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

-- AES3 / SPDIF Minimalistic Receiver

-- Version 0.9

-- Petr Nohavica (c) 2009

-- Released under GNU Lesser General Public License

-- Original target device: Xilinx Spartan-3AN family

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

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity aes3rx is

   generic (

      -- Registers width, determines minimal baud speed of input AES3 at given master clock frequency

      reg_width : integer := 5

   );

   port (

      -- Master clock

      clk   : in  std_logic;

      -- AES3/SPDIF compatible input signal

      aes3  : in  std_logic;

      -- Synchronous reset

      reset : in  std_logic;

      -- Serial data out

      sdata : out std_logic := '0'; -- output serial data

      -- AES3 clock out

      sclk  : out std_logic := '0'; -- output serial data clock

      -- Block start (asserted when Z subframe is being transmitted)

      bsync : out std_logic := '0';

      -- Frame sync (asserted for channel A, negated for B)

      lrck  : out std_logic := '0';

      -- Receiver has (probably) valid data on its outputs

      active: out std_logic := '0'

   );

end aes3rx;

architecture Behavioral of aes3rx is

   -- Locking state machine states enum type

   type lock_state_type is (locking, confirming, locked);

   -- Constants for preamble detection

   constant X_PREAMBLE     : std_logic_vector(7 downto 0) := "01000111";

   constant Y_PREAMBLE     : std_logic_vector(7 downto 0) := "00100111";

   constant Z_PREAMBLE     : std_logic_vector(7 downto 0) := "00010111";

   -- Input shift register for handling metastability issues and delaying input

   signal aes3_sync        : std_logic_vector(3 downto 0) := (others => '0');

   -- Change signal, active high on input transition

   signal change           : std_logic := '0';

   -- Recovered AES3 clock, stream of pulses

   signal aes3_clk         : std_logic := '0';

   -- Shift register for preamble detection and data decoding

   signal decoder_shift    : std_logic_vector(7 downto 0) := (others => '0');

   -- 1 bit counter used for correct decoder_shift alignment for data decoder

   signal align_counter    : std_logic := '0';

   -- Counter for AES3 clk regeneration

   signal clk_counter      : std_logic_vector(reg_width - 1 downto 0) := (others => '0');

   -- Counts aes3_clk pulses per frame (for locking reasons)

   signal sync_cnt         : std_logic_vector(5 downto 0) := (others => '0');

   -- Period register for clk_counter

   signal reg_clk_period   : std_logic_vector(reg_width - 1 downto 0) := (others => '1');

   -- Asserted when locking state machine is not in locked state

   signal sync_lost        : std_logic := '1';

   -- Asserted when preamble has been detected

   signal preamble_detected: std_logic := '0';

   -- Internal version of bsync signal

   signal bsync_int        : std_logic := '0';

   -- Internal version of lrck signal

   signal lrck_int         : std_logic := '0';

   -- Internal version of sdata signal

   signal sdata_int        : std_logic := '0';

   -- State of locking state machine

   signal lock_state       : lock_state_type := locking;

   -- Next state of locking state machine

   signal lock_state_next  : lock_state_type;

   -- Signal indicating some error in received AES3, used primarily by locking state machine

   signal lock_error       : std_logic;

   -- Asserted when sync_cnt is full (i.e. it has value of 63)

   signal sync_cnt_full    : std_logic;

   -- Asserted when there was at least one aes3_clk pulse since last input transition

   signal aes3_clk_activity: std_logic := '0';

   -- Signals indicating detection of X, Y and Z preambles

   signal x_detected       : std_logic := '0';

   signal y_detected       : std_logic := '0';

   signal z_detected       : std_logic := '0';

begin

   -- Carries out input double sampling in order to avoid metastable states on FFs and creation 

   -- of delayed signals for change detector (1 clk period) and decoder (2 clk periods).

   input_shift_reg_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            aes3_sync <= (others => '0');

         else

            aes3_sync <= aes3 & aes3_sync(3 downto 1); -- synthetizes  shift reg

         end if;

      end if;

   end process;

   -- Detects edge on sampled input in the way of comparsion of delayed input and its current 

