gbc_snd.vhd

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
use ieee.numeric_std.all;

library work;
entity gbc_snd is
    port (
        clk          : in std_logic;
        ce           : in std_logic;
        reset        : in std_logic;

        is_gbc       : in std_logic;

        s1_read      : in std_logic;
        s1_write     : in std_logic;
        s1_addr      : in std_logic_vector(6 downto 0);
        s1_readdata  : out std_logic_vector(7 downto 0);
        s1_writedata : in std_logic_vector(7 downto 0);

        snd_left     : out unsigned(11 downto 0);
        snd_right    : out unsigned(11 downto 0)
    );

end gbc_snd;

architecture SYN of gbc_snd is

    subtype wav_t is std_logic_vector(3 downto 0);
    type wav_arr_t is array(0 to 31) of wav_t;

    signal en_snd            : boolean; -- Enable at base sound frequency (4.19MHz)
    signal en_snd2           : boolean; -- Enable at clk/2
    signal en_snd4           : boolean; -- Enable at clk/4
    signal en_512            : boolean; -- 512Hz enable 

    signal en_snden2         : boolean; -- Enable at clk/2
    signal en_snden4         : boolean; -- Enable at clk/4

    signal en_len            : boolean; -- Sample length
    signal en_len_r          : boolean;
    signal en_env            : boolean; -- Envelope
    signal en_sweep          : boolean; -- Sweep

    signal snd_enable        : std_logic;

    signal sq1_swper         : std_logic_vector(2 downto 0);  -- Sq1 sweep period
    signal sq1_swdir         : std_logic;                     -- Sq1 sweep direction
    signal sq1_swdir_change  : std_logic;                     -- Sq1 sweep direction-change
    signal sq1_swshift       : std_logic_vector(2 downto 0);  -- Sq1 sweep frequency shift
    signal sq1_duty          : std_logic_vector(1 downto 0);  -- Sq1 duty cycle
    signal sq1_slen          : std_logic_vector(6 downto 0);  -- Sq1 play length
    signal sq1_svol          : std_logic_vector(3 downto 0);  -- Sq1 initial volume

    signal sq1_out         : std_logic;

    signal sq1_envsgn        : std_logic;                     -- Sq1 envelope sign
    signal sq1_envsgn_old    : std_logic;                     -- Sq1 old envelope sign (used in zombie mode)
    signal sq1_envper        : std_logic_vector(2 downto 0);  -- Sq1 envelope period
    signal sq1_envper_old    : std_logic_vector(2 downto 0);  -- Sq1 old envelope period  (used in zombie mode)
    signal sq1_freq          : std_logic_vector(10 downto 0); -- Sq1 frequency
    signal sq1_trigger       : std_logic;                     -- Sq1 trigger play note
    signal sq1_lenchk        : std_logic;                     -- Sq1 length check enable
    signal sq1_len_en_change : std_logic;                     -- Sq1 length off -> on

    signal sq1_fr2           : std_logic_vector(10 downto 0); -- Sq1 frequency (shadow copy)
    signal sq1_vol           : std_logic_vector(3 downto 0);  -- Sq1 initial volume
    signal sq1_nr2change     : std_logic;
    signal sq1_lenchange     : std_logic;
    signal sq1_lenquirk      : std_logic;
    signal sq1_freqchange    : std_logic;

    signal sq1_playing       : std_logic;                    -- Sq1 channel active
    signal sq1_wav           : std_logic_vector(5 downto 0); -- Sq1 output waveform
    signal sq1_suppressed    : std_logic;                    -- Sq1 channel 1st sample suppressed when triggered after poweron

    signal sq2_out         : std_logic;

    signal sq2_duty          : std_logic_vector(1 downto 0); -- Sq2 duty cycle
    signal sq2_slen          : std_logic_vector(6 downto 0); -- Sq2 play length
    signal sq2_svol          : std_logic_vector(3 downto 0); -- Sq2 initial volume
    signal sq2_nr2change     : std_logic;
    signal sq2_lenchange     : std_logic;
    signal sq2_lenquirk      : std_logic;
    signal sq2_envsgn        : std_logic;                     -- Sq2 envelope sign
    signal sq2_envper        : std_logic_vector(2 downto 0);  -- Sq2 envelope period
    signal sq2_envsgn_old    : std_logic;                     -- Sq2 old envelope sign (used in zombie mode)
    signal sq2_envper_old    : std_logic_vector(2 downto 0);  -- Sq2 old envelope period  (used in zombie mode)
    signal sq2_freq          : std_logic_vector(10 downto 0); -- Sq2 frequency
    signal sq2_trigger       : std_logic;                     -- Sq2 trigger play note
    signal sq2_lenchk        : std_logic;                     -- Sq2 length check enable

    signal sq2_vol           : std_logic_vector(3 downto 0);  -- Sq2 initial volume
    signal sq2_playing       : std_logic;                     -- Sq2 channel active
    signal sq2_wav           : std_logic_vector(5 downto 0);  -- Sq2 output waveform
    signal sq2_suppressed    : std_logic;                     -- Sq2 channel 1st sample suppressed when triggered after poweron
    
    signal wav_enable        : std_logic;                     -- Wave enable
    signal wav_slen          : std_logic_vector(8 downto 0);  -- Wave play length
    signal wav_lenchange     : std_logic;
    signal wav_lenquirk      : std_logic;
    signal wav_volsh         : std_logic_vector(1 downto 0);  -- Wave volume shift
    signal wav_freq          : std_logic_vector(10 downto 0); -- Wave frequency
    signal wav_trigger       : std_logic;                     -- Wave trigger play note
    signal wav_lenchk        : std_logic;                     -- Wave length check enable
    signal wav_index         : unsigned(4 downto 0);          -- Wave current sample index
    signal wav_access        : unsigned(1 downto 0);          -- Wave table access counter used for DMG
    signal wav_playing       : std_logic;
    signal wav_wav           : std_logic_vector(5 downto 0); -- Wave output waveform
    signal wav_ram           : wav_arr_t;                    -- Wave table

    signal noi_slen          : std_logic_vector(6 downto 0);
    signal noi_lenchange     : std_logic;
    signal noi_lenquirk      : std_logic;
    signal noi_svol          : std_logic_vector(3 downto 0);
    signal noi_nr2change     : std_logic;
    signal noi_envsgn        : std_logic;
    signal noi_envper        : std_logic_vector(2 downto 0);
    signal noi_envsgn_old    : std_logic;                    -- noi old envelope sign (used in zombie mode)
    signal noi_envper_old    : std_logic_vector(2 downto 0); -- noi old envelope period  (used in zombie mode)
    signal noi_freqsh        : std_logic_vector(3 downto 0);
    signal noi_freqchange    : std_logic;
    signal noi_short         : std_logic;
    signal noi_div           : std_logic_vector(2 downto 0);
    signal noi_trigger       : std_logic;
    signal noi_lenchk        : std_logic;

    signal noi_vol           : std_logic_vector(3 downto 0); -- Noise initial volume
    signal noi_playing       : std_logic;                    -- Noise channel active
    signal noi_wav           : std_logic_vector(5 downto 0); -- Noise output waveform

    signal ch_map            : std_logic_vector(7 downto 0);
    signal ch_vol            : std_logic_vector(7 downto 0);
    signal framecnt          : integer range 0 to 7 := 0;

    signal sq1_trigger_r     : std_logic;
    signal sq2_trigger_r     : std_logic;
    signal wav_trigger_r     : std_logic;
    -- signal noi_trigger_r     : std_logic;

begin

    en_snd2 <= en_snd and en_snden2;
    en_snd4 <= en_snd and en_snden4;

    process (clk, en_snd, reset)
    begin
        if reset = '1' then
            en_snd <= false;
        elsif rising_edge(clk) then
            if ce = '1' then
                en_snd <= not en_snd;
            end if;
        end if;
    end process;

    -- Calculate divided and frame sequencer clock enables
    process (clk, en_snd, snd_enable, reset)
        variable clkcnt   : unsigned(1 downto 0);
        variable cnt_512  : unsigned(12 downto 0);
        variable temp_512 : unsigned(13 downto 0);
    begin
        if reset = '1' or snd_enable = '0' then --only clock frame sequencer if sound is enabled, restart at 0 
            clkcnt  := "00";
            cnt_512 := (others => '0');
            framecnt <= 0;
            en_len_r <= false;
            en_snden2 <= false;
            en_snden4 <= false;
            en_len   <= false;
            en_env   <= false;
            en_sweep <= false;
            en_512 <= false;

        elsif rising_edge(clk) then
            if ce = '1' then
                -- Base clock divider
                if en_snd then
                    clkcnt := clkcnt + 1;
                    if clkcnt(0) = '1' then
                        en_snden2 <= true;
                    else
                        en_snden2 <= false;
                    end if;
                    if clkcnt = "11" then
                        en_snden4 <= true;
                    else
                        en_snden4 <= false;
                    end if;
                end if;

