🎶 第20课：背景音乐播放

// 音符序列器 - 从ROM读取音符序列并播放
module music_sequencer #(
    parameter CLK_FREQ = 50000000,
    parameter BPM      = 120          // 每分钟节拍数
)(
    input  wire        clk,
    input  wire        rst,
    input  wire        play,          // 1=播放
    output wire [7:0]  audio_out,     // 音频输出
    output wire [6:0]  note_index     // 当前音符序号(调试用)
);
    // 节拍定时: BPM=120 → 每拍0.5s → 25000000个时钟周期
    localparam BEAT_TICKS = CLK_FREQ * 60 / BPM / 2;
    reg [24:0] beat_counter;
    reg        beat_trigger;

    always @(posedge clk) begin
        if (rst) begin
            beat_counter <= 0;
            beat_trigger <= 0;
        end else begin
            beat_trigger <= 0;
            if (play && beat_counter >= BEAT_TICKS[24:0]) begin
                beat_counter <= 0;
                beat_trigger <= 1;
            end else begin
                beat_counter <= beat_counter + 1;
            end
        end
    end

    // 音符ROM: [7:4]=MIDI音符号(0~15映射C4~), [3:0]=时值(拍)
    // 小星星旋律: C C G G A A G - F F E E D D C
    reg [7:0] note_rom [0:31];
    initial begin
        // 小星星 (Twinkle Twinkle)
        note_rom[ 0] = 8'h18; // C4, 1拍
        note_rom[ 1] = 8'h18; // C4, 1拍
        note_rom[ 2] = 8'h28; // G4, 1拍
        note_rom[ 3] = 8'h28; // G4, 1拍
        note_rom[ 4] = 8'h30; // A4, 1拍
        note_rom[ 5] = 8'h30; // A4, 1拍
        note_rom[ 6] = 8'h40; // G4, 2拍
        note_rom[ 7] = 8'h24; // F4, 1拍
        note_rom[ 8] = 8'h24; // F4, 1拍
        note_rom[ 9] = 8'h14; // E4, 1拍
        note_rom[10] = 8'h14; // E4, 1拍
        note_rom[11] = 8'h0C; // D4, 1拍
        note_rom[12] = 8'h0C; // D4, 1拍
        note_rom[13] = 8'h08; // C4, 2拍
        // 重复
        note_rom[14] = 8'h18; note_rom[15] = 8'h18;
        note_rom[16] = 8'h28; note_rom[17] = 8'h28;
        note_rom[18] = 8'h30; note_rom[19] = 8'h30;
        note_rom[20] = 8'h40; note_rom[21] = 8'h24;
        note_rom[22] = 8'h24; note_rom[23] = 8'h14;
        note_rom[24] = 8'h14; note_rom[25] = 8'h0C;
        note_rom[26] = 8'h0C; note_rom[27] = 8'h08;
        note_rom[28] = 8'h00; // 结束标记
        note_rom[29] = 8'h00; note_rom[30] = 8'h00; note_rom[31] = 8'h00;
    end

    // 序列播放
    reg [4:0] seq_pos;
    reg [3:0] note_duration;
    reg [3:0] duration_cnt;
    reg [7:0] current_note;

    always @(posedge clk) begin
        if (rst) begin
            seq_pos <= 0; duration_cnt <= 0;
            current_note <= 0;
        end else if (beat_trigger) begin
            if (duration_cnt == 0) begin
                current_note <= note_rom[seq_pos];
                note_duration <= note_rom[seq_pos][3:0];
                duration_cnt <= note_rom[seq_pos][3:0];
                if (seq_pos < 27) seq_pos <= seq_pos + 1;
                else seq_pos <= 0;
            end else begin
                duration_cnt <= duration_cnt - 1;
            end
        end
    end

    // MIDI音符→频率查找表
    reg [19:0] freq_word;
    always @(*) begin
        case(current_note[7:4])
            4'h0: freq_word = 20'd0;     // 休止
            4'h1: freq_word = 20'd343;   // C4 ≈ 262Hz
            4'h2: freq_word = 20'd0;     // (reserved)
            4'h3: freq_word = 20'd431;   // D4 ≈ 294Hz
            4'h4: freq_word = 20'h0;     // reserved
            4'h5: freq_word = 20'd513;   // E4 ≈ 330Hz
            4'h6: freq_word = 20'h0;
            4'h7: freq_word = 20'd575;   // F4 ≈ 349Hz
            4'h8: freq_word = 20'h0;
            4'h9: freq_word = 20'd645;   // G4 ≈ 392Hz
            4'hA: freq_word = 20'h0;
            4'hB: freq_word = 20'd724;   // A4 ≈ 440Hz
            4'hC: freq_word = 20'h0;
            4'hD: freq_word = 20'd813;   // B4 ≈ 494Hz
            default: freq_word = 20'd0;
        endcase
    end

    // NCO生成音频
    reg [15:0] phase;
    always @(posedge clk) begin
        if (rst) phase <= 0;
        else     phase <= phase + freq_word[15:0];
    end

    // 简单正弦近似
    reg [7:0] sine_val;
    always @(*) begin
        case(phase[15:14])
            2'b00: sine_val = phase[13:6];
            2'b01: sine_val = 255 - phase[13:6];
            2'b10: sine_val = 255 - phase[13:6];
            2'b11: sine_val = phase[13:6];
        endcase
    end

    assign audio_out = (play && current_note[7:4] != 0) ? sine_val : 8'd128;
    assign note_index = seq_pos;

endmodule

module music_sequencer_tb;
    logic clk=0, rst=1, play=0;
    logic [7:0] audio_out;
    logic [6:0] note_index;

    music_sequencer #(.CLK_FREQ(1000), .BPM(120)) uut(.*);
    always #5 clk = ~clk; // 快速仿真

    initial begin
        rst=1; #50; rst=0;
        play=1;
        $display("--- 音乐序列器测试 ---");
        // 快速仿真: 等几个beat
        repeat(100) begin
            @(posedge clk);
        end
        $display("  note_index=%0d audio=%0d", note_index, audio_out);
        play=0; #100;
        $display("音乐序列器测试完成 ✓");
        $finish;
    end
endmodule