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

-- Simulation test bench -- not synthesizable.

--

-- Simulates the MCU core connected to a simulated external static RAM on a 

-- 16-bit bus, plus an optional 8-bit static ROM. This setup is more or less 

-- that of develoment board DE-1 from Terasic.

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

-- Console logging:

--

-- Console output (at addresses compatible to Plasma's) is logged to text file

-- "hw_sim_console_log.txt".

--

-- IMPORTANT: The code that echoes UART TX data to the simulation console does

-- line buffering; it will not print anything until it gets a CR (0x0d), and

-- will ifnore LFs (0x0a). Bear this in mind if you see no output when you 

-- expect it.

--

-- Console logging is done by monitoring CPU writes to the UART, NOT by looking

-- at the TxD pin. It will NOT catch baud-related problems, etc.

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

-- WARNING: Will only work on Modelsim; uses custom library SignalSpy.

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

use std.textio.all;

use work.mips_pkg.all;

use work.mips_tb_pkg.all;

use work.sim_params_pkg.all;

use work.txt_util.all;

entity mips_tb is

end;

architecture testbench of mips_tb is

-- NOTE: simulation parameters are defined in sim_params_pkg

-- External SRAM and interface signals -----------------------------------------

-- Static 16-bit wide RAM modelled as two separate byte-wide arrays foer easy 

-- simulation of byte enables.

-- Using shared variables for big memory arrays speeds up simulation a lot;

-- see Modelsim 6.3 User Manual, section on 'Modelling Memory'.

-- WARNING: I have only tested this construct with Modelsim SE 6.3.

shared variable sram1 : t_sram := ( others => X"00");

shared variable sram0 : t_sram := ( others => X"00");

signal sram_chip_addr :     std_logic_vector(SRAM_ADDR_SIZE downto 1);

signal sram_output :        std_logic_vector(15 downto 0);

-- PROM table and interface signals --------------------------------------------

-- We'll simulate a 16-bit-wide static PROM (e.g. a Flash) with some serious

-- cycle time (70 or 90 ns).

-- FIXME FLASH read cycle time not modelled yet.

signal prom_rd_addr :       t_prom_address;

signal prom_output :        std_logic_vector(7 downto 0);

signal prom_oe_n :          std_logic;

-- 8-bit wide FLASH modelled as read only block.

-- We don't simulate the actual FLASH chip: no FLASH writes, control regs, etc.

shared variable prom : t_prom := ( PROM_DATA );

-- I/O devices -----------------------------------------------------------------

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 clk :                std_logic := '0';

signal reset :              std_logic := '1';

signal interrupt :          std_logic := '0';

signal done :               std_logic := '0';

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

signal mpu_sram_address :   t_word;

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;

-- interface to i/o

signal io_rd_data :         std_logic_vector(31 downto 0);

signal io_wr_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_rd_vma :          std_logic;

signal io_byte_we :         std_logic_vector(3 downto 0);

signal rxd :                std_logic;

signal txd :                std_logic;

-- Other CPU signals 

signal cpu_irq :            std_logic_vector(7 downto 0);

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

-- Logging signals

-- Log file

file log_file: TEXT open write_mode is "hw_sim_log.txt";

-- Console output log file

file con_file: TEXT open write_mode is "hw_sim_console_log.txt";

-- All the info needed by the logger is here

signal log_info :           t_log_info;

-- IRQ trigger simulation ------------------------------------------------------

signal irq_trigger_addr :   std_logic_vector(2 downto 0);

signal irq_trigger_data :   std_logic_vector(31 downto 0);

signal irq_trigger_load :   std_logic;

subtype t_irq_countdown     is std_logic_vector(31 downto 0);

type t_irq_countdown_array  is array(0 to 7) of t_irq_countdown;

signal irq_countdown :      t_irq_countdown_array;

begin

    -- UUT instantiation -------------------------------------------------------

    mpu: entity work.mips_mpu

    generic map (

        CLOCK_FREQ     => 50000000,

        SRAM_ADDR_SIZE => 32

    )

    port map (

        interrupt       => cpu_irq,

        -- 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,

        debug_info      => OPEN,

        clk             => clk,

        reset           => reset

    );

    -- Master clock: free running clock used as main module clock --------------

    run_master_clock:

    process(done, clk)

    begin

        if done = '0' then

            clk <= not clk after T/2;

        end if;

    end process run_master_clock;

    -- Main simulation process: reset MCU and wait for fixed period ------------

    drive_uut:

    process

    variable l : line;

    begin

        wait for T*4;

        reset <= '0';

        wait for T*SIMULATION_LENGTH;

