT;
    nQ <= not QT;
end architecture behaviour;