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

-- #  << NEO430 - Custom Functions Unit >>                                                         #

-- # ********************************************************************************************* #

-- # This unit is a template for implementing custom functions, which are directly memory-mapped   #

-- # into the CPU's IO address space. The address space of this unit is 16 bytes large. This unit  #

-- # can only be accessed using full word (16-bit) accesses.                                       #

-- # In the original state, this unit only provides 8 16-bit register, that do not perform any     #

-- # kind of data manipulation.                                                                    #

-- # Exemplary applications: Cryptography, complex arithmetic, rocket science, ...                 #

-- # ********************************************************************************************* #

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2020, Stephan Nolting. All rights reserved.                                     #

-- #                                                                                               #

-- # Redistribution and use in source and binary forms, with or without modification, are          #

-- # permitted provided that the following conditions are met:                                     #

-- #                                                                                               #

-- # 1. Redistributions of source code must retain the above copyright notice, this list of        #

-- #    conditions and the following disclaimer.                                                   #

-- #                                                                                               #

-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of     #

-- #    conditions and the following disclaimer in the documentation and/or other materials        #

-- #    provided with the distribution.                                                            #

-- #                                                                                               #

-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

-- #    endorse or promote products derived from this software without specific prior written      #

-- #    permission.                                                                                #

-- #                                                                                               #

-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS   #

-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF               #

-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE    #

-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,     #

-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #

-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED    #

-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING     #

-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED  #

-- # OF THE POSSIBILITY OF SUCH DAMAGE.                                                            #

-- # ********************************************************************************************* #

-- # The NEO430 Processor - https://github.com/stnolting/neo430                                    #

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library neo430;

use neo430.neo430_package.all;

entity neo430_cfu is

  port (

    -- host access --

    clk_i       : in  std_ulogic; -- global clock line

    rden_i      : in  std_ulogic; -- read enable

    wren_i      : in  std_ulogic; -- write enable

    addr_i      : in  std_ulogic_vector(15 downto 0); -- address

    data_i      : in  std_ulogic_vector(15 downto 0); -- data in

    data_o      : out std_ulogic_vector(15 downto 0); -- data out

    -- clock generator --

    clkgen_en_o : out std_ulogic; -- enable clock generator

    clkgen_i    : in  std_ulogic_vector(07 downto 0)

    -- custom IOs --

--  ...

  );

end neo430_cfu;

architecture neo430_cfu_rtl of neo430_cfu is

  -- IO space: module base address --

  constant hi_abb_c : natural := index_size_f(io_size_c)-1; -- high address boundary bit

  constant lo_abb_c : natural := index_size_f(cfu_size_c); -- low address boundary bit

  -- access control --

  signal acc_en : std_ulogic; -- module access enable

  signal addr   : std_ulogic_vector(15 downto 0); -- access address

  signal wren   : std_ulogic; -- full word write enable

  signal rden   : std_ulogic; -- read enable

  -- accessible regs (8x16-bit) --

  signal cfu_ctrl_reg : std_ulogic_vector(15 downto 0);

  signal user_reg1    : std_ulogic_vector(15 downto 0);

  signal user_reg2    : std_ulogic_vector(15 downto 0);

  signal user_reg3    : std_ulogic_vector(15 downto 0);

  signal user_reg4    : std_ulogic_vector(15 downto 0);

  signal user_reg5    : std_ulogic_vector(15 downto 0);

  signal user_reg6    : std_ulogic_vector(15 downto 0);

  signal user_reg7    : std_ulogic_vector(15 downto 0);

begin

  -- Access Control -----------------------------------------------------------

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

  -- These assignments are required to check if this unit is accessed at all.

  -- Do NOT modify this for your custom application (unless you really know what you are doing)!

  acc_en <= '1' when (addr_i(hi_abb_c downto lo_abb_c) = cfu_base_c(hi_abb_c downto lo_abb_c)) else '0';

  addr   <= cfu_base_c(15 downto lo_abb_c) & addr_i(lo_abb_c-1 downto 1) & '0'; -- word aligned

  wren   <= acc_en and wren_i;

  rden   <= acc_en and rden_i;

  -- Clock System -------------------------------------------------------------

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

  -- The top unit implements a clock generator providing 8 "derived clocks"

  -- Actually, these signals must not be used as direct clock signals, but as clock enable signals.

  -- If wou want to drive a system at MAIN_CLK/8 use the following construct:

  -- if rising_edge(clk_i) then -- Always use the main clock for all clock processes!