   -- state on XOR gate.

   change_detect_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            change <= '0';

         else

            change <= aes3_sync(2) xor aes3_sync(1);

         end if;

      end if;

   end process;

   -- Counts number of aes3_clk pulses since last preamble detection, used by locking state machine

   sync_cnt_proc: process (clk)

   begin

      if clk'event and clk ='1' then

         if reset = '1' then

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

         elsif aes3_clk = '1' then

            if preamble_detected = '1' then

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

            else

               sync_cnt <= sync_cnt + 1;

            end if;

         end if;

      end if;

   end process;

   -- Comparator driving sync_cnt_full signal

   sync_cnt_comp: process(sync_cnt)

   begin

      if sync_cnt = 63 then

         sync_cnt_full <= '1';

      else

         sync_cnt_full <= '0';

      end if;

   end process;

   -- Lock error occurs when sync_cnt is full and no preamble has been detected (i.e. when the

   -- aes3_clk pulse generation speed is too high) or preamble is detected and sync_cnt is not full

   -- (when aes3_clk pulse rate is too low) or when since last input transition there was no 

   -- aes3_clk pulse at all (value of reg_clk_period is a way too high)

   lock_error <= (sync_cnt_full and not preamble_detected) or

                 (not sync_cnt_full and preamble_detected) or

                 (change and not aes3_clk_activity);

   -- Counter holding aes3_clk period duration. The receiver tries to receive a valid frame using 

   -- counter's (which is initially all ones) output, if it fails, counter is enabled to lower its

   -- value by one (counter is also enabled when aes3_clk_activity is low on input transition -

   -- which indicates that value of reg_clk_period is way too high and no aes3_clk pulses are being

   -- generated) and this new value is tried. This process is repeated until lock is not acquired. 

   -- This solution consumes less logic than direct measurement of shortest AES3 symbol and is actually

   -- more reliable. Lock time is fast enough (will always be under 2**(reg_width + 1) frames, but very

   -- likely much faster thanks to initial rapid speed of counting which is given by no activity on

   -- aes3_clk signal).

   aes3_clk_period_proc: process(clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            reg_clk_period <= (others => '1');

         elsif (lock_state = locked and lock_state_next = locking) then

            reg_clk_period <= (others => '1');

         elsif (aes3_clk = '1' and sync_cnt_full = '1' and lock_state_next = locking) or

               (change = '1' and aes3_clk_activity = '0') then

            reg_clk_period <= reg_clk_period - 1;

         end if;

      end if;

   end process;

   -- Locking state machine. While initialized in locking state, waits for preamble_detection - once one

   -- has been detected, state machine is transitioned to confirming state. When no locking error 

   -- occurs during one next frame, receiver is considered locked and state accordingly changes. Otherwise

   -- the state machine falls back to locking state.

   lock_state_machine: process (lock_state, preamble_detected, sync_cnt_full,

                                lock_error)

   begin

      case lock_state is

         when locking =>

            if preamble_detected = '1' then

               lock_state_next <= confirming;

            else

               lock_state_next <= locking;

            end if;

         when confirming =>

            if lock_error = '1' then

               lock_state_next <= locking;

            elsif sync_cnt_full = '1' and preamble_detected = '1' then

               lock_state_next <= locked;

            else

               lock_state_next <= confirming;

            end if;

         when locked =>

            if lock_error = '1' then

               lock_state_next <= locking;

            else

               lock_state_next <= locked;

            end if;

      end case;

   end process;

   -- When state of locking state machine is other than locked, sync_lost signal is asserted.

   sync_lost_proc: process (lock_state)

   begin

      if lock_state = locked then

         sync_lost <= '0';

      else

         sync_lost <= '1';

      end if;

   end process;

   -- Synchronization process for locking state machine

   lock_state_machinine_sync_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            lock_state <= locking;

         elsif aes3_clk = '1' or (change = '1' and aes3_clk_activity = '0') then

            lock_state <= lock_state_next;

         end if;

      end if;

   end process;

   -- Counter for aes3_clk generation. On input transition, counter is loaded with approx. half

   -- of reg_clk_period, which should create 90 degrees phase shift of regenerated clock in respect

   -- to delayed input and thus ensure that input will be sampled in approximate middle of 1UI symbol

   -- (or in the middle of one half of 2UI symbol or in the middle of one third of 3UI symbol).

