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

-- #  << NEO430 - Two Wire Serial Interface Master (I2C) >>                                        #

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

-- # Supports START and STOP conditions, 8 bit data + ACK/NACK transfers and clock stretching.     #

-- # Supports ACKs by the master. No multi-master support and no slave mode support yet!           #

-- # Interrupt: TWI_transfer_done                                                                  #

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

-- # 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.                                                                                #

-- #                                                                                               #

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

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-- # 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_twi 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);

    -- com lines --

    twi_sda_io  : inout std_logic; -- serial data line

    twi_scl_io  : inout std_logic; -- serial clock line

    -- interrupt --

    twi_irq_o   : out std_ulogic -- transfer done IRQ

  );

end neo430_twi;

architecture neo430_twi_rtl of neo430_twi 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(twi_size_c); -- low address boundary bit

  -- control reg bits --

  constant ctrl_twi_en_c     : natural := 0; -- r/w: TWI enable

  constant ctrl_twi_start_c  : natural := 1; -- -/w: Generate START condition

  constant ctrl_twi_stop_c   : natural := 2; -- -/w: Generate STOP condition

  constant ctrl_twi_busy_c   : natural := 3; -- r/-: Set if TWI unit is busy

  constant ctrl_twi_prsc0_c  : natural := 4; -- r/w: CLK prsc bit 0

  constant ctrl_twi_prsc1_c  : natural := 5; -- r/w: CLK prsc bit 1

  constant ctrl_twi_prsc2_c  : natural := 6; -- r/w: CLK prsc bit 2

  constant ctrl_twi_irq_en_c : natural := 7; -- r/w: transmission done interrupt

  constant ctrl_twi_mack_c   : natural := 8; -- r/w: generate ACK by master for transmission

  -- data register flags --

  constant data_twi_ack_c    : natural := 15; -- r/-: Set if ACK received

  -- access control --

  signal acc_en : std_ulogic; -- module access enable

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

  signal wr_en  : std_ulogic; -- word write enable

  signal rd_en  : std_ulogic; -- read enable

  -- twi clocking --

  signal twi_clk        : std_ulogic;

  signal twi_phase_gen  : std_ulogic_vector(3 downto 0);

  signal twi_clk_phase  : std_ulogic_vector(3 downto 0);

  -- twi clock stretching --

  signal twi_clk_halt : std_ulogic;

  -- twi transceiver core --

  signal ctrl         : std_ulogic_vector(8 downto 0); -- unit's control register

  signal arbiter      : std_ulogic_vector(2 downto 0);

  signal twi_bitcnt   : std_ulogic_vector(3 downto 0);

  signal twi_rtx_sreg : std_ulogic_vector(8 downto 0); -- main rx/tx shift reg

  -- tri-state I/O --

  signal twi_sda_i_ff0, twi_sda_i_ff1 : std_ulogic; -- sda input sync

  signal twi_scl_i_ff0, twi_scl_i_ff1 : std_ulogic; -- sda input sync

  signal twi_sda_i,     twi_sda_o     : std_ulogic;

  signal twi_scl_i,     twi_scl_o     : std_ulogic;

begin

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

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

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

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

  wr_en  <= acc_en and wren_i;

  rd_en  <= acc_en and rden_i;

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

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

  wr_access: process(clk_i)

  begin

    if rising_edge(clk_i) then

      if (wr_en = '1') then

        if (addr = twi_ctrl_addr_c) then

          ctrl <= data_i(ctrl'left downto 0);

        end if;

      end if;

    end if;

  end process wr_access;

  -- Clock Generation ---------------------------------------------------------

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

  -- clock generator enable --

  clkgen_en_o <= ctrl(ctrl_twi_en_c);

  -- main twi clock select --

  twi_clk <= clkgen_i(to_integer(unsigned(ctrl(ctrl_twi_prsc2_c downto ctrl_twi_prsc0_c))));

  -- generate four non-overlapping clock ticks at twi_clk/4 --

  clock_phase_gen: process(clk_i)

  begin

    if rising_edge(clk_i) then

      if (arbiter(2) = '0') or (arbiter = "100") then -- offline or idle

        twi_phase_gen <= "0001"; -- make sure to start with a new phase, 0,1,2,3 stepping

      elsif (twi_clk = '1') and (twi_clk_halt = '0') then -- enabled and no clock stretching detected

        twi_phase_gen <= twi_phase_gen(2 downto 0) & twi_phase_gen(3); -- shift left

      end if;

    end if;

  end process clock_phase_gen;

  twi_clk_phase(0) <= twi_phase_gen(0) and twi_clk; -- first step

  twi_clk_phase(1) <= twi_phase_gen(1) and twi_clk;

  twi_clk_phase(2) <= twi_phase_gen(2) and twi_clk;

  twi_clk_phase(3) <= twi_phase_gen(3) and twi_clk; -- last step

  -- TWI transceiver ----------------------------------------------------------

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

  twi_rtx_unit: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- input synchronizer & sampler --

      twi_sda_i_ff0 <= twi_sda_i;

      twi_sda_i_ff1 <= twi_sda_i_ff0;

      twi_scl_i_ff0 <= twi_scl_i;

      twi_scl_i_ff1 <= twi_scl_i_ff0;

      -- defaults --

      twi_irq_o  <= '0';

      arbiter(2) <= ctrl(ctrl_twi_en_c); -- still activated?

