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---------------------------------------------------------------------

-- Program RAM

--

-- Part of the LXP32 test platform

--

-- Copyright (c) 2016 by Alex I. Kuznetsov

--

-- Program RAM for the LXP32 test platform. Has two interfaces:

-- WISHBONE (for data access) and LLI (for LXP32 instruction bus).

-- Optionally performs throttling.

--

-- Note: regardless of whether this description is synthesizable,

-- it was designed exclusively for simulation purposes.

---------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use work.common_pkg.all;

entity program_ram is

        generic(

                THROTTLE: boolean

        );

        port(

                clk_i: in std_logic;

                rst_i: in std_logic;

                wbs_cyc_i: in std_logic;

                wbs_stb_i: in std_logic;

                wbs_we_i: in std_logic;

                wbs_sel_i: in std_logic_vector(3 downto 0);

                wbs_ack_o: out std_logic;

                wbs_adr_i: in std_logic_vector(27 downto 2);

                wbs_dat_i: in std_logic_vector(31 downto 0);

                wbs_dat_o: out std_logic_vector(31 downto 0);

                lli_re_i: in std_logic;

                lli_adr_i: in std_logic_vector(29 downto 0);

                lli_dat_o: out std_logic_vector(31 downto 0);

                lli_busy_o: out std_logic

        );

end entity;

architecture rtl of program_ram is

signal ram_a_we: std_logic_vector(3 downto 0);

signal ram_a_rdata: std_logic_vector(31 downto 0);

signal ram_b_re: std_logic;

signal ram_b_rdata: std_logic_vector(31 downto 0);

signal ack_write: std_logic;

signal ack_read: std_logic;

signal prbs: std_logic;

signal lli_busy: std_logic:='0';

begin

-- The total memory size is 16384 words, i.e. 64K bytes

gen_dprams: for i in 3 downto 0 generate

        generic_dpram_inst: entity work.generic_dpram(rtl)

                generic map(

                        DATA_WIDTH=>8,

                        ADDR_WIDTH=>14,

                        SIZE=>16384,

                        MODE=>"DONTCARE"

                )

                port map(

                        clka_i=>clk_i,

                        cea_i=>'1',

                        wea_i=>ram_a_we(i),

                        addra_i=>wbs_adr_i(15 downto 2),

                        da_i=>wbs_dat_i(i*8+7 downto i*8),

                        da_o=>ram_a_rdata(i*8+7 downto i*8),

                        clkb_i=>clk_i,

                        ceb_i=>ram_b_re,

                        addrb_i=>lli_adr_i(13 downto 0),

                        db_o=>ram_b_rdata(i*8+7 downto i*8)

                );

end generate;

-- WISHBONE interface

gen_ram_a_we: for i in 3 downto 0 generate

        ram_a_we(i)<='1' when wbs_cyc_i='1' and wbs_stb_i='1' and wbs_we_i='1'

                and wbs_sel_i(i)='1' and wbs_adr_i(27 downto 16)="000000000000" else '0';

end generate;

process (clk_i) is

begin

        if rising_edge(clk_i) then

                ack_read<=wbs_cyc_i and wbs_stb_i and not wbs_we_i and not ack_read;

        end if;

end process;

ack_write<=wbs_cyc_i and wbs_stb_i and wbs_we_i;

wbs_ack_o<=ack_read or ack_write;

wbs_dat_o<=ram_a_rdata;

-- Low Latency Interface (with optional pseudo-random throttling)

process (clk_i) is

begin

        if rising_edge(clk_i) then

                assert lli_re_i='0' or lli_adr_i(lli_adr_i'high downto 14)=X"0000"

                        report "Attempted to fetch instruction from a non-existent address 0x"&

                                hex_string(lli_adr_i&"00")

                        severity failure;

        end if;

end process;

gen_throttling: if THROTTLE generate

        throttle_inst: entity work.scrambler(rtl)

                generic map(TAP1=>9,TAP2=>11)

                port map(clk_i=>clk_i,rst_i=>rst_i,ce_i=>'1',d_o=>prbs);

end generate;

gen_no_throttling: if not THROTTLE generate

        prbs<='0';

end generate;

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if rst_i='1' then

                        lli_busy<='0';

                elsif prbs='1' and lli_re_i='1' then

                        lli_busy<='1';

                elsif prbs='0' then

                        lli_busy<='0';

                end if;

        end if;

end process;

ram_b_re<=lli_re_i and not lli_busy;

lli_busy_o<=lli_busy;

lli_dat_o<=ram_b_rdata when lli_busy='0' else (others=>'-');

end architecture;