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--

--      Input file         : sram_4en.vhd

--      Design name        : sram_4en

--      Author             : Tamar Kranenburg

--      Company            : Delft University of Technology

--                         : Faculty EEMCS, Department ME&CE

--                         : Systems and Circuits group

--

--      Description          : Single Port Synchronous Random Access Memory with 4 write enable

--                             ports.

--      Architecture 'arch'  : Default implementation

--      Architecture 'arch2' : Alternative implementation

--

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LIBRARY ieee;

USE ieee.std_logic_1164.ALL;

USE ieee.std_logic_unsigned.ALL;

LIBRARY mblite;

USE mblite.std_Pkg.ALL;

ENTITY sram_4en IS GENERIC

(

    WIDTH : positive := 32;

    SIZE  : positive := 16

);

PORT

(

    dat_o : OUT std_ulogic_vector(WIDTH - 1 DOWNTO 0);

    dat_i : IN std_ulogic_vector(WIDTH - 1 DOWNTO 0);

    adr_i : IN std_ulogic_vector(SIZE - 1 DOWNTO 0);

    wre_i : IN std_ulogic_vector(WIDTH/8 - 1 DOWNTO 0);

    ena_i : IN std_ulogic;

    clk_i : IN std_ulogic

);

END sram_4en;

-- Although this memory is very easy to use in conjunction with Modelsims mem load, it is not

-- supported by many devices (although it comes straight from the library. Many devices give

-- cryptic synthesization errors on this implementation, so it is not the default.

ARCHITECTURE arch2 OF sram_4en IS

    TYPE ram_type IS array(2 ** SIZE - 1 DOWNTO 0) OF std_ulogic_vector(WIDTH - 1 DOWNTO 0);

    TYPE sel_type IS array(WIDTH/8 - 1 DOWNTO 0) OF std_ulogic_vector(7 DOWNTO 0);

    SIGNAL ram: ram_type;

    SIGNAL di: sel_type;

BEGIN

    PROCESS(wre_i, dat_i, adr_i)

    BEGIN

        FOR i IN 0 TO WIDTH/8 - 1 LOOP

            IF wre_i(i) = '1' THEN

                di(i) <= dat_i((i+1)*8 - 1 DOWNTO i*8);

            ELSE

                di(i) <= ram(my_conv_integer(adr_i))((i+1)*8 - 1 DOWNTO i*8);

            END IF;

        END LOOP;

    END PROCESS;

    PROCESS(clk_i)

    BEGIN

        IF rising_edge(clk_i) THEN

            IF ena_i = '1' THEN

                ram(my_conv_integer(adr_i)) <= di(3) & di(2) & di(1) & di(0);

                dat_o <= di(3) & di(2) & di(1) & di(0);

            END IF;

        END IF;

    END PROCESS;

END arch2;

-- Less convenient but very general memory block with four separate write

-- enable signals. (4x8 bit)

ARCHITECTURE arch OF sram_4en IS

BEGIN

   mem: FOR i IN 0 TO WIDTH/8 - 1 GENERATE

       mem : sram GENERIC MAP

       (

           WIDTH   => 8,

           SIZE    => SIZE

       )

       PORT MAP

       (

           dat_o   => dat_o((i+1)*8 - 1 DOWNTO i*8),

           dat_i   => dat_i((i+1)*8 - 1 DOWNTO i*8),

           adr_i   => adr_i,

           wre_i   => wre_i(i),

           ena_i   => ena_i,

           clk_i   => clk_i

       );

   END GENERATE;

END arch;