Hi,
As the improved version of my last post, Sequential Detector has states. 
Difference of this one is it has output called  as "q". The states goes 
through zero to three. When the input is "1" and clock comes three times
it reaches the state 'three' then it makes the output as "1". If the input
still "1" when new clocks arise, output keeps as same. After that if we 
change the input "d" from "1" to "0" state changes to "zero" and output 
become "0". You can see the pattern at below.

Sequence Pattern

library ieee;
use ieee.std_logic_1164.all;
entity string_detector is
    port
    (
        clk         : in    std_logic;
        d          : in    std_logic;
        rst        : in    std_logic;
        q           : out std_logic:='0');
end entity;

architecture behavioral of string_detector is

    type state_type is (zero, one, two, three);
    signal state : state_type;

begin

    process (clk)
    begin

    if (rising_edge(clk)) then
        if rst = '1' then
            state <= zero;
                    q <= '0'; 
        else

            case state is

                when zero=>
                    if d = '0' then
                        state <= zero;
                    elsif d='1' then
                        state <= one;
                    end if;

                when one=>
                    if d = '0' then
                        state <= zero;
                    elsif d='1' then
                        state <= two;
                    end if;

                when two=>
                    if d = '0' then
                        state <= zero;
                    elsif d='1' then
                        state <= three;
                             q <= '1';
                    end if;

                when three=>
                    if d = '0' then
                        state <= zero;
                                q <= '0';
                    elsif d='1' then
                        state <= three;
                    end if;

            end case;

        end if;
        end if;
    end process;

end behavioral;
--ozturkgokhan.com

Modelsim Output
best wishes
gökhan öztürk

        
    
			

			
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