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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.

-- Synthesize with 'balanced' optimization for best results.

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

-- NOTE: See note at bottom of file about optional use of PLL.

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

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;

        clk_27MHz     : 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

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

-- Parameters

-- Address size (FIXME: not tested with other values)

constant SRAM_ADDR_SIZE : integer := 32;

-- Clock rate selection (affects UART configuration)

-- Acceptable values: {27000000, 50000000, 45000000(pll config)}

constant CLOCK_FREQ : integer := 50000000;

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

-- 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;

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

-- I/O registers

signal sd_clk_reg :         std_logic;

signal sd_cs_reg :          std_logic;

signal sd_cmd_reg :         std_logic;

signal sd_do_reg :          std_logic;

-- CPU access to hex display

signal reg_display :        std_logic_vector(31 downto 0);

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

-- DE-1 board interface signals

-- Synchronization FF chain for asynchronous reset input

signal reset_sync :         std_logic_vector(3 downto 0);

-- Reset pushbutton debouncing logic

subtype t_debouncer is integer range 0 to CLOCK_FREQ;

constant DEBOUNCING_DELAY : t_debouncer := 500;

signal debouncing_counter : t_debouncer := (CLOCK_FREQ/1000) * DEBOUNCING_DELAY;

-- 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);

-- 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;

-- Master clock signal

signal clk :                std_logic;

-- Clock from PLL, is a PLL is used

signal clk_pll :            std_logic;

-- '1' when PLL is locked or when no PLL is used

signal pll_locked :         std_logic;

-- Altera PLL component declaration (in case it's used)

-- Note that the MegaWizard component needs to be called 'pll' or the component

-- name should be changed in this file.

--component pll

--    port (

--        areset      : in std_logic  := '0';

--        inclk0      : in std_logic  := '0';

--        c0          : out std_logic ;

--        locked      : out std_logic

--    );

--end component;

-- 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-1 downto 0);

signal mpu_sram_data_rd :   std_logic_vector(15 downto 0);

signal mpu_sram_data_wr :   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;

-- Converts hex nibble to 7-segment

-- Segments ordered as "GFEDCBA"; '0' is ON, '1' is OFF

function nibble_to_7seg(nibble : std_logic_vector(3 downto 0))

                        return std_logic_vector is

begin

    case nibble is

    when X"0"       => return "0000001";

    when X"1"       => return "1001111";

    when X"2"       => return "0010010";

    when X"3"       => return "0000110";

    when X"4"       => return "1001100";

    when X"5"       => return "0100100";

    when X"6"       => return "0100000";

    when X"7"       => return "0001111";

    when X"8"       => return "0000000";

    when X"9"       => return "0000100";

    when X"a"       => return "0001000";

    when X"b"       => return "1100000";

    when X"c"       => return "0110001";

    when X"d"       => return "1000010";

    when X"e"       => return "0110000";

    when X"f"       => return "0111000";

    when others     => return "0111111"; -- can't happen

    end case;

end function nibble_to_7seg;

begin

    mpu: entity work.mips_mpu

    generic map (

        CLOCK_FREQ     => CLOCK_FREQ,

        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_data_rd    => mpu_sram_data_rd,

        sram_data_wr    => mpu_sram_data_wr,

        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

    );

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

-- I/O registers

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

hex_display_register:

process(clk)

begin

    if clk'event and clk='1' then

        if io_byte_we/="0000" and io_wr_addr(15 downto 12)=X"2" then

            reg_display(15 downto 0) <= io_wr_data(15 downto 0);

            --reg_display <= mpu_sram_address;

        end if;

    end if;

end process hex_display_register;

sd_control_register:

process(clk)

begin

    if clk'event and clk='1' then

        if io_byte_we/="0000" and io_wr_addr(15 downto 12)=X"1" then

            if io_wr_addr(5)='1' then

                sd_clk_reg <= io_wr_addr(4);

            end if;

            if io_wr_addr(7)='1' then

                sd_cs_reg <= io_wr_addr(6);

            end if;

            if io_wr_addr(11)='1' then

                sd_do_reg <= io_wr_data(0);

            end if;

        end if;

    end if;

end process sd_control_register;

-- Show the SD interface signals on the green leds for debug

reg_gleds <= sd_clk_reg & sd_in & sd_do_reg & "000" & sd_cmd_reg & sd_cs_reg;

io_rd_data(0) <= sd_in;

io_rd_data(31 downto 22) <= switches;

-- 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 (connected to the same mup bus as the sram)

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

flash_we_n <= '1'; -- all write control signals inactive

flash_reset_n <= '1';

flash_addr(21 downto 18) <= (others => '0');

flash_addr(17 downto  0) <= mpu_sram_address(17 downto 0); -- FIXME

-- Flash is decoded at 0xb0000000

flash_oe_n <= '0'

    when mpu_sram_address(31 downto 27)="10110" and mpu_sram_oe_n='0'

    else '1';