                -- Frame sequencer (length, envelope, sweep) clock enables
                en_len   <= false;
                en_env   <= false;
                en_sweep <= false;
                if en_512 then
                    en_len_r <= not en_len_r;
                    if framecnt = 0 or framecnt = 2 or framecnt = 4 or framecnt = 6 then
                        en_len   <= true;
                        en_len_r <= not en_len_r;
                    end if;
                    if framecnt = 2 or framecnt = 6 then
                        en_sweep <= true;
                    end if;
                    if framecnt = 7 then
                        en_env <= true;
                    end if;

                    if framecnt < 7 then
                        framecnt <= framecnt + 1;
                    else
                        framecnt <= 0;
                    end if;
                end if;

                --
                en_512 <= false;
                if en_snd then
                    temp_512 := ('0' & cnt_512) + to_unsigned(1, temp_512'length);
                    cnt_512  := temp_512(temp_512'high - 1 downto temp_512'low);
                    en_512 <= (temp_512(13) = '1');
                end if;
            end if;
        end if;
    end process;

    -- Registers
    registers : process (clk, snd_enable, reset, is_gbc)
        variable sq1_trigger_cnt : unsigned(2 downto 0);
        variable sq2_trigger_cnt : unsigned(2 downto 0);
        variable wav_trigger_cnt : unsigned(3 downto 0);
        variable wave_index_write : std_logic_vector(3 downto 0);
    begin
      if rising_edge(clk) then
        -- Registers
        if reset = '1' then
            -- Reset register values
            sq1_swper   <= (others => '0');
            sq1_swdir   <= '0';
            sq1_swshift <= (others => '0');
            sq1_duty    <= (others => '0');
            sq1_slen    <= (others => '0');
            sq1_svol    <= (others => '0');
            sq1_envsgn  <= '0';
            sq1_envper  <= (others => '0');
            sq1_freq    <= (others => '0');
            sq1_lenchk  <= '0';
            sq1_trigger <= '0';

            sq2_duty    <= (others => '0');
            sq2_slen    <= (others => '0');
            sq2_svol    <= (others => '0');
            sq2_envsgn  <= '0';
            sq2_envper  <= (others => '0');
            sq2_freq    <= (others => '0');
            sq2_lenchk  <= '0';
            sq2_trigger <= '0';

            wav_enable  <= '0';
            wav_volsh   <= (others => '0');
            wav_freq    <= (others => '0');
            wav_trigger <= '0';
            wav_lenchk  <= '0';
            sq1_trigger_cnt := (others => '0'); --counter to delay the trigger
            sq2_trigger_cnt := (others => '0'); --counter to delay the trigger
            wav_trigger_cnt := (others => '0'); --counter to delay the trigger
            noi_slen    <= (others     => '0');
            noi_svol    <= (others     => '0');
            noi_envsgn  <= '0';
            noi_envper  <= (others => '0');
            noi_freqsh  <= (others => '0');
            noi_short   <= '0';
            noi_div     <= (others => '0');
            noi_trigger <= '0';
            noi_lenchk  <= '0';
            
            ch_map      <= (others => '0');
            ch_vol      <= (others => '0');

            --      Wave table   https://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware#Power_Control
            if is_gbc = '1' then
                wav_ram(0) <=  X"0";
                wav_ram(1) <=  X"0";
                wav_ram(2) <=  X"F";
                wav_ram(3) <=  X"F";
                wav_ram(4) <=  X"0";
                wav_ram(5) <=  X"0";
                wav_ram(6) <=  X"F";
                wav_ram(7) <=  X"F";
                wav_ram(8) <=  X"0";
                wav_ram(9) <=  X"0";
                wav_ram(10) <= X"F";
                wav_ram(11) <= X"F";
                wav_ram(12) <= X"0";
                wav_ram(13) <= X"0";
                wav_ram(14) <= X"F";
                wav_ram(15) <= X"F";
                wav_ram(16) <= X"0";
                wav_ram(17) <= X"0";
                wav_ram(18) <= X"F";
                wav_ram(19) <= X"F";
                wav_ram(20) <= X"0";
                wav_ram(21) <= X"0";
                wav_ram(22) <= X"F";
                wav_ram(23) <= X"F";
                wav_ram(24) <= X"0";
                wav_ram(25) <= X"0";
                wav_ram(26) <= X"F";
                wav_ram(27) <= X"F";
                wav_ram(28) <= X"0";
                wav_ram(29) <= X"0";
                wav_ram(30) <= X"F";
                wav_ram(31) <= X"F";
            else
                wav_ram(0) <=  X"8";
                wav_ram(1) <=  X"4";
                wav_ram(2) <=  X"4";
                wav_ram(3) <=  X"0";
                wav_ram(4) <=  X"4";
                wav_ram(5) <=  X"3";
                wav_ram(6) <=  X"A";
                wav_ram(7) <=  X"A";
                wav_ram(8) <=  X"2";
                wav_ram(9) <=  X"D";
                wav_ram(10) <= X"7";
                wav_ram(11) <= X"8";
                wav_ram(12) <= X"9";
                wav_ram(13) <= X"2";
                wav_ram(14) <= X"3";
                wav_ram(15) <= X"C";
                wav_ram(16) <= X"6";
                wav_ram(17) <= X"0";
                wav_ram(18) <= X"5";
                wav_ram(19) <= X"9";
                wav_ram(20) <= X"5";
                wav_ram(21) <= X"9";
                wav_ram(22) <= X"B";
                wav_ram(23) <= X"0";
                wav_ram(24) <= X"3";
                wav_ram(25) <= X"4";
                wav_ram(26) <= X"B";
                wav_ram(27) <= X"8";
                wav_ram(28) <= X"2";
                wav_ram(29) <= X"E";
                wav_ram(30) <= X"D";
                wav_ram(31) <= X"A";          
            end if;           

            snd_enable <= '0';

        elsif ce = '1' then
                
                -- TODO: align wav and noi triggers                 
                if en_snd2 then             
                    if sq1_trigger_cnt = "000" then
                        sq1_trigger <= '0';
                    else
                        sq1_trigger_cnt := sq1_trigger_cnt - 1;
                    end if;
                
                    if sq2_trigger_cnt = "000" then
                        sq2_trigger <= '0';
                    else
                        sq2_trigger_cnt := sq2_trigger_cnt - 1;
                    end if;
                end if;

                if wav_trigger_cnt = "0000" then
                  wav_trigger <= '0';
                else
                  wav_trigger_cnt := wav_trigger_cnt - 1;
                end if;
                
                if en_snd then
                    noi_trigger <= '0';
                end if;

                sq2_nr2change    <= '0';
                sq1_nr2change    <= '0';
                noi_nr2change    <= '0';
                noi_freqchange   <= '0';

                sq2_lenchange    <= '0';
                sq1_lenchange    <= '0';
                wav_lenchange    <= '0';
                noi_lenchange    <= '0';
                sq1_lenquirk     <= '0';
                sq2_lenquirk     <= '0';
                wav_lenquirk     <= '0';
                noi_lenquirk     <= '0';
                sq1_swdir_change <= '0';

                if sq1_freqchange = '1' then
                    sq1_freq <= sq1_fr2;
                end if;
                
