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

-- # << NEORV32 - Custom Functions Subsystem (CFS) >>                                              #

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

-- # Intended for tightly-coupled, application-specific custom co-processors. This module provides #

-- # 32x 32-bit memory-mapped interface registers, one interrupt request signal and custom IO      #

-- # conduits for processor-external or chip-external interface.                                   #

-- #                                                                                               #

-- # NOTE: This is just an example/illustration template. Modify/replace this file to implement    #

-- #       your own custom design logic.                                                           #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2022, 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               #

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

-- # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting #

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library neorv32;

use neorv32.neorv32_package.all;

entity neorv32_cfs is

  generic (

    CFS_CONFIG   : std_ulogic_vector(31 downto 0); -- custom CFS configuration generic

    CFS_IN_SIZE  : positive; -- size of CFS input conduit in bits

    CFS_OUT_SIZE : positive  -- size of CFS output conduit in bits

  );

  port (

    -- host access --

    clk_i       : in  std_ulogic; -- global clock line

    rstn_i      : in  std_ulogic; -- global reset line, low-active, use as async

    addr_i      : in  std_ulogic_vector(31 downto 0); -- address

    rden_i      : in  std_ulogic; -- read enable

    wren_i      : in  std_ulogic; -- word write enable

    data_i      : in  std_ulogic_vector(31 downto 0); -- data in

    data_o      : out std_ulogic_vector(31 downto 0); -- data out

    ack_o       : out std_ulogic; -- transfer acknowledge

    err_o       : out std_ulogic; -- transfer error

    -- clock generator --

    clkgen_en_o : out std_ulogic; -- enable clock generator

    clkgen_i    : in  std_ulogic_vector(07 downto 0); -- "clock" inputs

    -- interrupt --

    irq_o       : out std_ulogic; -- interrupt request

    -- custom io (conduits) --

    cfs_in_i    : in  std_ulogic_vector(CFS_IN_SIZE-1 downto 0);  -- custom inputs

    cfs_out_o   : out std_ulogic_vector(CFS_OUT_SIZE-1 downto 0)  -- custom outputs

  );

end neorv32_cfs;

architecture neorv32_cfs_rtl of neorv32_cfs is

  -- IO space: module base address (DO NOT MODIFY!) --

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

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

  -- access control --

  signal acc_en : std_ulogic; -- module access enable

  signal addr   : std_ulogic_vector(31 downto 0); -- access address

  signal wren   : std_ulogic; -- word write enable

  signal rden   : std_ulogic; -- read enable

  -- default CFS interface registers --

  type cfs_regs_t is array (0 to 3) of std_ulogic_vector(31 downto 0); -- just implement 4 registers for this example

  signal cfs_reg_wr : cfs_regs_t; -- interface registers for WRITE accesses

  signal cfs_reg_rd : cfs_regs_t; -- interface registers for READ accesses

begin

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

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

  -- This logic is required to handle the CPU accesses.

  -- DO NOT MODIFY this unless you really know what you are doing.

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

  addr   <= cfs_base_c(31 downto lo_abb_c) & addr_i(lo_abb_c-1 downto 2) & "00"; -- word aligned

  wren   <= acc_en and wren_i; -- write accesses always write a full 32-bit word

  rden   <= acc_en and rden_i; -- read accesses always return a full 32-bit word

  -- NOTE: Do not modify the CFS base address or the CFS' occupied address space as this might cause access

  -- collisions with other modules.

  -- CFS Generics ---------------------------------------------------------------------------

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

  -- In it's default version the CFS provides three configuration generics:

  -- CFS_IN_SIZE  - configures the size (in bits) of the CFS input conduit cfs_in_i

  -- CFS_OUT_SIZE - configures the size (in bits) of the CFS output conduit cfs_out_o

  -- CFS_CONFIG   - is a blank 32-bit generic. It is intended as a "generic conduit" to propagate

  --                custom configuration flags from the top entity down to this module.

  -- CFS IOs --------------------------------------------------------------------------------

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

  -- By default, the CFS provides two IO signals (cfs_in_i and cfs_out_o) that are available at the processor's top entity.

  -- These are intended as "conduits" to propagate custom signals from this module and the processor top entity.

  cfs_out_o <= (others => '0'); -- not used for this minimal example

  -- Reset System ---------------------------------------------------------------------------

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

  -- The CFS can be reset using the global rstn_i signal. This signal should be used as asynchronous reset and is active-low.

  -- Note that rstn_i can be asserted by a processor-external reset, the on-chip debugger and also by the watchdog.

  --

  -- Most default peripheral devices of the NEORV32 do NOT use a dedicated hardware reset at all. Instead, these units are

  -- reset by writing ZERO to a specific "control register" located right at the beginning of the device's address space

  -- (so this register is cleared at first). The crt0 start-up code writes ZERO to every single address in the processor's

  -- IO space - including the CFS. Make sure that this initial clearing does not cause any unintended CFS actions.

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

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

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

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

  -- clkgen_i is always synchronous to the main system clock (clk_i).