      -- arbiter FSM --

      -- TWI bus signals are set/sampled using 4 clock phases

      case arbiter is

        when "100" => -- IDLE: waiting for requests, bus might be still claimed by this master if no STOP condition was generated

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

          if (wr_en = '1') then

            if (addr = twi_ctrl_addr_c) then

              if (data_i(ctrl_twi_start_c) = '1') then -- issue START condition

                arbiter(1 downto 0) <= "01";

              elsif (data_i(ctrl_twi_stop_c) = '1') then  -- issue STOP condition

                arbiter(1 downto 0) <= "10";

              end if;

            elsif (addr = twi_rtx_addr_c) then -- start a data transmission

              -- one bit extra for ack, issued by master if ctrl_twi_mack_c is set,

              -- sampled from slave if ctrl_twi_mack_c is cleared

              twi_rtx_sreg <= data_i(7 downto 0) & (not ctrl(ctrl_twi_mack_c));

              arbiter(1 downto 0) <= "11";

            end if;

          end if;

        when "101" => -- START: generate START condition

          if (twi_clk_phase(0) = '1') then

            twi_sda_o <= '1';

          elsif (twi_clk_phase(1) = '1') then

            twi_sda_o <= '0';

          end if;

          if (twi_clk_phase(0) = '1') then

            twi_scl_o <= '1';

          elsif (twi_clk_phase(3) = '1') then

            twi_scl_o <= '0';

            arbiter(1 downto 0) <= "00"; -- go back to IDLE

          end if;

        when "110" => -- STOP: generate STOP condition

          if (twi_clk_phase(0) = '1') then

            twi_sda_o <= '0';

          elsif (twi_clk_phase(3) = '1') then

            twi_sda_o <= '1';

            arbiter(1 downto 0) <= "00"; -- go back to IDLE

          end if;

          if (twi_clk_phase(0) = '1') then

            twi_scl_o <= '0';

          elsif (twi_clk_phase(1) = '1') then

            twi_scl_o <= '1';

          end if;

        when "111" => -- TRANSMISSION: transmission in progress

          if (twi_clk_phase(0) = '1') then

            twi_bitcnt   <= std_ulogic_vector(unsigned(twi_bitcnt) + 1);

            twi_scl_o    <= '0';

            twi_sda_o    <= twi_rtx_sreg(8); -- MSB first

          elsif (twi_clk_phase(1) = '1') then -- first half + second half of valid data strobe

            twi_scl_o    <= '1';

          elsif (twi_clk_phase(3) = '1') then

            twi_rtx_sreg <= twi_rtx_sreg(7 downto 0) & twi_sda_i_ff1; -- sample and shift left

            twi_scl_o    <= '0';

          end if;

          if (twi_bitcnt = "1010") then -- 8 data bits + 1 bit for ACK + 1 tick delay

            arbiter(1 downto 0) <= "00"; -- go back to IDLE

            twi_irq_o <= ctrl(ctrl_twi_irq_en_c); -- fire IRQ if enabled

          end if;

        when others => -- "0--" OFFLINE: TWI deactivated

          twi_sda_o <= '1';

          twi_scl_o <= '1';

          arbiter   <= ctrl(ctrl_twi_en_c) & "00"; -- stay here, go to idle when activated

      end case;

    end if;

  end process twi_rtx_unit;

  -- Clock Stretching Detector ------------------------------------------------

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

  clock_stretching: process(arbiter, twi_scl_o, twi_scl_i_ff1)

  begin

    -- clock stretching by the slave can happen at "any time"

    if (arbiter(2) = '1') and     -- module enabled

       (twi_scl_o = '1') and      -- master wants to pull scl high

       (twi_scl_i_ff1 = '0') then -- but scl is pulled low by slave

      twi_clk_halt <= '1';

    else

      twi_clk_halt <= '0';

    end if;

  end process clock_stretching;

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

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

  rd_access: process(clk_i)

  begin

    if rising_edge(clk_i) then

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

      if (rd_en = '1') then

        if (addr = twi_ctrl_addr_c) then

          data_o(ctrl_twi_en_c)     <= ctrl(ctrl_twi_en_c);

          data_o(ctrl_twi_prsc0_c)  <= ctrl(ctrl_twi_prsc0_c);

          data_o(ctrl_twi_prsc1_c)  <= ctrl(ctrl_twi_prsc1_c);

          data_o(ctrl_twi_prsc2_c)  <= ctrl(ctrl_twi_prsc2_c);

          data_o(ctrl_twi_irq_en_c) <= ctrl(ctrl_twi_irq_en_c);

          data_o(ctrl_twi_busy_c)   <= arbiter(1) or arbiter(0);

          data_o(ctrl_twi_mack_c)   <= ctrl(ctrl_twi_mack_c);

        else -- twi_rtx_addr_c =>

          data_o(7 downto 0)        <= twi_rtx_sreg(8 downto 1);

          data_o(data_twi_ack_c)    <= not twi_rtx_sreg(0);

        end if;

      end if;

    end if;

  end process rd_access;

  -- Tri-State Driver ---------------------------------------------------------

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

  -- SDA and SCL need to be of type std_logic to be correctly resolved in simulation

  twi_sda_io <= '0' when (twi_sda_o = '0') else 'Z';

  twi_scl_io <= '0' when (twi_scl_o = '0') else 'Z';

  -- read-back --

  twi_sda_i <= std_ulogic(twi_sda_io);

  twi_scl_i <= std_ulogic(twi_scl_io);

end neo430_twi_rtl;