                -- write to registers ignored when the apu is off , Wave memory can be read back freely , NR52 power control is always writable 
                if s1_write = '1' and (snd_enable = '1' or s1_addr = "0100110" or s1_addr(5 downto 4) = "11" or (is_gbc = '0' and (s1_addr = "0010001" or  s1_addr = "0010110" or s1_addr = "0011011" or s1_addr = "0100000" ))) then
                    case s1_addr is
                            -- Square 1
                        when "0010000" => -- NR10 FF10 -PPP NSSS Sweep period, negate, shift
                            sq1_swper <= s1_writedata(6 downto 4);
                            if s1_writedata(3) = '0' then -- only neg to pos, 1 -> 0
                                sq1_swdir_change <= '1';
                            end if;
                            sq1_swdir   <= s1_writedata(3);
                            sq1_swshift <= s1_writedata(2 downto 0);
                        when "0010001" => -- NR11 FF11 DDLL LLLL Duty, Length load (64-L)
                            if snd_enable = '1' then
                                sq1_duty      <= s1_writedata(7 downto 6);
                            end if;
                            sq1_slen      <= std_logic_vector("1000000" - unsigned(s1_writedata(5 downto 0)));
                            sq1_lenchange <= '1';
                        when "0010010" => -- NR12 FF12 VVVV APPP Starting volume, Envelope add mode, period
                            -- zombie mode copy old values
                            sq1_envsgn_old <= sq1_envsgn;
                            sq1_envper_old <= sq1_envper;
                            -- write to registers
                            sq1_nr2change  <= '1';
                            sq1_svol       <= s1_writedata(7 downto 4);
                            sq1_envsgn     <= s1_writedata(3);
                            sq1_envper     <= s1_writedata(2 downto 0);
                        when "0010011" => -- NR13 FF13 FFFF FFFF Frequency LSB
                            sq1_freq(7 downto 0) <= s1_writedata;
                        when "0010100" => -- NR14 FF14 TL-- -FFF Trigger, Length enable, Frequency MSB                           
                            sq1_trigger <= s1_writedata(7);
                            if s1_writedata(7) = '1' then
                                if sq1_playing = '1' then
                                    sq1_trigger_cnt := "010";  -- --according to sameboy : Timing quirk: if already active, sound starts 2 (2MHz) ticks earlier.
                                else
                                    sq1_trigger_cnt := "100";
                                end if;
                            end if;
                            if sq1_lenchk = '0' and s1_writedata(6) = '1' and en_len_r then
                                sq1_lenquirk <= '1';
                            end if;
                            sq1_lenchk            <= s1_writedata(6);
                            sq1_freq(10 downto 8) <= s1_writedata(2 downto 0);

                            -- Square 2
                        when "0010110" => -- NR21 FF16 DDLL LLLL Duty, Length load (64-L)
                            if snd_enable = '1' then
                                sq2_duty      <= s1_writedata(7 downto 6);
                            end if;
                            sq2_slen      <= std_logic_vector("1000000" - unsigned(s1_writedata(5 downto 0)));
                            sq2_lenchange <= '1';
                        when "0010111" => -- NR22 FF17 VVVV APPP Starting volume, Envelope add mode, period
                            -- zombie mode copy old values
                            sq2_envsgn_old <= sq2_envsgn;
                            sq2_envper_old <= sq2_envper;
                            -- write to registers
                            sq2_svol       <= s1_writedata(7 downto 4);
                            sq2_nr2change  <= '1';
                            sq2_envsgn     <= s1_writedata(3);
                            sq2_envper     <= s1_writedata(2 downto 0);
                        when "0011000" => -- NR23 FF18 FFFF FFFF Frequency LSB
                            sq2_freq(7 downto 0) <= s1_writedata;
                        when "0011001" => -- NR24 FF19 TL-- -FFF Trigger, Length enable, Frequency MSB                                                   
                            sq2_trigger <= s1_writedata(7);
                            if s1_writedata(7) = '1' then
                                if sq2_playing = '1' then
                                    sq2_trigger_cnt := "010"; -- according to sameboy : Timing quirk: if already active, sound starts 2 (2MHz) ticks earlier.
                                else
                                    sq2_trigger_cnt := "100";
                                end if;
                            end if;
                            if sq2_lenchk = '0' and s1_writedata(6) = '1' and en_len_r then
                                sq2_lenquirk <= '1';
                            end if;
                            sq2_lenchk            <= s1_writedata(6);
                            sq2_freq(10 downto 8) <= s1_writedata(2 downto 0);

                            -- Wave
                        when "0011010" => -- NR30 FF1A E--- ---- DAC power
                            wav_enable <= s1_writedata(7);
                        when "0011011" => -- NR31 FF1B LLLL LLLL Length load (256-L)
                            -- wav_slen <= s1_writedata;
                            wav_slen      <= std_logic_vector("100000000" - unsigned(s1_writedata));
                            wav_lenchange <= '1';
                        when "0011100" => -- NR32 FF1C -VV- ---- Volume code (00=0%, 01=100%, 10=50%, 11=25%)
                            wav_volsh <= s1_writedata(6 downto 5);
                        when "0011101" => -- NR33 FF1D FFFF FFFF Frequency LSB
                            wav_freq(7 downto 0) <= s1_writedata;
                        when "0011110" => -- NR34 FF1E TL-- -FFF Trigger, Length enable, Frequency MSB
                            wav_trigger <= s1_writedata(7);
                            if s1_writedata(7) = '1' then
                                wav_trigger_cnt := "1001";
                            end if;
                            if wav_lenchk = '0' and s1_writedata(6) = '1' and en_len_r then
                                wav_lenquirk <= '1';
                            end if;
                            wav_lenchk            <= s1_writedata(6);
                            wav_freq(10 downto 8) <= s1_writedata(2 downto 0);

                            -- Noise
                        when "0100000" => -- NR41 FF20 --LL LLLL Length load (64-L)
                            noi_slen      <= std_logic_vector("1000000" - unsigned(s1_writedata(5 downto 0)));
                            noi_lenchange <= '1';
                        when "0100001" => -- NR42 FF21 VVVV APPP Starting volume, Envelope add mode, period
                            -- zombie mode copy old values
                            noi_envsgn_old <= noi_envsgn;
                            noi_envper_old <= noi_envper;
                            -- write to registers
                            noi_svol       <= s1_writedata(7 downto 4);
                            noi_nr2change  <= '1';
                            noi_envsgn     <= s1_writedata(3);
                            noi_envper     <= s1_writedata(2 downto 0);
                        when "0100010" => -- NR43 FF22 SSSS WDDD Clock shift, Width mode of LFSR, Divisor code
                            noi_freqsh     <= s1_writedata(7 downto 4);
                            noi_short      <= s1_writedata(3);
                            noi_div        <= s1_writedata(2 downto 0);
                            noi_freqchange <= '1';
                        when "0100011" => -- NR44 FF23 TL-- ---- Trigger, Length enable
                            noi_trigger <= s1_writedata(7);
                            if noi_lenchk = '0' and s1_writedata(6) = '1' and en_len_r then
                                noi_lenquirk <= '1';
                            end if;
                            noi_lenchk              <= s1_writedata(6);

                            -- Control/Status
                        when "0100100" => ch_vol <= s1_writedata; -- NR50 FF24
                        when "0100101" => ch_map <= s1_writedata; -- NR51 FF25

                            -- Wave Table
                        when "0110000" | "0110001" | "0110010" | "0110011" | "0110100" | "0110101" | "0110110" | "0110111" | "0111000" | "0111001" | "0111010" | "0111011" | "0111100" | "0111101" | "0111110" | "0111111" => -- FF30 to FF3F
                            if wav_playing = '1' then
                                wave_index_write := std_logic_vector(wav_index(4 downto 1));
                            else
                                wave_index_write := s1_addr(3 downto 0);
                            end if;
            
                            if is_gbc = '1' or wav_access > 0 or wav_playing = '0' then
                                case wave_index_write is
                                when "0000" => --      FF30 0000 1111 Samples 0 and 1
                                    wav_ram(0) <= s1_writedata(7 downto 4);
                                    wav_ram(1) <= s1_writedata(3 downto 0);
                                when "0001" => --      FF31 0000 1111 Samples 2 and 3
                                    wav_ram(2) <= s1_writedata(7 downto 4);
                                    wav_ram(3) <= s1_writedata(3 downto 0);
                                when "0010" => --      FF32 0000 1111 Samples 4 and 5
                                    wav_ram(4) <= s1_writedata(7 downto 4);
                                    wav_ram(5) <= s1_writedata(3 downto 0);
                                when "0011" => --      FF33 0000 1111 Samples 6 and 31
                                    wav_ram(6) <= s1_writedata(7 downto 4);
                                    wav_ram(7) <= s1_writedata(3 downto 0);
                                when "0100" => --      FF34 0000 1111 Samples 8 and 31
                                    wav_ram(8) <= s1_writedata(7 downto 4);
                                    wav_ram(9) <= s1_writedata(3 downto 0);
                                when "0101" => --      FF35 0000 1111 Samples 10 and 11
                                    wav_ram(10) <= s1_writedata(7 downto 4);
                                    wav_ram(11) <= s1_writedata(3 downto 0);
                                when "0110" => --      FF36 0000 1111 Samples 12 and 13
                                    wav_ram(12) <= s1_writedata(7 downto 4);
                                    wav_ram(13) <= s1_writedata(3 downto 0);
                                when "0111" => --      FF37 0000 1111 Samples 14 and 15
                                    wav_ram(14) <= s1_writedata(7 downto 4);
                                    wav_ram(15) <= s1_writedata(3 downto 0);
                                when "1000" => --      FF38 0000 1111 Samples 16 and 17
                                    wav_ram(16) <= s1_writedata(7 downto 4);
                                    wav_ram(17) <= s1_writedata(3 downto 0);
                                when "1001" => --      FF39 0000 1111 Samples 18 and 19
                                    wav_ram(18) <= s1_writedata(7 downto 4);
                                    wav_ram(19) <= s1_writedata(3 downto 0);
                                when "1010" => --      FF3A 0000 1111 Samples 20 and 21
                                    wav_ram(20) <= s1_writedata(7 downto 4);
                                    wav_ram(21) <= s1_writedata(3 downto 0);
                                when "1011" => --      FF3B 0000 1111 Samples 22 and 23
                                    wav_ram(22) <= s1_writedata(7 downto 4);
                                    wav_ram(23) <= s1_writedata(3 downto 0);
                                when "1100" => --      FF3C 0000 1111 Samples 24 and 25
                                    wav_ram(24) <= s1_writedata(7 downto 4);
                                    wav_ram(25) <= s1_writedata(3 downto 0);
                                when "1101" => --      FF3D 0000 1111 Samples 26 and 27
                                    wav_ram(26) <= s1_writedata(7 downto 4);
                                    wav_ram(27) <= s1_writedata(3 downto 0);
                                when "1110" => --      FF3E 0000 1111 Samples 28 and 29
                                    wav_ram(28) <= s1_writedata(7 downto 4);
                                    wav_ram(29) <= s1_writedata(3 downto 0);
                                when "1111" => --      FF3F 0000 1111 Samples 30 and 31
                                    wav_ram(30) <= s1_writedata(7 downto 4);
                                    wav_ram(31) <= s1_writedata(3 downto 0);
                                end case;
                            end if;