   -- Otherwise, when no transition has been detected on input and clk_counter counts to zero, full 

   -- reg_clk_period is loaded into the counter to create aes3_clk pulses when 2UI and 3UI symbols are 

   -- being received.

   aes3_clk_cnt_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

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

         elsif change = '1' or clk_counter = 0 then

            if change = '1' then

               clk_counter <= '0' & reg_clk_period(reg_width - 1 downto 1);

            else

               clk_counter <= reg_clk_period;

            end if;

         else

            clk_counter <= clk_counter - 1;

         end if;

      end if;

   end process;

   -- Generates aes3_clk pulse when clk_counter counts to zero.

   process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            aes3_clk <= '0';

         else

            if clk_counter = 0 then

               aes3_clk <= '1';

            else

               aes3_clk <= '0';

            end if;

         end if;

      end if;

   end process;

   -- Monitors activity on aes3_clk

   process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            aes3_clk_activity <= '0';

         else

            if change = '1' then

               aes3_clk_activity <= '0';

            elsif clk_counter = 0 then

               aes3_clk_activity <= '1';

            end if;

         end if;

      end if;

   end process;

   -- Eight bit shift register for preamble detection and decoder functionality.

   decoder_shift_reg_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

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

         elsif aes3_clk = '1' then

            decoder_shift <= aes3_sync(0) & decoder_shift(7  downto 1);

         end if;

      end if;

   end process;

   -- Preamble detectors (implemented using comparators)

   x_preamble_detector: process(decoder_shift)

   begin

      if decoder_shift = X_PREAMBLE or decoder_shift = not X_PREAMBLE then

         x_detected <= '1';

      else

         x_detected <= '0';

      end if;

   end process;

   y_preamble_detector: process(decoder_shift)

   begin

      if decoder_shift = Y_PREAMBLE or decoder_shift = not Y_PREAMBLE then

         y_detected <= '1';

      else

         y_detected <= '0';

      end if;

   end process;

   z_preamble_detector: process(decoder_shift)

   begin

      if decoder_shift = Z_PREAMBLE or decoder_shift = not Z_PREAMBLE then

         z_detected <= '1';

      else

         z_detected <= '0';

      end if;

   end process;

   preamble_detected <= x_detected or y_detected or z_detected;

   -- One bit counter used for correct bit alignment on bit decoder. Align_counter is reset on 

   -- preamble detection and thus allows decoder to be correctly aligned with sampled symbol beginning.

   align_cnt_proc: process(clk)

   begin

      if clk'event and clk = '1' then

         if reset = '1' then

            align_counter <= '0';

         elsif aes3_clk = '1' then

            if preamble_detected = '1' then

               align_counter <= '0';

            else

               align_counter <= not align_counter;

            end if;

         end if;

      end if;

   end process;

   -- Drives lrck and bsync signals

   frame_block_sync_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if aes3_clk = '1' and preamble_detected = '1' then

            lrck_int <= x_detected or z_detected;

            bsync_int <= z_detected;

         end if;

      end if;

   end process;

   -- Two consecutive sampled symbols, when equal, are considered as logical 0. Two consecutive sampled 

   -- symbols, when differs, are considered as logical 1. This logical value is then shifted into 

   -- data_shift_reg.

   data_shift_reg_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         if aes3_clk = '1' and align_counter = '1' then

            sdata_int <= decoder_shift(1) xor decoder_shift(0);

         end if;

      end if;

   end process;

   -- Synchronization and activity signals outputs

   activity_eval_proc: process (clk)

   begin

      if clk'event and clk = '1' then

         active <= not sync_lost;

         lrck <= lrck_int and not sync_lost;

         bsync <= bsync_int and not sync_lost;

         sclk <= align_counter and not sync_lost;

         sdata <= sdata_int and not sync_lost;

      end if;

   end process;

end Behavioral;