module freq_select(
    input   wire sys_clk    ,
    input   wire sys_rst_n  ,

    output  wire pwm
);
parameter MAX   = 24'd15_000_000;//300ms最大数
parameter NOTE  = 6'd34;//音符个数
parameter DO = 16'd47750,//1
          RE = 16'd42550,//2
          MI = 16'd37900,//3
          FA = 16'd37550,//4
          SO = 16'd31850,//5
          LA = 16'd28400,//6
          XI = 16'd25400;//7

          
reg [23: 0] cnt_300 ;
reg [5: 0]  cnt_note;
reg [15: 0] cnt_freq;
wire [15: 0] duty   ;//占空比
reg [15: 0] X;
//300ms计数器设计
always @(posedge sys_clk or negedge sys_rst_n) begin
    if(!sys_rst_n)begin
        cnt_300 <= 24'd0;
    end 
    else if(cnt_300 == MAX - 1'd1)begin
        cnt_300 <= 24'd0;
    end 
    else begin
        cnt_300 <= cnt_300 + 1'd1;
    end 
end

//34个音符计数器设计
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        cnt_note <= 6'd0;
    end 
    else if((cnt_note == NOTE - 1'd1) && (cnt_300 == MAX - 1'd1))begin
        cnt_note <= 6'd0;
    end 
    else if(cnt_300 == MAX - 1'd1)begin
        cnt_note <= cnt_note + 1'd1;
    end 
    else begin
        cnt_note <= cnt_note;
    end 
end 

//每个音符频率计数器
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        cnt_freq <= 16'd0;
    end 
    else if(cnt_freq == X - 1)begin
        cnt_freq <= 16'd0;
    end 
    else begin
        cnt_freq <= cnt_freq + 1'd1;
    end 
end 

//查找表
always @(*)begin
    case(cnt_note)
        6'd0   :  X = DO;//1
        6'd1   :  X = RE;//2
        6'd2   :  X = MI;//3
        6'd3   :  X = DO;//1
        6'd4   :  X = DO;//1
        6'd5   :  X = RE;//2
        6'd6   :  X = MI;//3
        6'd7   :  X = DO;//1
        6'd8   :  X = MI;//3
        6'd9   :  X = FA;//4
        6'd10  :  X = SO;//5
        6'd11  :  X = MI;//3
        6'd12  :  X = FA;//4
        6'd13  :  X = SO;//5
        6'd14  :  X = SO;//5
        6'd15  :  X = LA;//6
        6'd16  :  X = SO;//5
        6'd17  :  X = FA;//4
        6'd18  :  X = MI;//3
        6'd19  :  X = DO;//1
        6'd20  :  X = SO;//5
        6'd21  :  X = LA;//6
        6'd22  :  X = SO;//5
        6'd23  :  X = FA;//4
        6'd24  :  X = MI;//3
        6'd25  :  X = DO;//1
        6'd26  :  X = RE;//2
        6'd27  :  X = SO;//5
        6'd28  :  X = DO;//1
        6'd29  :  X = DO;//1
        6'd30  :  X = RE;//2
        6'd31  :  X = SO;//5
        6'd32  :  X = DO;//1
        6'd33  :  X = DO;//1
        default:  X = DO;//1
    endcase
end 
assign duty = X >> 1;//占空比是1/2
assign pwm = (cnt_freq >= duty) ? 1'b0: 1'b1; 
endmodule