  --

  -- The following clock dividers 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

  --

  -- For instance, if you want to drive a clock process at MAIN_CLK/8 clock speed you can use the following construct:

  --

  --   if (rstn_i = '0') then -- async and low-active reset (if required at all)

  --   ...

  --   elsif 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 clkgen_i input clocks are available when at least one IO/peripheral device (for example the UART) requires the clocks

  -- generated by the clock generator. The CFS can enable the clock generator by itself by setting the clkgen_en_o signal high.

  -- The CFS cannot ensure to deactivate the clock generator by setting the clkgen_en_o signal low as other peripherals might

  -- still keep the generator activated. Make sure to deactivate the CFS's clkgen_en_o if no clocks are required in here to

  -- reduce dynamic power consumption.

  clkgen_en_o <= '0'; -- not used for this minimal example

  -- Interrupt ------------------------------------------------------------------------------

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

  -- The CFS features a single interrupt signal, which is connected to the CPU's "fast interrupt" channel 1 (FIRQ1).

  -- The interrupt is triggered by a one-cycle high-level. After triggering, the interrupt appears as "pending" in the CPU's

  -- mip CSR ready to trigger execution of the according interrupt handler. It is the task of the application to programmer

  -- to enable/clear the CFS interrupt using the CPU's mie and mip registers when required.

  irq_o <= '0'; -- not used for this minimal example

  -- Read/Write Access ----------------------------------------------------------------------

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

  -- Here we are reading/writing from/to the interface registers of the module. Please note that the peripheral/IO modules

  -- of the NEORV32 can only be written in full word mode (32-bit). Any other write accesses (half-word or byte) will raise

  -- a store bus access fault exception.

  --

  -- The CFS provides up to 32 memory-mapped 32-bit interface register. For instance, these could be used to provide a

  -- <control register> for global control of the unit, a <data register> for reading/writing from/to a data FIFO, a

  -- <command register> for issuing commands and a <status register> for status information.

  --

  -- Following the interface protocol, each read or write access has to be acknowledged in the following cycle using the ack_o

  -- signal (or even later if the module needs additional time; exceeding the maximum ACK latency will raise a bus exception).

  -- If no ACK is generated at all, the bus access will time out and cause a bus access fault exception.

  --

  -- This module also provides an optional ERROR signal to indicate a faulty access operation (for example when accessing an

  -- unused, read-only or "locked" CFS register address). This signal may only be set when the module is actually accessed

  -- and is asserted INSTEAD of the ACK signal. Setting the ERR signal will raise a bus access exception that can be handled

  -- by the application software.

  err_o <= '0'; -- Tie to zero if not explicitly used.

  -- Host access: Read and write access to the interface registers + bus transfer acknowledge. This example only implements

  -- four physical r/w register (the four lowest CFS registers). The remaining addresses of the CFS are not associated with

  -- any physical registers - any access to those is simply ignored but still acknowledged.

  host_access: process(clk_i)

  begin

    if rising_edge(clk_i) then -- synchronous interface for read and write accesses

      -- transfer/access acknowledge --

      ack_o <= rden or wren; -- default: required for the CPU to check the CFS is answering a bus read OR write request;

                             -- all r/w accesses (to any cfs_reg) will succeed

      -- write access --

      if (wren = '1') then

        if (addr = cfs_reg0_addr_c) then -- make sure to use the internal "addr" signal for the read/write interface

          cfs_reg_wr(0) <= data_i; -- physical register, for example: control register

        end if;

        if (addr = cfs_reg1_addr_c) then

          cfs_reg_wr(1) <= data_i; -- physical register, for example: data in/out fifo

        end if;

        if (addr = cfs_reg2_addr_c) then

          cfs_reg_wr(2) <= data_i; -- physical register, for example: command fifo

        end if;

        if (addr = cfs_reg3_addr_c) then

          cfs_reg_wr(3) <= data_i; -- physical register, for example: status register

        end if;

      end if;

      -- read access --

      data_o <= (others => '0'); -- the output has to be zero if there is no actual read access

      if (rden = '1') then -- the read access is always a full 32-bit word wide

        case addr is -- make sure to use the internal 'addr' signal for the read/write interface

          when cfs_reg0_addr_c => data_o <= cfs_reg_rd(0);

          when cfs_reg1_addr_c => data_o <= cfs_reg_rd(1);

          when cfs_reg2_addr_c => data_o <= cfs_reg_rd(2);

          when cfs_reg3_addr_c => data_o <= cfs_reg_rd(3);

          when others          => data_o <= (others => '0'); -- the remaining registers are not implemented and will read as zero

        end case;

      end if;

    end if;

  end process host_access;

  -- CFS Function Core ----------------------------------------------------------------------

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

  -- This is where the actual functionality can be implemented.

  -- In this example we are just implementing four r/w registers that invert any value written to them.

  cfs_core_logic: process(cfs_reg_wr)

  begin

    cfs_reg_rd(0) <= not cfs_reg_wr(0);

    cfs_reg_rd(1) <= not cfs_reg_wr(1);

    cfs_reg_rd(2) <= not cfs_reg_wr(2);

    cfs_reg_rd(3) <= not cfs_reg_wr(3);

  end process cfs_core_logic;

end neorv32_cfs_rtl;