        -- Flush console output to log console file (in case the end of the

        -- simulation caugh an unterminated line in the buffer)

        if log_info.con_line_ix > 1 then

            write(l, log_info.con_line_buf(1 to log_info.con_line_ix));

            writeline(con_file, l);

        end if;

        print("TB finished");

        done <= '1';

        wait;

    end process drive_uut;

    -- SRAM/FLASH mux (on a real board this would be a simple address decoder)

    mpu_sram_data_rd <=

        X"00" & prom_output when mpu_sram_address(31 downto 27)="10110" else

        sram_output;

    -- Do a very basic simulation of an external SRAM --------------------------

    sram_chip_addr <= mpu_sram_address(SRAM_ADDR_SIZE downto 1);

    -- FIXME should add some verification of /WE 

    sram_output <=

        sram1(conv_integer(unsigned(sram_chip_addr))) &

        sram0(conv_integer(unsigned(sram_chip_addr)))   when mpu_sram_oe_n='0'

        else (others => 'Z');

    simulated_sram_write:

    process(mpu_sram_byte_we_n, mpu_sram_address, mpu_sram_oe_n)

    begin

        -- Write cycle

        -- FIXME should add OE\ to write control logic

        if mpu_sram_byte_we_n'event or mpu_sram_address'event then

            if mpu_sram_byte_we_n(1)='0' then

                sram1(conv_integer(unsigned(sram_chip_addr))) := mpu_sram_data_wr(15 downto  8);

            end if;

            if mpu_sram_byte_we_n(0)='0' then

                sram0(conv_integer(unsigned(sram_chip_addr))) := mpu_sram_data_wr( 7 downto  0);

            end if;

        end if;

    end process simulated_sram_write;

    -- Do a very basic simulation of an external PROM (FLASH) ------------------

    -- (wired to the same bus as the sram and both are static).

    prom_rd_addr <= mpu_sram_address(PROM_ADDR_SIZE+1 downto 2);

    prom_oe_n <= mpu_sram_oe_n;

    prom_output <=

        prom(conv_integer(unsigned(prom_rd_addr)))(31 downto 24) when prom_oe_n='0' and mpu_sram_address(1 downto 0)="00" else

        prom(conv_integer(unsigned(prom_rd_addr)))(23 downto 16) when prom_oe_n='0' and mpu_sram_address(1 downto 0)="01" else

        prom(conv_integer(unsigned(prom_rd_addr)))(15 downto  8) when prom_oe_n='0' and mpu_sram_address(1 downto 0)="10" else

        prom(conv_integer(unsigned(prom_rd_addr)))( 7 downto  0) when prom_oe_n='0' and mpu_sram_address(1 downto 0)="11" else

        (others => 'Z');

    -- Simulate dummy I/O traffic external to the MCU --------------------------

    -- the only IO present is the test interrupt trigger registers

    simulated_io:

    process(clk)

    variable i : integer;

    variable uart_data : integer;

    begin

        if clk'event and clk='1' then

            if io_byte_we /= "0000" then

                if io_wr_addr(31 downto 16)=X"2001" then

                    irq_trigger_load <= '1';

                    irq_trigger_data <= io_wr_data;

                    irq_trigger_addr <= io_wr_addr(4 downto 2);

                else

                    irq_trigger_load <= '0';

                end if;

            else

                irq_trigger_load <= '0';

            end if;

        end if;

    end process simulated_io;

    -- Simulate IRQs -----------------------------------------------------------

    irq_trigger_registers:

    process(clk)

    variable index : integer range 0 to 7;

    begin

        if clk'event and clk='1' then

            if reset='1' then

                cpu_irq <= "00000000";

            else

                if irq_trigger_load='1' then

                    index := conv_integer(irq_trigger_addr);

                    irq_countdown(index) <= irq_trigger_data;

                else

                    for index in 0 to 7 loop

                        if irq_countdown(index) = X"00000001" then

                            cpu_irq(index) <= '1';

                            irq_countdown(index) <= irq_countdown(index) - 1;

                        elsif irq_countdown(index)/=X"00000000" then

                            irq_countdown(index) <= irq_countdown(index) - 1;

                            cpu_irq(index) <= '0';

                        else

                            cpu_irq(index) <= '0';

                        end if;

                    end loop;

                end if;

            end if;

        end if;

    end process irq_trigger_registers;

    -- This is useless (the simulated UART will not be actually used)

    -- but at least prevents the simulator from optimizing the logic away.

    rxd <= txd;

    -- Logging process: launch logger function ---------------------------------

    log_execution:

    process

    begin

        log_cpu_activity(clk, reset, done,

                         "mips_tb/mpu", "cpu",

                         log_info, "log_info",

                         LOG_TRIGGER_ADDRESS, log_file, con_file);

        wait;

    end process log_execution;

end architecture testbench;