You are an expert Verilog hardware designer working on a complete FPGA or ASIC project. You are given the full source code of a Verilog project. One of the modules in the project is incomplete — its implementation is missing and marked with:

// >>> Module Implementation Begin

// <<< Module Implementation End

Your task is to complete the missing implementation so that the entire project functions correctly. You must infer what the missing module or logic is supposed to do by analyzing the rest of the codebase: module names, signal connections, instantiations, comments, and usage patterns.

Follow these rules:

1. Only generate Verilog code to go between the `// >>> Module Implementation Begin` and `// <<< Module Implementation End` comments.
2. Use Verilog-2001 syntax. Ensure the code is synthesizable.
3. Assume the goal is a working, integrated hardware design — your code must make the whole system function as intended.
4. Analyze the naming, I/O ports, and how the module is instantiated or used in other parts of the project.
5. Implement correct behavior based on design context. Do not guess randomly — reason it out.
6. Avoid vendor-specific features unless explicitly required.
7. Do not output comments, explanation, or anything outside the required code region unless explicitly asked.

Your output should contain **only the Verilog code** that fits inside the marked region.

/*
 * Copyright (c) 2024 Jeremiasz Dados
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

module tt_um_DaDDS (
    input  wire [7:0] ui_in,    // Dedicated inputs
    output wire [7:0] uo_out,   // Dedicated outputs
    input  wire [7:0] uio_in,   // IOs: Input path
    output wire [7:0] uio_out,  // IOs: Output path
    output wire [7:0] uio_oe,   // IOs: Enable path (active high: 0=input, 1=output)
    input  wire ena,      // always 1 when the design is powered, so you can ignore it
    input  wire clk,      // clock
    input  wire rst_n     // reset_n - low to reset
);

  // All output pins must be assigned. If not used, assign to 0.
  assign uio_out [7:0] = 0;
  assign uio_oe [7:0] = 8'hFF;
  // List all unused inputs to prevent warnings
  wire _unused = &{ena, ui_in[2:0], ui_in[7:6], uio_in[7:0], 1'b0};
  
  DaDDS top_level(
    .clk(clk),
    .rst(!rst_n),
    .rx(ui_in[3]),
    .freq_sel(ui_in[4]),
    .rf_data(ui_in[5]),
    .dac(uo_out)
  );

endmodule


module DaDDS(
    input wire clk,
    input wire rst,
    input wire rx,
    input wire rf_data,
    input wire freq_sel,
    output wire [7:0] dac
);
    wire [15:0] adder_out;
    wire [15:0] freq0_reg_out;
    wire [15:0] freq1_reg_out;
    wire [15:0] sig_freq_reg_out;
    wire [15:0] phase_reg_out;

    uart_rx uart (
        .clk(clk),                  
        .rx(rx),                   
        .rst(rst),                 
        .freq_sel(freq_sel),              
        .freq0(freq0_reg_out),
        .freq1(freq1_reg_out)                  
    );

    // Frequency switching for FSK
    assign sig_freq_reg_out = (rf_data == 0) ? freq0_reg_out : freq1_reg_out;

    sum16 adder (
        .a(sig_freq_reg_out),
        .b(phase_reg_out),
        .sum(adder_out)
    );

    reg16 phase_reg (
        .clk(clk),
        .en(1'b1),
        .rst(rst),
        .d(adder_out),
        .q(phase_reg_out)
    );

    wire [7:0] phase;
    wire [7:0] lut_out;   
    wire [7:0] sine [63:0];

    assign sine[0] = 8'h7F; assign sine[1] = 8'h82; assign sine[2] = 8'h85;
    assign sine[3] = 8'h88; assign sine[4] = 8'h8B; assign sine[5] = 8'h8F;
    assign sine[6] = 8'h92; assign sine[7] = 8'h95; assign sine[8] = 8'h98;
    assign sine[9] = 8'h9B; assign sine[10] = 8'h9E; assign sine[11] = 8'hA1;
    assign sine[12] = 8'hA4; assign sine[13] = 8'hA7; assign sine[14] = 8'hAA;
    assign sine[15] = 8'hAD; assign sine[16] = 8'hB0; assign sine[17] = 8'hB2;
    assign sine[18] = 8'hB5; assign sine[19] = 8'hB8; assign sine[20] = 8'hBB;
    assign sine[21] = 8'hBE; assign sine[22] = 8'hC0; assign sine[23] = 8'hC3;
    assign sine[24] = 8'hC6; assign sine[25] = 8'hC8; assign sine[26] = 8'hCB;
    assign sine[27] = 8'hCD; assign sine[28] = 8'hD0; assign sine[29] = 8'hD2;
    assign sine[30] = 8'hD4; assign sine[31] = 8'hD7; assign sine[32] = 8'hD9;
    assign sine[33] = 8'hDB; assign sine[34] = 8'hDD; assign sine[35] = 8'hDF;
    assign sine[36] = 8'hE1; assign sine[37] = 8'hE3; assign sine[38] = 8'hE5;
    assign sine[39] = 8'hE7; assign sine[40] = 8'hE9; assign sine[41] = 8'hEA;
    assign sine[42] = 8'hEC; assign sine[43] = 8'hEE; assign sine[44] = 8'hEF;
    assign sine[45] = 8'hF0; assign sine[46] = 8'hF2; assign sine[47] = 8'hF3;
    assign sine[48] = 8'hF4; assign sine[49] = 8'hF5; assign sine[50] = 8'hF7;
    assign sine[51] = 8'hF8; assign sine[52] = 8'hF9; assign sine[53] = 8'hF9;
    assign sine[54] = 8'hFA; assign sine[55] = 8'hFB; assign sine[56] = 8'hFC;
    assign sine[57] = 8'hFC; assign sine[58] = 8'hFD; assign sine[59] = 8'hFD;
    assign sine[60] = 8'hFD; assign sine[61] = 8'hFE; assign sine[62] = 8'hFE;
    assign sine[63] = 8'hFE;