                        -- NR52 FF26 P--- NW21 Power control/status, Channel length statuses
                        when "0100110" => 
                            -- TODO: maybe check if event on poweroff or poweron
                            snd_enable <= s1_writedata(7);
                            if s1_writedata(7) = '0' then
                                -- Reset register values
                                sq1_swper   <= (others => '0');
                                sq1_swdir   <= '0';
                                sq1_swshift <= (others => '0');
                                sq1_duty    <= (others => '0');
                                
                                sq1_svol    <= (others => '0');
                                sq1_envsgn  <= '0';
                                sq1_envper  <= (others => '0');
                                sq1_freq    <= (others => '0');
                                sq1_lenchk  <= '0';
                                sq1_trigger <= '0';

                                sq2_duty    <= (others => '0');
                                sq2_svol    <= (others => '0');
                                sq2_envsgn  <= '0';
                                sq2_envper  <= (others => '0');
                                sq2_freq    <= (others => '0');
                                sq2_lenchk  <= '0';
                                sq2_trigger <= '0';

                                wav_enable  <= '0';
                                wav_volsh   <= (others => '0');
                                wav_freq    <= (others => '0');
                                wav_trigger <= '0';
                                wav_lenchk  <= '0';

                                sq1_trigger_cnt := (others => '0'); --counter to leave the trigger high for 6 cycles
                                sq2_trigger_cnt := (others => '0'); --counter to leave the trigger high for 6 cycles
                                wav_trigger_cnt := (others => '0'); --counter to leave the trigger high for 6 cycles

                                noi_svol    <= (others     => '0');
                                noi_envsgn  <= '0';
                                noi_envper  <= (others => '0');
                                noi_freqsh  <= (others => '0');
                                noi_short   <= '0';
                                noi_div     <= (others => '0');
                                noi_trigger <= '0';
                                noi_lenchk  <= '0';
                                
                                ch_map      <= (others => '0');
                                ch_vol      <= (others => '0');
                                
                                if is_gbc = '1' then
                                    sq1_slen    <= (others => '0');
                                    sq2_slen    <= (others => '0');
                                    wav_slen    <= (others => '0');
                                    noi_slen    <= (others => '0');
                                end if;

                            end if;

                        when others =>
                            null;
                    end case;
                end if;
            end if;

        end if;

    end process;

    process (s1_addr, sq1_swper, sq1_swdir, sq1_swshift, sq1_duty, sq1_svol, sq1_envsgn, sq1_envper, sq1_lenchk,
        noi_playing, wav_playing, sq2_playing, sq1_playing, wav_enable, wav_volsh, wav_ram, wav_index, wav_access,
        sq2_duty, sq2_svol, sq2_envsgn, sq2_envper, sq2_lenchk, snd_enable, wav_lenchk, noi_svol, noi_envsgn, noi_envper,
        noi_freqsh, noi_short, noi_div, noi_lenchk, ch_vol, ch_map, is_gbc, sq1_wav, sq2_wav, wav_wav, noi_wav)
        variable wave_index_read : std_logic_vector(3 downto 0);
    begin
        case s1_addr is
                -- Square 1
            when "0010000" => -- NR10 FF10 -PPP NSSS Sweep period, negate, shift
                s1_readdata <= '1' & sq1_swper & sq1_swdir & sq1_swshift;
            when "0010001" => -- NR11 FF11 DDLL LLLL Duty, Length load (64-L)
                s1_readdata <= sq1_duty & "111111";
            when "0010010" => -- NR12 FF12 VVVV APPP Starting volume, Envelope add mode, period
                s1_readdata <= sq1_svol & sq1_envsgn & sq1_envper;
            when "0010011" => -- NR13 FF13 FFFF FFFF Frequency LSB
                s1_readdata <= X"FF";
            when "0010100" => -- NR14 FF14 TL-- -FFF Trigger, Length enable, Frequency MSB
                s1_readdata <= '1' & sq1_lenchk & "111111";

                -- Square 2
            when "0010110" => -- NR21 FF16 DDLL LLLL Duty, Length load (64-L)
                s1_readdata <= sq2_duty & "111111";
            when "0010111" => -- NR22 FF17 VVVV APPP Starting volume, Envelope add mode, period
                s1_readdata <= sq2_svol & sq2_envsgn & sq2_envper;
            when "0011000" => -- NR23 FF18 FFFF FFFF Frequency LSB
                s1_readdata <= X"FF";
            when "0011001" => -- NR24 FF19 TL-- -FFF Trigger, Length enable, Frequency MSB
                s1_readdata <= '1' & sq2_lenchk & "111111";

            when "0100110" => -- NR52 FF26 P--- NW21 Power control/status, Channel length statuses
                s1_readdata <= snd_enable & "111" & noi_playing & wav_playing & sq2_playing & sq1_playing;

                -- Wave
            when "0011010" => -- NR30 FF1A E--- ---- DAC power
                s1_readdata <= wav_enable & "1111111";
            when "0011011" => -- NR31 FF1B LLLL LLLL Length load (256-L)
                s1_readdata <= X"FF";
            when "0011100" => -- NR32 FF1C -VV- ---- Volume code (00=0%, 01=100%, 10=50%, 11=25%)
                s1_readdata <= '1' & wav_volsh & "11111";
            when "0011101" => -- NR33 FF1D FFFF FFFF Frequency LSB
                s1_readdata <= X"FF";
            when "0011110" => -- NR34 FF1E TL-- -FFF Trigger, Length enable, Frequency MSB
                s1_readdata <= '1' & wav_lenchk & "111111";

                -- Noise
            when "0100000" => -- NR41 FF20 --LL LLLL Length load (64-L)
                s1_readdata <= X"FF";
            when "0100001" => -- NR42 FF21 VVVV APPP Starting volume, Envelope add mode, period
                s1_readdata <= noi_svol & noi_envsgn & noi_envper;
            when "0100010" => -- NR43 FF22 SSSS WDDD Clock shift, Width mode of LFSR, Divisor code
                s1_readdata <= noi_freqsh & noi_short & noi_div;
            when "0100011" => -- NR44 FF23 TL-- ---- Trigger, Length enable
                s1_readdata <= '1' & noi_lenchk & "111111";

