vhdl Memories Shift register


Example

A shift register of generic length. With serial in and serial out.

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity SHIFT_REG is
    generic(
        LENGTH: natural := 8
    );
    port(
        SHIFT_EN : in  std_logic;
        SO       : out std_logic;
        SI       : in  std_logic;
        clk      : in  std_logic;
        rst      : in  std_logic
    );
end entity SHIFT_REG;

architecture Behavioral of SHIFT_REG is
    signal reg : std_logic_vector(LENGTH-1 downto 0) := (others => '0');
begin
    main_process : process(clk) is
    begin
        if rising_edge(clk) then
            if rst = '1' then
                reg <= (others => '0');
            else
                if SHIFT_EN = '1' then
                    --Shift 
                    reg <= reg(LENGTH-2 downto 0) & SI;
                else
                    reg <= reg;
                end if;
            end if;
        end if;
    end process main_process;
    
    SO <= reg(LENGTH-1);
end architecture Behavioral;

For Parallel out,

--In port
DOUT: out std_logic_vector(LENGTH-1 downto 0);
----------------------------------------------
--In architecture
DOUT <= REG;

Shift register with direction control, parallel load, parallel out. (Using Variable instead of signal)

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity SHIFT_REG_UNIVERSAL is
    generic(
        LENGTH : integer := 8
    );
    port(
        DIN  : in  std_logic_vector(LENGTH - 1 downto 0);
        DOUT : out std_logic_vector(LENGTH - 1 downto 0);
        MODE : in  std_logic_vector(1 downto 0);
        SI   : in  std_logic;
        clk  : in  std_logic;
        rst  : in  std_logic
    );
end entity SHIFT_REG_UNIVERSAL;

architecture RTL of SHIFT_REG_UNIVERSAL is
begin
    main : process(clk, rst) is
        variable reg : std_logic_vector(LENGTH - 1 downto 0) := (others => '0');
    begin
        if rst = '1' then
            reg := (others => '0');
        elsif rising_edge(clk) then
            case MODE is
                when "00" =>
                    -- Hold Value
                    reg := reg;
                when "01" =>
                    -- Shift Right
                    reg := SI & reg(LENGTH - 1 downto 1);
                when "10" =>
                    -- Shift Left
                    reg := reg(LENGTH - 2 downto 0) & SI;
                when "11" =>
                    -- Parallel Load
                    reg := DIN;
                when others =>
                    null;
            end case;
        end if;
        DOUT <= reg;
    end process main;

end architecture RTL;