  --   if (clkgen_i(clk_div8_c) = '1') then -- the div8 "clock" is actually a clock enable

  --     ...

  --   end if;

  -- end if;

  -- The following clock divider rates are available:

  -- clkgen_i(clk_div2_c)    -> MAIN_CLK/2

  -- clkgen_i(clk_div4_c)    -> MAIN_CLK/4

  -- clkgen_i(clk_div8_c)    -> MAIN_CLK/8

  -- clkgen_i(clk_div64_c)   -> MAIN_CLK/64

  -- clkgen_i(clk_div128_c)  -> MAIN_CLK/128

  -- clkgen_i(clk_div1024_c) -> MAIN_CLK/1024

  -- clkgen_i(clk_div2048_c) -> MAIN_CLK/2048

  -- clkgen_i(clk_div4096_c) -> MAIN_CLK/4096

  -- this signal enabled the generator driving the clkgen_i

  -- set this signal to '0' when you do not need the clkgen_i signal or when your CFU is disabled

  -- to reduce dynamic power consumption

  clkgen_en_o <= '0';

  -- Write access -------------------------------------------------------------

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

  -- Here we are writing to the interface registers of the module. This unit can only be accessed

  -- in full 16-bit word mode!

  -- Please note, that all register of every unit are cleared during the processor boot sequence.

  -- Make cfu_reg0_addr_c the CFU's control register. This register is cleared first during booting.

  -- If the control register is cleared no actions should be taken when writing to other CFU registers.

  wr_access: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- write access to user registers --

      if (wren = '1') then -- valid write access

        -- use full-parallel IFs instead of a CASE to prevent some EDA tools from complaining (GHDL)

        if (addr = cfu_reg0_addr_c) then

          cfu_ctrl_reg <= data_i;

        end if;

        if (addr = cfu_reg1_addr_c) then

          user_reg1 <= data_i;

        end if;

        if (addr = cfu_reg2_addr_c) then

          user_reg2 <= data_i;

        end if;

        if (addr = cfu_reg3_addr_c) then

          user_reg3 <= data_i;

        end if;

        if (addr = cfu_reg4_addr_c) then

          user_reg4 <= data_i;

        end if;

        if (addr = cfu_reg5_addr_c) then

          user_reg5 <= data_i;

        end if;

        if (addr = cfu_reg6_addr_c) then

          user_reg6 <= data_i;

        end if;

        if (addr = cfu_reg7_addr_c) then

          user_reg7 <= data_i;

        end if;

      end if;

    end if;

  end process wr_access;

  -- >>> UNIT HARDWARE RESET <<< --

  -- The IO devices DO NOT feature a dedicated reset signal, so make sure your CFU does not require a defined initial state.

  -- If you really require a defined initial state, implement a software reset by implementing a control register with an

  -- enable bit, which resets all internal states when cleared.

  -- Read access --------------------------------------------------------------

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

  -- This is the read access process. Data must be asserted synchronously to the output data bus

  -- and thus, with exactly 1 cycle delay. The units always output a full 16-bit word, no matter if we want to

  -- read 8- or 16-bit. For actual 8-bit read accesses the corresponding byte is selected in the

  -- hardware of the CPU core.

  rd_access: process(clk_i)

  begin

    if rising_edge(clk_i) then

      data_o <= (others => '0'); -- this is crucial for the final OR-ing of all IO device's outputs

      if (rden = '1') then -- valid read access

        -- use IFs instead of a CASE to prevent some EDA tools from complaining (GHDL)

        if (addr = cfu_reg0_addr_c) then

          data_o <= cfu_ctrl_reg;

        elsif (addr = cfu_reg1_addr_c) then

          data_o <= user_reg1;

        elsif (addr = cfu_reg2_addr_c) then

          data_o <= user_reg2;

        elsif (addr = cfu_reg3_addr_c) then

          data_o <= user_reg3;

        elsif (addr = cfu_reg4_addr_c) then

          data_o <= user_reg4;

        elsif (addr = cfu_reg5_addr_c) then

          data_o <= user_reg5;

        elsif (addr = cfu_reg6_addr_c) then

          data_o <= user_reg6;

        elsif (addr = cfu_reg7_addr_c) then

          data_o <= user_reg7;

        else

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

        end if;

      end if;

    end if;

  end process rd_access;

end neo430_cfu_rtl;