                -- Wave Table
            when "0110000" | "0110001" | "0110010" | "0110011" | "0110100" | "0110101" | "0110110" | "0110111" | "0111000" | "0111001" | "0111010" | "0111011" | "0111100" | "0111101" | "0111110" | "0111111" => -- FF30 to FF3F
                if wav_playing = '1' then
                    wave_index_read := std_logic_vector(wav_index(4 downto 1));
                else
                    wave_index_read := s1_addr(3 downto 0);
                end if;

                if is_gbc = '1' or wav_access > 0 or wav_playing = '0' then
                    case wave_index_read is
                        when "0000" => --      FF30 0000 1111 Samples 0 and 1
                            s1_readdata <= wav_ram(0) & wav_ram(1);
                        when "0001" => --      FF31 0000 1111 Samples 2 and 3
                            s1_readdata <= wav_ram(2) & wav_ram(3);
                        when "0010" => --      FF32 0000 1111 Samples 4 and 5
                            s1_readdata <= wav_ram(4) & wav_ram(5);
                        when "0011" => --      FF33 0000 1111 Samples 6 and 7
                            s1_readdata <= wav_ram(6) & wav_ram(7);
                        when "0100" => --      FF34 0000 1111 Samples 8 and 9
                            s1_readdata <= wav_ram(8) & wav_ram(9);
                        when "0101" => --      FF35 0000 1111 Samples 10 and 11
                            s1_readdata <= wav_ram(10) & wav_ram(11);
                        when "0110" => --      FF36 0000 1111 Samples 12 and 13
                            s1_readdata <= wav_ram(12) & wav_ram(13);
                        when "0111" => --      FF37 0000 1111 Samples 14 and 15
                            s1_readdata <= wav_ram(14) & wav_ram(15);
                        when "1000" => --      FF38 0000 1111 Samples 16 and 17
                            s1_readdata <= wav_ram(16) & wav_ram(17);
                        when "1001" => --      FF39 0000 1111 Samples 18 and 19
                            s1_readdata <= wav_ram(18) & wav_ram(19);
                        when "1010" => --      FF3A 0000 1111 Samples 20 and 21
                            s1_readdata <= wav_ram(20) & wav_ram(21);
                        when "1011" => --      FF3B 0000 1111 Samples 22 and 23
                            s1_readdata <= wav_ram(22) & wav_ram(23);
                        when "1100" => --      FF3C 0000 1111 Samples 24 and 25
                            s1_readdata <= wav_ram(24) & wav_ram(25);
                        when "1101" => --      FF3D 0000 1111 Samples 26 and 27
                            s1_readdata <= wav_ram(26) & wav_ram(27);
                        when "1110" => --      FF3E 0000 1111 Samples 28 and 29
                            s1_readdata <= wav_ram(28) & wav_ram(29);
                        when "1111" => --      FF3F 0000 1111 Samples 30 and 31
                            s1_readdata <= wav_ram(30) & wav_ram(31);
                    end case;
                else
                    s1_readdata <= X"FF";
                end if;

                -- Control/Status
            when "0100100" =>
                s1_readdata <= ch_vol; -- NR50 FF24
            when "0100101" =>
                s1_readdata <= ch_map; -- NR51 FF25
            
                -- Undocumented Registers
            when "1110110"  =>         -- PCM12 FF76
                  if is_gbc = '1' then
                    s1_readdata <= (others => '0');
                    if  sq2_playing = '1' then
                        s1_readdata(7 downto 4) <= sq2_wav(5 downto 2);
                    end if;
                    if  sq1_playing = '1' then
                        s1_readdata(3 downto 0) <= sq1_wav(5 downto 2);
                    end if;
                  else
                    s1_readdata <= X"FF";
                  end if;
            when "1110111"  =>         -- PCM34 FF77
                  if is_gbc = '1' then
                    s1_readdata <= (others => '0');
                    if  noi_playing = '1' then
                        s1_readdata(7 downto 4) <= noi_wav(5 downto 2);
                    end if;
                    if  wav_playing = '1' then
                        s1_readdata(3 downto 0) <= wav_wav(5 downto 2);  
                    end if;
                  else
                    s1_readdata <= X"FF";
                  end if;

            
            when others =>
                s1_readdata <= X"FF";
        end case;

    end process;

    sound : process (clk, snd_enable, en_snd, en_len, en_env, en_sweep, wav_access, sq1_out, sq1_vol, sq2_out, sq2_vol, noi_vol, sq1_suppressed, sq2_suppressed, reset)
        constant duty_0          : std_logic_vector(0 to 7) := "00000001";
        constant duty_1          : std_logic_vector(0 to 7) := "10000001";
        constant duty_2          : std_logic_vector(0 to 7) := "10000111";
        constant duty_3          : std_logic_vector(0 to 7) := "01111110";
        variable sq1_fcnt        : unsigned(10 downto 0);
        variable sq1_phase       : integer range 0 to 7;
        variable sq1_len         : std_logic_vector(6 downto 0);
        variable sq1_envcnt      : std_logic_vector(3 downto 0); -- Sq1 envelope timer count
        variable sq1_swcnt       : std_logic_vector(3 downto 0); -- Sq1 sweep timer count
        variable sq1_swoffs      : unsigned(11 downto 0);
        variable sq1_swfr        : unsigned(11 downto 0);

        variable sq1_sweep_en    : boolean;
        variable sweep_calculate : boolean;
        variable sweep_update    : boolean;
        variable sweep_negate    : boolean;
        variable sq1_envoff      : boolean; -- check if envelope is on (zombiemode)
        variable sq1_sduty       : std_logic_vector(1 downto 0); -- Sq1 duty cycle shadow register

        variable sq2_fcnt        : unsigned(10 downto 0);
        variable sq2_phase       : integer range 0 to 7;
        variable sq2_len         : std_logic_vector(6 downto 0);
        variable sq2_envcnt      : std_logic_vector(3 downto 0); -- Sq2 envelope timer count
        variable sq2_envoff      : boolean;                      -- check if envelope is on (zombiemode)
        variable sq2_sduty       : std_logic_vector(1 downto 0); -- Sq2 duty cycle shadow register

        variable wav_fcnt        : unsigned(10 downto 0);
        variable wav_len         : std_logic_vector(8 downto 0);
        variable wav_shift_r     : boolean;
        variable wav_shift       : boolean;

        variable noi_divisor     : unsigned(10 downto 0); -- Noise frequency divisor
        variable noi_period      : unsigned(10 downto 0); -- Noise period    (calculated)
        variable noi_fcnt        : unsigned(10 downto 0);
        variable noi_lfsr        : unsigned(14 downto 0); -- 15 bits
        variable noi_len         : std_logic_vector(6 downto 0);
        variable noi_envcnt      : std_logic_vector(3 downto 0); -- Noise envelope timer count
        variable noi_envoff      : boolean;                      -- check if envelope is on (zombiemode)
        variable noi_out         : std_logic;
        variable noi_xor         : std_logic;

        variable tmp_volume      : unsigned(7 downto 0); -- used in zombie mode

        variable acc_fcnt        : unsigned(11 downto 0);
    begin

        if sq1_out = '1' and sq1_suppressed = '0' then
            sq1_wav <= sq1_vol & "00";
        else
            sq1_wav <= "000000";
        end if;
        
        if sq2_out = '1' and sq2_suppressed = '0' then
            sq2_wav <= sq2_vol & "00";
        else
            sq2_wav <= "000000";
        end if;

        if noi_out = '1' then
            noi_wav <= noi_vol & "00";
        else
            noi_wav <= "000000";
        end if;

        -- Sound processing
        if reset = '1' then
            sq1_playing <= '0';
            sq1_fr2     <= (others => '0');
            sq1_fcnt  := (others   => '0');
            sq1_phase := 0;
            sq1_vol <= "0000";
            sq1_envcnt := "0000";
            sq1_swcnt  := "0000";
            sq1_swoffs := (others => '0');
            sq1_swfr   := (others => '0');
            sq1_out    <= '0';
            sq2_playing <= '0';
            sq2_fcnt  := (others => '0');
            sq2_phase := 0;
            sq2_vol <= "0000";
            sq2_envcnt := "0000";
            sq2_out    <= '0';
            
            sq1_suppressed <= '1';
            sq2_suppressed <= '1';

            sq1_sduty    := (others => '0');
            sq2_sduty    := (others => '0');

            wav_playing <= '0';
            wav_fcnt    := (others => '0');
            wav_shift_r := false;
            wav_index   <= (others => '0');
            wav_access  <= (others => '0');

            noi_playing <= '0';
            noi_fcnt := (others => '0');
            noi_lfsr := (others => '1');
            noi_vol <= "0000";
            noi_envcnt      := "0000";
            noi_out         := '0';
            sweep_calculate := false;
            sweep_update    := false;
            sq1_sweep_en    := false;
            sweep_negate    := false;