    assign phase = phase_reg_out[15:8]; 

    wire [5:0] phase_index; 
    assign phase_index = (phase < 8'd64) ? phase[5:0] :
                         (phase < 8'd128) ? 6'd63 - phase[5:0] :  
                         (phase < 8'd192) ? phase[5:0]:   
                         6'd63 - phase[5:0];  

    assign lut_out = (phase < 8'd64) ? sine[phase_index] : 
                     (phase < 8'd128) ? sine[phase_index] :  
                     (phase < 8'd192) ? 8'd255 - sine[phase_index] : 
                     8'd255 - sine[phase_index];  

    //if the second register wasn't programmed, switch to OOK.
    assign dac = ((freq1_reg_out == 0) && (rf_data == 0)) ? 8'd128 : lut_out; 
    
endmodule


module reg32 (
    input clk,    
    input rst,     
    input en,  
    input [31:0] d,       
    output reg [31:0] q         
);
    always @(posedge clk or posedge rst) begin
        if (rst)
            q <= 32'b0;         
        else if (en)
            q <= d;             
    end
endmodule

module reg16 (
    input clk,    
    input rst,     
    input en,  
    input [15:0] d,       
    output reg [15:0] q         
);
    always @(posedge clk or posedge rst) begin
        if (rst)
            q <= 16'b0;         
        else if (en)
            q <= d;             
    end
endmodule

module sum32 (              
    input wire [31:0] a,          
    input wire [31:0] b,         
    output reg [31:0] sum      
);
    assign sum = a + b;
endmodule

module sum16 (              
    input wire [15:0] a,          
    input wire [15:0] b,         
    output wire [15:0] sum      
);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



module uart_rx (
    input wire clk,
    input wire rx,
    input wire rst,
    input wire freq_sel,
    output wire [15:0] freq0,
    output wire [15:0] freq1
  );

  parameter idle = 3'b000;
  parameter start_bit = 3'b001;
  parameter byte_num_bit = 3'b010;
  parameter data_bits = 3'b011;
  parameter stop_bit  = 3'b100;
  parameter complete  = 3'b101;

  reg rx_data_reg;
  reg [11:0] clk_count_reg;
  reg [3:0] bit_index_reg; 
  reg byte_number;
  reg [2:0] state_reg;
  
  // Clock cycles per UART bit (115200 baud rate)
  integer clk_cycles_per_bit = 521; 

  // Output registers
  reg [15:0] freq0_reg;
  reg [15:0] freq1_reg;

  always @(posedge clk) begin
    rx_data_reg <= rx;
  end

  always @(posedge clk or posedge rst) begin
    if (rst) begin
      state_reg <= idle;
      clk_count_reg <= 0;
      bit_index_reg <= 0;
      byte_number <= 0;
      freq0_reg <= 0;
      freq1_reg <= 0;
    end else begin
      case (state_reg)
        idle: begin
          clk_count_reg <= 0;
          bit_index_reg <= 0;

          if (rx_data_reg == 1'b0)
            state_reg <= start_bit;
          else
            state_reg <= idle;
        end

        start_bit: begin
          if (clk_count_reg == (clk_cycles_per_bit - 1) / 2) begin
            if (rx_data_reg == 1'b0) begin
              clk_count_reg <= 0;
              bit_index_reg <= 0;
              state_reg <= byte_num_bit;
            end else
              state_reg <= idle;
          end else
            clk_count_reg <= clk_count_reg + 1;
        end

        byte_num_bit : begin
          if (clk_count_reg == (clk_cycles_per_bit - 1) / 2) begin
              clk_count_reg <= 0;
              byte_number <= rx_data_reg;
              state_reg <= data_bits;
          end else begin
              clk_count_reg <= clk_count_reg + 1;
          end
        end

        data_bits: begin
          if (clk_count_reg < clk_cycles_per_bit - 1) begin
              clk_count_reg <= clk_count_reg + 1;
          end else begin
              clk_count_reg <= 0;

              if (byte_number == 1'b1) begin
                  if (freq_sel == 1'b0) begin
                    freq0_reg[8 + bit_index_reg] <= rx_data_reg; 
                  end else begin
                    freq1_reg[8 + bit_index_reg] <= rx_data_reg; 
                  end
              end else begin
                  if (freq_sel == 1'b0) begin
                    freq0_reg[bit_index_reg] <= rx_data_reg; 
                  end else begin
                    freq1_reg[bit_index_reg] <= rx_data_reg; 
                  end
              end

              if (bit_index_reg < 7) begin
                  bit_index_reg <= bit_index_reg + 1;
              end else begin
                  bit_index_reg <= 0;
                  state_reg <= stop_bit;
              end
          end
        end

        stop_bit: begin
          if (clk_count_reg < clk_cycles_per_bit - 1) begin
            clk_count_reg <= clk_count_reg + 1;
          end else begin
            clk_count_reg <= 0;
            bit_index_reg <= 0;
            state_reg <= complete;
          end
        end

        complete: begin
          state_reg <= idle;
        end

        default: state_reg <= idle;
      endcase
    end
  end

  assign freq0 = freq0_reg;
  assign freq1 = freq1_reg;
endmodule

Reference solution:
rtl/generated.v:     assign sum = a + b;