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--##############################################################################

-- ION MIPS-compatible CPU demo on Terasic DE-1 Cyclone-II starter board

--##############################################################################

-- This module is little more than a wrapper around the CPU and its memories.

--##############################################################################

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

-- FPGA i/o for Terasic DE-1 board

-- (Many of the board's i/o devices will go unused in this demo)

entity c2sb_demo is

    port (

        -- ***** Clocks

        clk_50MHz     : in std_logic;

        -- ***** Flash 4MB

        flash_addr    : out std_logic_vector(21 downto 0);

        flash_data    : in std_logic_vector(7 downto 0);

        flash_oe_n    : out std_logic;

        flash_we_n    : out std_logic;

        flash_reset_n : out std_logic;

        -- ***** SRAM 256K x 16

        sram_addr     : out std_logic_vector(17 downto 0);

        sram_data     : inout std_logic_vector(15 downto 0);

        sram_oe_n     : out std_logic;

        sram_ub_n     : out std_logic;

        sram_lb_n     : out std_logic;

        sram_ce_n     : out std_logic;

        sram_we_n     : out std_logic;

        -- ***** RS-232

        rxd           : in std_logic;

        txd           : out std_logic;

        -- ***** Switches and buttons

        switches      : in std_logic_vector(9 downto 0);

        buttons       : in std_logic_vector(3 downto 0);

        -- ***** Quad 7-seg displays

        hex0          : out std_logic_vector(0 to 6);

        hex1          : out std_logic_vector(0 to 6);

        hex2          : out std_logic_vector(0 to 6);

        hex3          : out std_logic_vector(0 to 6);

        -- ***** Leds

        red_leds      : out std_logic_vector(9 downto 0);

        green_leds    : out std_logic_vector(7 downto 0);

        -- ***** SD Card

        sd_data       : in  std_logic;

        sd_cs         : out std_logic;

        sd_cmd        : out std_logic;

        sd_clk        : out std_logic

    );

end c2sb_demo;

architecture minimal of c2sb_demo is

--##############################################################################

-- 

constant SRAM_ADDR_SIZE : integer := 18;

--##############################################################################

-- RS232 interface signals

signal rx_rdy :             std_logic;

signal tx_rdy :             std_logic;

signal rs232_data_rx :      std_logic_vector(7 downto 0);

signal rs232_status :       std_logic_vector(7 downto 0);

signal data_io_out :        std_logic_vector(7 downto 0);

signal io_port :            std_logic_vector(7 downto 0);

signal read_rx :            std_logic;

signal write_tx :           std_logic;

--##############################################################################

-- 

-- CPU access to hex display (unused by Altair SW)

signal reg_display :        std_logic_vector(15 downto 0);

--##############################################################################

-- DE-1 board interface signals

-- Quad 7-segment display (non multiplexed) & LEDS

signal display_data :       std_logic_vector(15 downto 0);

signal reg_gleds :          std_logic_vector(7 downto 0);

-- i/o signals

signal data_io_in :         std_logic_vector(7 downto 0);

signal data_mem_in :        std_logic_vector(7 downto 0);

signal data_rom_in :        std_logic_vector(7 downto 0);

signal rom_access :         std_logic;

signal rom_space :          std_logic;

signal breakpoint :         std_logic;

-- Clock & reset signals

signal clk_1hz :            std_logic;

signal clk_master :         std_logic;

signal counter_1hz :        std_logic_vector(25 downto 0);

signal reset :              std_logic;

signal clk :                std_logic;

-- SD control signals

signal sd_in :              std_logic;

signal reg_sd_dout :        std_logic;

signal reg_sd_clk :         std_logic;

signal reg_sd_cs :          std_logic;

-- MPU interface signals

signal data_uart :          std_logic_vector(31 downto 0);

signal data_uart_status :   std_logic_vector(31 downto 0);

signal uart_tx_rdy :        std_logic := '1';

signal uart_rx_rdy :        std_logic := '1';

signal io_rd_data :         std_logic_vector(31 downto 0);

signal io_rd_addr :         std_logic_vector(31 downto 2);

signal io_wr_addr :         std_logic_vector(31 downto 2);

signal io_wr_data :         std_logic_vector(31 downto 0);

signal io_rd_vma :          std_logic;

signal io_byte_we :         std_logic_vector(3 downto 0);

signal mpu_sram_address :   std_logic_vector(SRAM_ADDR_SIZE downto 1);

signal mpu_sram_databus :   std_logic_vector(15 downto 0);

signal mpu_sram_byte_we_n : std_logic_vector(1 downto 0);

signal mpu_sram_oe_n :      std_logic;

begin

    mpu: entity work.mips_mpu

    generic map (

        SRAM_ADDR_SIZE => SRAM_ADDR_SIZE

    )

    port map (

        interrupt   => '0',

        -- interface to FPGA i/o devices

        io_rd_data  => io_rd_data,

        io_rd_addr  => io_rd_addr,

        io_wr_addr  => io_wr_addr,

        io_wr_data  => io_wr_data,

        io_rd_vma   => io_rd_vma,

        io_byte_we  => io_byte_we,

        -- interface to asynchronous 16-bit-wide EXTERNAL SRAM

        sram_address    => mpu_sram_address,

        sram_databus    => sram_data,

        sram_byte_we_n  => mpu_sram_byte_we_n,

        sram_oe_n       => mpu_sram_oe_n,

        uart_rxd    => rxd,

        uart_txd    => txd,

        clk         => clk,

        reset       => reset

    );

reg_display <= io_wr_data(15 downto 0);

reg_gleds <= io_rd_vma & "000" & io_byte_we;