            -- zombie mode check if env is still updating
            sq2_envoff      := true;
            sq1_envoff      := true;
            noi_envoff      := true;

            sq1_len  := (others => '0');
            sq2_len  := (others => '0');
            wav_len  := (others => '0');
            noi_len  := (others => '0');

            sq1_trigger_r   <= '0'; 
            sq2_trigger_r   <= '0'; 
            wav_trigger_r   <= '0'; 
            -- noi_trigger_r   <= '0'; 

        elsif rising_edge(clk) then
            if ce = '1' then

                if sq1_lenchange = '1' then
                    sq1_len := sq1_slen;
                end if;
                
                if sq2_lenchange = '1' then
                    sq2_len := sq2_slen;
                end if;

                if noi_lenchange = '1' then
                    noi_len := noi_slen;
                end if;

                if wav_lenchange = '1' then
                    wav_len := wav_slen;
                end if;
                
                -- used to detect trigger negedge
                sq1_trigger_r <= sq1_trigger;
                sq2_trigger_r <= sq2_trigger;
                wav_trigger_r <= wav_trigger;
                -- noi_trigger_r <= noi_trigger;

                if snd_enable = '1' then

                    if wav_access > 0 then
                        wav_access <= wav_access - 1;
                    end if;

                    ----------------------- Square channel 1	-----------------------------	

                    if en_snd4 then
                        -- Sq1 frequency timer Frequency = 131072/(2048-x) Hz
                        if sq1_playing = '1' then
                            acc_fcnt := ('0' & sq1_fcnt) + to_unsigned(1, acc_fcnt'length);
                            if acc_fcnt(acc_fcnt'high) = '1' then
                                sq1_suppressed <= '0';
                                sq1_phase := sq1_phase + 1;
                                sq1_fcnt := unsigned(sq1_freq);
                                sq1_sduty := sq1_duty; -- only change duty after the sample is finished
                            else
                                sq1_fcnt := acc_fcnt(sq1_fcnt'range);
                            end if;
                        end if;

                        case sq1_sduty is
                            when "00"   => sq1_out <= duty_0(sq1_phase);
                            when "01"   => sq1_out <= duty_1(sq1_phase);
                            when "10"   => sq1_out <= duty_2(sq1_phase);
                            when "11"   => sq1_out <= duty_3(sq1_phase);
                            when others => null;
                        end case;
                    end if;

                    -- Length counter
                    if en_len or sq1_lenquirk = '1' then
                        if sq1_len > 0 and sq1_lenchk = '1' then
                            sq1_len := std_logic_vector(unsigned(sq1_len) - 1);
                        end if;
                    end if;

                    -- Sweep processing

                    -- sweep counter
                    if en_sweep then
                        sq1_swcnt := std_logic_vector(unsigned(sq1_swcnt) - 1);
                        if sq1_swcnt = 0 then

                            -- reload counter with period
                            if sq1_swper = "000" then
                                sq1_swcnt := "1000"; -- set to 8
                            else
                                sq1_swcnt := '0' & sq1_swper; -- set to period
                            end if;

                            -- check if update needed
                            if sq1_sweep_en and sq1_swper /= "000" then
                                sweep_calculate := true;
                                sweep_update    := true;
                            end if;
                        end if;
                    end if;

                    sq1_freqchange <= '0';

                    if sq1_sweep_en then

                        -- Calculate next sweep frequency
                        if sweep_calculate then
                            case sq1_swshift is
                                when "000"  => sq1_swoffs  := unsigned('0' & sq1_fr2);
                                when "001"  => sq1_swoffs  := "00" & unsigned(sq1_fr2(10 downto 1));
                                when "010"  => sq1_swoffs  := "000" & unsigned(sq1_fr2(10 downto 2));
                                when "011"  => sq1_swoffs  := "0000" & unsigned(sq1_fr2(10 downto 3));
                                when "100"  => sq1_swoffs  := "00000" & unsigned(sq1_fr2(10 downto 4));
                                when "101"  => sq1_swoffs  := "000000" & unsigned(sq1_fr2(10 downto 5));
                                when "110"  => sq1_swoffs  := "0000000" & unsigned(sq1_fr2(10 downto 6));
                                when "111"  => sq1_swoffs  := "00000000" & unsigned(sq1_fr2(10 downto 7));
                                when others => sq1_swoffs := unsigned('0' & sq1_fr2);
                            end case;
                            if sq1_swdir = '1' then
                                sq1_swfr     := ('0' & unsigned(sq1_fr2)) - sq1_swoffs;
                                sweep_negate := true;
                            else
                                sq1_swfr     := ('0' & unsigned(sq1_fr2)) + sq1_swoffs;
                                sweep_negate := false;
                            end if;
                            sweep_calculate := false;
                        end if;

                        -- update registers, and calculate next frequency
                        if sweep_update then
                            sweep_update := false;
                            if (sq1_swper /= "000" and sq1_swshift /= "000") then
                                sq1_fr2        <= std_logic_vector(sq1_swfr(10 downto 0));
                                sq1_freqchange <= '1';
                                sweep_calculate := true; -- when updating calculate 2nd time
                            end if;
                        end if;

                    end if;

                    if sq1_playing = '1' then

                        -- Envelope counter
                        if en_env and sq1_envper /= "000" then

                            sq1_envcnt := std_logic_vector(unsigned(sq1_envcnt) - 1); -- decrement counter 

                            if sq1_envcnt = 0 then
                                if sq1_envsgn = '1' then
                                    if sq1_vol /= "1111" then -- sq1_vol < 15
                                        sq1_vol <= std_logic_vector(unsigned(sq1_vol) + 1);
                                    else
                                        sq1_envoff := true; -- envelope done
                                    end if;
                                else
                                    if sq1_vol /= "0000" then -- sq1_vol > 
                                        sq1_vol <= std_logic_vector(unsigned(sq1_vol) - 1);
                                    else
                                        sq1_envoff := true; -- envelope done
                                    end if;
                                end if;

                                -- reload counter with period
                                if sq1_envper = "000" then
                                    sq1_envcnt := "1000"; -- set to 8
                                else
                                    sq1_envcnt := '0' & sq1_envper; -- set to period
                                end if;

                            end if;
                        end if;

                        -- Check for end of playing conditions
                        if (sq1_lenchk = '1' and sq1_len = 0)                         -- Play length timer overrun
                            or (sq1_sweep_en and sq1_swfr(11) = '1')                      -- Sweep frequency overrun
                            or (sq1_sweep_en and sq1_swdir_change = '1' and sweep_negate) -- sweep direction change after trigger
                            then
                            sq1_playing <= '0';
                            sq1_envcnt   := (others => '0');
                            sq1_swcnt    := (others => '0');
                            sq1_swfr     := (others => '0');
                            sq1_sweep_en := false;
                        end if;
                    end if;

                    if ((sq1_trigger_r = '1' and sq1_trigger = '0') or sq1_nr2change = '1') and sq1_playing = '1' then  -- falling edge of trigger

                        -- "zombie" mode
                        tmp_volume := "0000" & unsigned(sq1_vol);
                        if sq1_envper_old = "000" and not sq1_envoff then
                            tmp_volume := "0000" & unsigned(sq1_vol) + 1;
                        else
                            if sq1_envsgn_old = '0' then
                                tmp_volume := "0000" & unsigned(sq1_vol) + 2;
                            end if;
                        end if;

                        if (sq1_envsgn xor sq1_envsgn_old) = '1' then
                            tmp_volume := X"10" - tmp_volume;
                        end if;

                        sq1_vol <= std_logic_vector(tmp_volume(3 downto 0));

                        -- check if dac is enabled
                        if sq1_svol = "00000" and sq1_envsgn = '0' then -- dac disabled
                            sq1_playing <= '0';
                        end if;
                    end if;

                    -- if sound was triggered and already playing reset/stop frequency counter
                    if sq1_trigger = '1' and sq1_playing = '1' then
                        sq1_fcnt  := (others   => '0');
                    end if;

                    -- Check sample trigger and start playing
                    if sq1_trigger_r = '1' and sq1_trigger = '0' then -- falling edge of trigger
                        -- from sameboy:
                        -- Current sample index remains unchanged when restarting channels 1 or 2. It is only reset by turning the APU off.

