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

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

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

-- # For tightly-coupled custom co-processors. Provides 32x32-bit memory-mapped registers.         #

-- # This is just an "example/illustration template". Modify this file to implement your own       #

-- # custom design logic.                                                                          #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2021, Stephan Nolting. All rights reserved.                                     #

-- #                                                                                               #

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

-- #                                                                                               #

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

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

-- #    provided with the distribution.                                                            #

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-- #    permission.                                                                                #

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

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

  -- These assignments are required to check if the CFS is accessed at all.

  -- 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; -- full 32-bit word write enable

  rden   <= acc_en and rden_i; -- the read access is always a full 32-bit word wide; if required, the byte/half-word select/masking is done in the CPU

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

  -- collisions with other modules.

  -- This module provides an ERROR signal to signal a faulty access operation to the CPU.

  -- It can be used to indicate an invalid access (for example to an unused CFS register address) or to signal

  -- a faulty state (like "not operational yet"). The error signal can be checked be checked by the applications

  -- "bus access fault" exception handler (provided by the system's BUSKEEPER module).

  -- This signal may only be set when the module is actually accessed! Tie to zero if not explicitly used.

  err_o <= '0';

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

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

  -- In its default version, the CFS provides the configuration generics. single generic:

  -- 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 entiy.

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

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

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

  -- These are intended as "conduits" to propagate custom signals this entity <=> 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 an external reset and also by a watchdog-cause reset.

  --

  -- Most default peripheral devices of the NEORV32 do NOT use a dedicated 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 write ZERO to every single address in the processor's IO space - including the CFS.

  -- Make sure that this clearing does not cause any unintended actions in the CFS.

  -- 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 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

  --

  -- 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 SPI) 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.

  -- The interrupt is high-level-active. When set, the interrupt appears as "pending" in the CPU's mie register

  -- ready to trigger execution of the according interrupt handler.

  -- Once set, the irq_o signal **has to stay set** until explicitly acknowledged by the CPU

  -- (for example by reading/writing from/to a specific CFS interface register address).

  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 access (half-word or byte)

  -- will trigger 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; the maximum latency until an unacknowledged access will trigger a bus exception is defined via the package's

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

  -- 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 CF register). The remaining addresses of the CFS are not

  -- associated with any writable or readable register - an access to those is simply ignored but still acknowledged.

  host_access: process(clk_i)

  begin

    if rising_edge(clk_i) then -- synchronous interface for reads and writes

      -- 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

--    ack_o <= rden; -- use this construct if your CFS is read-only

--    ack_o <= wren; -- use this construct if your CFS is write-only

--    ack_o <= ... -- or define the ACK by yourself (example: some registers are read-only, some others can only be written, ...)

      -- write access --

      if (wren = '1') then -- word-wide write-access only!

        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; -- for example: control register

        end if;

        if (addr = cfs_reg1_addr_c) then

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

        end if;

        if (addr = cfs_reg2_addr_c) then

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

        end if;

        if (addr = cfs_reg3_addr_c) then

          cfs_reg_wr(3) <= data_i; -- 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; if required, the byte/half-word select/masking is done in the CPU

        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: process(cfs_reg_wr)

  begin

    cfs_reg_rd(0) <= not cfs_reg_wr(0); -- just invert the written value

    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;

end neorv32_cfs_rtl;