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

-- SRAM

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

sram_addr <= mpu_sram_address(sram_addr'high+1 downto 1);

sram_oe_n <= '0'

    when mpu_sram_address(31 downto 27)="00000" and mpu_sram_oe_n='0'

    else '1';

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);

sram_data <= mpu_sram_data_wr when mpu_sram_byte_we_n/="11" else (others => 'Z');

-- The only reason we need this mux is because we have the static RAM and the

-- static flash in separate FPGA pins, whereas in a real world application they

-- would be on the same data+address bus

mpu_sram_data_rd <=

    -- SRAM is decoded at 0x00000000

    sram_data when mpu_sram_address(31 downto 27)="00000" else

    X"00" & flash_data;

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

-- RESET, CLOCK

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

-- Use button 3 as reset

-- This FF chain only prevents metastability trouble, it does not help with

-- switching bounces.

-- (NOTE: the anti-metastability logic is probably not needed when we include 

-- the debouncing logic)

reset_synchronization:

process(clk)

begin

    if clk'event and clk='1' then

        reset_sync(3) <= not buttons(2);

        reset_sync(2) <= reset_sync(3);

        reset_sync(1) <= reset_sync(2);

        reset_sync(0) <= reset_sync(1);

    end if;

end process reset_synchronization;

reset_debouncing:

process(clk)

begin

    if clk'event and clk='1' then

        if reset_sync(0)='1' and reset_sync(1)='0' then

            debouncing_counter <= (CLOCK_FREQ/1000) * DEBOUNCING_DELAY;

        else

            if debouncing_counter /= 0 then

                debouncing_counter <= debouncing_counter - 1;

            end if;

        end if;

    end if;

end process reset_debouncing;

--

reset <= '1' when debouncing_counter /= 0 or pll_locked='0' else '0';

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

process(clk)

begin

  if clk'event and clk='1' then

    if reset = '1' then

      clk_1hz <= '0';

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

    else

      if conv_integer(counter_1hz) = CLOCK_FREQ-1 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 or 27MHz oscillator

slow_clock:

if CLOCK_FREQ = 27000000 generate

clk <= clk_27MHz;

pll_locked <=  '1';

end generate;

fast_clock:

if CLOCK_FREQ = 50000000 generate

clk <= clk_50MHz;

pll_locked <=  '1';

end generate;

--pll_clock:

--if CLOCK_FREQ /= 27000000 and CLOCK_FREQ/=50000000 generate

---- Assume PLL black box is properly configured for whatever the clock rate is...

--input_clock_pll: component pll

--    port map(

--        areset  => '0',

--        inclk0  => clk_50MHz,

--        c0      => clk_pll,

--        locked  => pll_locked

--    );

--

----clk <= clk_1hz when reg_display(31 downto 27)="10110" else clk_pll;

--clk <= clk_pll;

--end generate;

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

-- LEDS, SWITCHES

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

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

green_leds <= reg_gleds;

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

-- QUAD 7-SEGMENT DISPLAYS

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

-- Show contents of debug register in hex display

display_data <=

    reg_display(15 downto 0);-- when switches(0)='0' else 

    --reg_display(31 downto 16);

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

hex3 <= nibble_to_7seg(display_data(15 downto 12));

hex2 <= nibble_to_7seg(display_data(11 downto  8));

hex1 <= nibble_to_7seg(display_data( 7 downto  4));

hex0 <= nibble_to_7seg(display_data( 3 downto  0));

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

-- SD card interface

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

-- Connect to FFs for use in bit-banged interface (still unused)

sd_cs       <= sd_cs_reg;

sd_cmd      <= sd_do_reg;

sd_clk      <= sd_clk_reg;

sd_in       <= sd_data;

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

-- SERIAL

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

--  Embedded in the MPU entity

end minimal;

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

-- NOTE: Optional use of a PLL

-- 

-- In order to try the core with any clock other the 50 and 27MHz oscillators 

-- readily available onboard we need to use a PLL.

-- Unfortunately, Quartus-II won't let you just instantiate a PLL like ISE does.

-- Instead, you have to build a PLL module using the MegaWizard tool.

-- A nasty consequence of this is that the PLL can't be reconfigured without

-- rebuilding it with the MW tool, and a bunch of ugly binary files have to be 

-- committed to SVN if the project is to be complete.

-- When I figure up what files need to be committed to SVN I will. Meanwhile you

-- have to build the module yourself if you want to u se a PLL -- Sorry!

-- At least it is very straightforward -- create an ALTPLL variation (from the 

-- IO module library) named 'pll' with a 45MHz clock at output c0, that's it.

--

-- Please note that the system will run at >50MHz when using 'balanced' 

-- synthesis. Only the 'area optimized' synthesis may give you trouble.

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