                        if sq1_playing = '0' then
                          sq1_suppressed <= '1';  -- suppress 1st sample if channel wasn't active
                        end if;

                        sq1_vol <= sq1_svol;
                        sq1_fr2 <= sq1_freq;                                        -- shadow frequency register for sweep unit
                        sq1_sweep_en := sq1_swper /= "000" or sq1_swshift /= "000"; -- sweep unit enabled ?
                        ---- sweep quirks ---
                        if sq1_swshift /= "000" then
                            sweep_calculate := true;
                        end if;
                        if sq1_swper = "000" then
                            sq1_swcnt := "1000"; -- set to 8
                        else
                            sq1_swcnt := '0' & sq1_swper; -- set to period
                        end if;
                        sweep_negate := false;
                        ---- sweep quirks ---

                        sq1_fcnt     := unsigned(sq1_freq);
                        if not (sq1_svol = "00000" and sq1_envsgn = '0') then -- dac enabled
                            sq1_playing <= '1';
                        end if;

                        -- reload envelope counter with period
                        if sq1_envper = "000" then
                            sq1_envcnt := "1000"; -- set to 8
                        else
                            sq1_envcnt := '0' & sq1_envper; -- set to period
                        end if;
                        sq1_envoff := false;
                                               
                        if sq1_len = 0 then -- trigger quirks
                            if sq1_lenchk = '1' and en_len_r then
                                sq1_len := "0111111"; -- 63
                            else
                                sq1_len := "1000000"; -- 64
                            end if;
                        end if;

                    end if;

                    ----------------------- Square channel 2	-----------------------------	

                    if en_snd4 then
                        -- Sq2 frequency timer  Frequency = 131072/(2048-x) Hz
                        if sq2_playing = '1' then
                            acc_fcnt := ('0' & sq2_fcnt) + to_unsigned(1, acc_fcnt'length);
                            if acc_fcnt(acc_fcnt'high) = '1' then
                                sq2_suppressed <= '0';
                                sq2_phase := sq2_phase + 1;
                                sq2_fcnt := unsigned(sq2_freq);
                                sq2_sduty := sq2_duty;  -- only change duty after the sample is finished
                            else
                                sq2_fcnt := acc_fcnt(sq2_fcnt'range);
                            end if;
                        end if;

                        case sq2_sduty is
                            when "00"   => sq2_out <= duty_0(sq2_phase);
                            when "01"   => sq2_out <= duty_1(sq2_phase);
                            when "10"   => sq2_out <= duty_2(sq2_phase);
                            when "11"   => sq2_out <= duty_3(sq2_phase);
                            when others => null;
                        end case;
                    end if;

                    -- Length counter
                    if en_len or sq2_lenquirk = '1' then
                        if sq2_len > 0 and sq2_lenchk = '1' then
                            sq2_len := std_logic_vector(unsigned(sq2_len) - 1);
                        end if;
                    end if;

                    if sq2_playing = '1' then

                        -- Envelope counter
                        if en_env and sq2_envper /= "000" then

                            sq2_envcnt := std_logic_vector(unsigned(sq2_envcnt) - 1); -- decrement counter 

                            if sq2_envcnt = 0 then
                                if sq2_envsgn = '1' then
                                    if sq2_vol /= "1111" then -- sq2_vol < 15
                                        sq2_vol <= std_logic_vector(unsigned(sq2_vol) + 1);
                                    else
                                        sq2_envoff := true;
                                    end if;
                                else
                                    if sq2_vol /= "0000" then -- sq2_vol > 0
                                        sq2_vol <= std_logic_vector(unsigned(sq2_vol) - 1);
                                    else
                                        sq2_envoff := true;
                                    end if;
                                end if;

                                -- reload counter with period
                                if sq2_envper = "000" then
                                    sq2_envcnt := "1000"; -- set to 8
                                else
                                    sq2_envcnt := '0' & sq2_envper; -- set to period
                                end if;
                            end if;
                        end if;

                        -- Check for end of playing conditions
                        -- if sq2_vol = X"0" 										 -- Volume == 0              	
                        if sq2_lenchk = '1' and sq2_len = 0 -- Play length timer overrun
                            then
                            sq2_playing <= '0';
                            sq2_envcnt := "0000";
                        end if;
                    end if;

                    if ((sq2_trigger_r = '1' and sq2_trigger = '0') or sq2_nr2change = '1') and sq2_playing = '1' then  -- falling edge of trigger

                        -- "zombie" mode
                        tmp_volume := "0000" & unsigned(sq2_vol);
                        if sq2_envper_old = "000" and not sq2_envoff then
                            tmp_volume := "0000" & unsigned(sq2_vol) + 1;
                        else
                            if sq2_envsgn_old = '0' then
                                tmp_volume := "0000" & unsigned(sq2_vol) + 2;
                            end if;
                        end if;

                        if (sq2_envsgn xor sq2_envsgn_old) = '1' then
                            tmp_volume := X"10" - tmp_volume;
                        end if;

                        sq2_vol <= std_logic_vector(tmp_volume(3 downto 0));

                        -- check if dac is enabled
                        if sq2_svol = "00000" and sq2_envsgn = '0' then -- dac disabled
                            sq2_playing <= '0';
                        end if;
                    end if;

                    -- if sound was triggered and already playing reset/stop frequency counter
                    if sq2_trigger = '1' and sq2_playing = '1' then
                        sq2_fcnt  := (others   => '0');
                    end if;

                    -- Check sample trigger and start playing
                    if sq2_trigger_r = '1' and sq2_trigger = '0' then -- falling edge of trigger
                        
                        -- from sameboy:
                        -- Current sample index remains unchanged when restarting channels 1 or 2. It is only reset by turning the APU off.

                        if sq2_playing = '0' then
                            sq2_suppressed <= '1';  -- suppress 1st sample if channel wasn't active
                        end if;

                        sq2_vol <= sq2_svol;

                        -- sq2_fr2 <= sq2_freq;
                        sq2_fcnt := unsigned(sq2_freq);

                        if not (sq2_svol = "00000" and sq2_envsgn = '0') then -- dac enabled
                            sq2_playing <= '1';
                        end if;

                        -- reload envelope counter with period
                        if sq2_envper = "000" then
                            sq2_envcnt := "1000"; -- set to 8
                        else
                            sq2_envcnt := '0' & sq2_envper; -- set to period
                        end if;
                        sq2_envoff := false;

                        if sq2_len = 0 then -- trigger quirks 
                            if sq2_lenchk = '1' and en_len_r then
                                sq2_len := "0111111"; -- 63
                            else
                                sq2_len := "1000000"; -- 64
                            end if;
                        end if;
                    end if;
                    ----------------------- Noise channel -----------------------------	

                    -- Noi frequency timer Frequency = 524288 Hz / r / 2^(s+1) ;For r=0 assume r=0.5 instead  
                    if en_snd4 then
                        if noi_playing = '1' then
                            acc_fcnt := ('0' & noi_fcnt) - to_unsigned(1, acc_fcnt'length);
                            if acc_fcnt = 0 then
                                -- Noise LFSR
                                noi_xor  := noi_lfsr(0) xor noi_lfsr(1);
                                noi_lfsr := noi_xor & noi_lfsr(14 downto 1);

                                if noi_short = '1' then
                                    noi_lfsr(6) := noi_xor;
                                end if;

                                noi_out  := not noi_lfsr(0);
                                noi_fcnt := noi_period;
                            else
                                noi_fcnt := acc_fcnt(noi_fcnt'range);
                            end if;
                        end if;
                    end if;

                    -- Length counter
                    if en_len or noi_lenquirk = '1' then
                        if noi_len > 0 and noi_lenchk = '1' then
                            noi_len := std_logic_vector(unsigned(noi_len) - 1);
                        end if;
                    end if;

                    if noi_playing = '1' then

                        -- Envelope counter
                        if en_env and noi_envper /= "000" then

                            noi_envcnt := std_logic_vector(unsigned(noi_envcnt) - 1); -- decrement counter 

                            if noi_envcnt = 0 then
                                if noi_envsgn = '1' then
                                    if noi_vol /= "1111" then -- noi_vol < 15
                                        noi_vol <= std_logic_vector(unsigned(noi_vol) + 1);
                                    else
                                        noi_envoff := true;
                                    end if;
                                else
                                    if noi_vol /= "0000" then -- noi_vol > 0
                                        noi_vol <= std_logic_vector(unsigned(noi_vol) - 1);
                                    else
                                        noi_envoff := true;
                                    end if;
                                end if;

                                -- reload counter with period
                                if noi_envper = "000" then
                                    noi_envcnt := "1000"; -- set to 8
                                else
                                    noi_envcnt := '0' & noi_envper; -- set to period
                                end if;
                            end if;
                        end if;

                        -- Check for end of playing conditions            	
                        if noi_lenchk = '1' and noi_len = 0 -- Play length timer overrun
                            then
                            noi_playing <= '0';
                            noi_envcnt := "0000";
                            --sq2_wav <= "000000";
                        end if;
                    end if;

                    if (noi_trigger = '1' or noi_nr2change = '1') and noi_playing = '1' then

                        -- "zombie" mode
                        tmp_volume := "0000" & unsigned(noi_vol);
                        if noi_envper_old = "000" and not noi_envoff then
                            tmp_volume := "0000" & unsigned(noi_vol) + 1;
                        else
                            if noi_envsgn_old = '0' then
                                tmp_volume := "0000" & unsigned(noi_vol) + 2;
                            end if;
                        end if;