-- red leds (light with '1') -- some CPU control signals 

red_leds(0) <= '0';

red_leds(1) <= '0';

red_leds(2) <= '0';

red_leds(3) <= '0';

red_leds(4) <= '0';

red_leds(5) <= '0';

red_leds(6) <= '0';

red_leds(7) <= '0';

red_leds(8) <= '0';

red_leds(9) <= clk_1hz;

--##############################################################################

-- terasIC Cyclone II STARTER KIT BOARD -- interface to on-board devices

--##############################################################################

--##############################################################################

-- FLASH (flash is unused in this demo)

--##############################################################################

flash_addr <= (others => '0');

flash_we_n <= '1'; -- all enable signals inactive

flash_oe_n <= '1';

flash_reset_n <= '1';

--##############################################################################

-- SRAM (used as 64K x 8)

--

-- NOTE: All writes go to SRAM independent of rom paging status

--##############################################################################

sram_addr <= mpu_sram_address;

sram_oe_n <= mpu_sram_oe_n;

sram_ub_n <= mpu_sram_byte_we_n(1) and mpu_sram_oe_n;

sram_lb_n <= mpu_sram_byte_we_n(0) and mpu_sram_oe_n;

sram_ce_n <= '0';

sram_we_n <= mpu_sram_byte_we_n(1) and mpu_sram_byte_we_n(0);

--##############################################################################

-- RESET, CLOCK

--##############################################################################

-- Use button 3 as reset

reset <= not buttons(3);

-- Generate a 1-Hz 'clock' to flash a LED for visual reference.

process(clk_50MHz)

begin

  if clk_50MHz'event and clk_50MHz='1' then

    if reset = '1' then

      clk_1hz <= '0';

      counter_1hz <= (others => '0');

    else

      if conv_integer(counter_1hz) = 50000000 then

        counter_1hz <= (others => '0');

        clk_1hz <= not clk_1hz;

      else

        counter_1hz <= counter_1hz + 1;

      end if;

    end if;

  end if;

end process;

-- Master clock is external 50MHz oscillator

clk <= clk_50MHz;

--##############################################################################

-- LEDS, SWITCHES

--##############################################################################

-- Display the contents of a debug register at the green leds bar

green_leds <= reg_gleds;

--##############################################################################

-- QUAD 7-SEGMENT DISPLAYS

--##############################################################################

-- So far, nothing to display

display_data <= reg_display;

-- 7-segment encoders; the dev board displays are not multiplexed or encoded

with display_data(15 downto 12) select hex3 <=

"0000001" when X"0","1001111" when X"1","0010010" when X"2","0000110" when X"3",

"1001100" when X"4","0100100" when X"5","0100000" when X"6","0001111" when X"7",

"0000000" when X"8","0000100" when X"9","0001000" when X"a","1100000" when X"b",

"0110001" when X"c","1000010" when X"d","0110000" when X"e","0111000" when others;

with display_data(11 downto 8) select hex2 <=

"0000001" when X"0","1001111" when X"1","0010010" when X"2","0000110" when X"3",

"1001100" when X"4","0100100" when X"5","0100000" when X"6","0001111" when X"7",

"0000000" when X"8","0000100" when X"9","0001000" when X"a","1100000" when X"b",

"0110001" when X"c","1000010" when X"d","0110000" when X"e","0111000" when others;

with display_data(7 downto 4) select hex1 <=

"0000001" when X"0","1001111" when X"1","0010010" when X"2","0000110" when X"3",

"1001100" when X"4","0100100" when X"5","0100000" when X"6","0001111" when X"7",

"0000000" when X"8","0000100" when X"9","0001000" when X"a","1100000" when X"b",

"0110001" when X"c","1000010" when X"d","0110000" when X"e","0111000" when others;

with display_data(3 downto 0) select hex0 <=

"0000001" when X"0","1001111" when X"1","0010010" when X"2","0000110" when X"3",

"1001100" when X"4","0100100" when X"5","0100000" when X"6","0001111" when X"7",

"0000000" when X"8","0000100" when X"9","0001000" when X"a","1100000" when X"b",

"0110001" when X"c","1000010" when X"d","0110000" when X"e","0111000" when others;

--##############################################################################

-- SD card interface

--##############################################################################

-- unused in this demo, but I did not bother to cut away the attached registers

sd_cs     <= '0';

sd_cmd    <= '0';

sd_clk    <= '0';

sd_in     <= 'Z';

--##############################################################################

-- SERIAL

--##############################################################################

--  Embedded in the MPU entity

end minimal;