                        if (noi_envsgn xor noi_envsgn_old) = '1' then
                            tmp_volume := X"10" - tmp_volume;
                        end if;

                        noi_vol <= std_logic_vector(tmp_volume(3 downto 0));

                        -- check if dac is enabled
                        if noi_svol = "00000" and noi_envsgn = '0' then -- dac disabled
                            noi_playing <= '0';
                        end if;
                    end if;

                    if noi_trigger = '1'or noi_freqchange = '1' then

                        -- Calculate noise frequency
                        case noi_div is
                            when "000"  => noi_period  := to_unsigned(2, noi_period'length);
                            when "001"  => noi_period  := to_unsigned(4, noi_period'length);
                            when "010"  => noi_period  := to_unsigned(8, noi_period'length);
                            when "011"  => noi_period  := to_unsigned(12, noi_period'length);
                            when "100"  => noi_period  := to_unsigned(16, noi_period'length);
                            when "101"  => noi_period  := to_unsigned(20, noi_period'length);
                            when "110"  => noi_period  := to_unsigned(24, noi_period'length);
                            when others => noi_period := to_unsigned(28, noi_divisor'length);
                        end case;

                        noi_period := noi_period sll to_integer(unsigned(noi_freqsh));
                        noi_fcnt   := noi_period;

                    end if;

                    -- Check sample trigger and start playing
                    if noi_trigger = '1' then
                        noi_vol <= noi_svol;

                        noi_lfsr := (others => '1');

                        -- reload envelope counter with period
                        if noi_envper = "000" then
                            noi_envcnt := "1000"; -- set to 8
                        else
                            noi_envcnt := '0' & noi_envper; -- set to period
                        end if;
                        noi_envoff := false;

                        if not (noi_svol = "00000" and noi_envsgn = '0') then -- dac enabled
                            noi_playing <= '1';
                        end if;

                        if noi_len = 0 then -- trigger quirks 
                            if noi_lenchk = '1' and en_len_r then
                                noi_len := "0111111"; -- 63
                            else
                                noi_len := "1000000"; -- 64
                            end if;
                        end if;
                    end if;

                    ----------------------------- Wave channel -----------------------------------	

                    if en_snd2 then
                        -- Wave frequency timer Frequency = 4194304/(64*(2048-x)) Hz = 65536/(2048-x) Hz
                        wav_shift := false;
                        if wav_playing = '1' then
                            acc_fcnt := ('0' & wav_fcnt) + to_unsigned(1, acc_fcnt'length);
                            if acc_fcnt(acc_fcnt'high) = '1' then
                                wav_shift := true;
                                wav_fcnt  := unsigned(wav_freq);
                            else
                                wav_fcnt := acc_fcnt(wav_fcnt'range);
                            end if;
                        end if;
                    end if;

                    if wav_trigger_r = '1' and wav_trigger = '0' then
                        wav_index <= (others => '0');
                        wav_shift_r := false;
                    else
                        -- Rotate wave table on rising edge of wav_shift
                        if wav_shift and not wav_shift_r then
                            wav_index  <= wav_index + 1;
                            wav_access <= "10";
                        end if;
                        wav_shift_r := wav_shift;
                    end if;

                    -- Length counter
                    if en_len or wav_lenquirk = '1' then
                        if wav_len > 0 and wav_lenchk = '1' then
                            wav_len := std_logic_vector(unsigned(wav_len) - 1);
                        end if;
                    end if;

                    if wav_playing = '1' then
                        -- Check for end of playing conditions
                        if (wav_lenchk = '1' and wav_len = 0) or wav_enable = '0' then
                            wav_playing <= '0';
                            wav_wav     <= "000000";
                        end if;
                    end if;

                    -- Check sample trigger and start playing
                    if wav_trigger_r = '1' and wav_trigger = '0' then
                        wav_fcnt := unsigned(wav_freq);
                        wav_playing <= '1';
                        if wav_len = 0 then -- trigger quirks 
                            if wav_lenchk = '1' and en_len_r then
                                wav_len := "011111111"; -- 255
                            else
                                wav_len := "100000000"; -- 256
                            end if;
                        end if;

                    end if;

                    if wav_enable = '1' and wav_volsh /= "00" then
                        case wav_volsh is
                            when "01" => wav_wav   <= wav_ram(to_integer(wav_index)) & "00";
                            when "10" => wav_wav   <= '0' & (wav_ram(to_integer(wav_index))(3 downto 1)) & "00";
                            when "11" => wav_wav   <= "00" & (wav_ram(to_integer(wav_index))(3 downto 2)) & "00";
                            when others => wav_wav <= (others => 'X');
                        end case;
                    else
                        wav_wav <= "000000";
                    end if;
                else  -- snd_enable = '0'
                    sq1_playing <= '0';
                    sq1_fr2     <= (others => '0');
                    sq1_fcnt  := (others   => '0');
                    sq1_phase := 0;
                    sq1_vol <= "0000";
                    sq1_envcnt := "0000";
                    sq1_swcnt  := "0000";
                    sq1_swoffs := (others => '0');
                    sq1_swfr   := (others => '0');
                    sq1_out   <= '0';
                    sq2_playing <= '0';
                    sq2_fcnt  := (others => '0');
                    sq2_phase := 0;
                    sq2_vol <= "0000";
                    sq2_envcnt := "0000";
                    sq2_out    <= '0';

                    sq1_suppressed <= '1';
                    sq2_suppressed <= '1';

                    sq1_sduty    := (others => '0');
                    sq2_sduty    := (others => '0');
        
                    wav_playing <= '0';
                    wav_fcnt    := (others => '0');
                    wav_shift_r := false;
                    wav_index   <= (others => '0');
                    wav_access  <= (others => '0');
        
                    noi_playing <= '0';
                    noi_fcnt := (others => '0');
                    noi_lfsr := (others => '1');
                    noi_vol <= "0000";
                    noi_envcnt      := "0000";
                    noi_out         := '0';
                    sweep_calculate := false;
                    sweep_update    := false;
                    sq1_sweep_en    := false;
                    sweep_negate    := false;
        
                    -- zombie mode check if env is still updating
                    sq2_envoff      := true;
                    sq1_envoff      := true;
                    noi_envoff      := true;
        
                    if is_gbc = '1' then
                        sq1_len  := (others => '0');
                        sq2_len  := (others => '0');
                        wav_len  := (others => '0');
                        noi_len  := (others => '0');
                    end if;

                    sq1_trigger_r   <= '0'; 
                    sq2_trigger_r   <= '0'; 
                    wav_trigger_r   <= '0'; 
                    --noi_trigger_r   <= '0'; 
                
                end if; 

            end if; -- ce
        end if; -- rising edge
    end process sound;

    -- Test
    process (clk, en_512, reset)
        variable l : std_logic_vector(15 downto 0);
    begin
        if reset = '1' then
            l := x"4000";

        elsif rising_edge(clk) then
            if ce = '1' then
                if en_512 then
                    l := not l;
                end if;
                --			snd_left <= l;
            end if;
        end if;
    end process;

    -- Mixer
    mixer : process (sq1_wav, sq2_wav, noi_wav, wav_wav, ch_map, ch_vol)
        variable snd_left_in  : unsigned(8 downto 0);
        variable snd_right_in : unsigned(8 downto 0);
    begin
        snd_left_in  := (others => '0');
        snd_right_in := (others => '0');

        if ch_map(0) = '1' then
            snd_right_in := snd_right_in + unsigned(sq1_wav);
        end if;
        if ch_map(1) = '1' then
            snd_right_in := snd_right_in + unsigned(sq2_wav);
        end if;
        if ch_map(2) = '1' then
            snd_right_in := snd_right_in + unsigned(wav_wav);
        end if;
        if ch_map(3) = '1' then
            snd_right_in := snd_right_in + unsigned(noi_wav);
        end if;

        if ch_map(4) = '1' then
            snd_left_in := snd_left_in +  unsigned(sq1_wav);
        end if;
        if ch_map(5) = '1' then
            snd_left_in := snd_left_in +  unsigned(sq2_wav);
        end if;
        if ch_map(6) = '1' then
            snd_left_in := snd_left_in +  unsigned(wav_wav);
        end if;
        if ch_map(7) = '1' then
            snd_left_in := snd_left_in + unsigned(noi_wav);
        end if;

        snd_right <= snd_right_in * unsigned(ch_vol(2 downto 0));
        snd_left  <= snd_left_in  * unsigned(ch_vol(6 downto 4));
    end process;

end SYN;