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.

/*
 * tt_um_loopback_ericsmi.v
 *
 * Loopback test user module, with basic skew measurement,
 * and clock jitter measurement flop
 *
 * Extended from:
 * https://github.com/TinyTapeout/tt05-loopback
 *
 * Author: Eric Smith
 */

`default_nettype none
`timescale 1ns/10ps

/////////////////////////////////////////////////////////////
`define BEHAV `COCOTB_SIM
`define LB_DELAY
//`define TECH_SKY130
`define TECH_IHP130
/////////////////////////////////////////////////////////////
// helper modules

module buff2(input I, output Z);
`ifdef BEHAV
  assign `LB_DELAY Z = I;
`elsif TECH_SKY130
  sky130_fd_sc_hd__buf_2 sky130_fd_sc_hd__buf_2(.A(I),.X(Z));
`elsif TECH_IHP130
  sg13g2_buf_2 sg13g2_buf_2(.A(I),.X(Z));
`else
  $error("TECH_xxxxxx");
`endif
endmodule

module split(input I, output A,B,C);
  wire t;
  buff2 buff2(.I(I),.Z(t));
  buff2 buff2a(.I(t),.Z(A));
  buff2 buff2b(.I(t),.Z(B));
  buff2 buff2c(.I(t),.Z(C));
endmodule

module rdffe(input D,E,CLK,RSTN, output Q);
  reg FF;
  always @(negedge RSTN or posedge CLK)
    if( ~RSTN )
	  FF <= 0;
	else begin
	  if (E) FF <= D;
	end
  assign `LB_DELAY Q = FF;
endmodule

module dff(input D,CLK, output Q);
  reg FF;
  always @(posedge CLK)
    FF <= D;
  assign `LB_DELAY Q = FF;
endmodule

module mux8(input [7:0] I, input [2:0] S, output Z);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



/////////////////////////////////////////////////////////////

module tt_um_dlmiles_loopback (
	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,
	input  wire       clk,
	input  wire       rst_n
);

    wire [7:0] ui_ina; // for measurement
    wire [7:0] ui_inb; // for internal use, avoid loading measurement
    wire [7:0] ui_inc; // for measurement

    wire rst_n_a, rst_n_b, rst_n_c;

    wire en,D,mCLK,Qp,Qp_bypassed,Qn,Qn_bypassed,Qj;

    wire default_mode_n; // when 0, match the behavior of tt05-loopback
    wire bypass;

    reg [3:0] clk_div;

    split split_rst_n (
        .I(rst_n),
        .A(rst_n_a), 
        .B(rst_n_b),
        .C(rst_n_c) 
    );

    split split_ui_in [7:0] (
        .I(ui_in[7:0]),
        .A(ui_ina[7:0]), 
        .B(ui_inb[7:0]),
        .C(ui_inc[7:0]) 
    );

    rdffe rdffe_uio [7:0] (
        .D(uio_in[7:0]),
        .E({8{en}}),
        .CLK(clk),
        .RSTN({8{rst_n_a}}),
        .Q(uio_out[7:0])
    );

    rdffe rdffe_jitter (
        .D(~Qj),
      	.E(en & default_mode_n),
        .CLK(clk),
	      .RSTN(rst_n_b),
	      .Q(Qj)
    );

    assign bypass = uio_out[7];
    assign default_mode_n = uio_out[6];

    assign en = &ui_inb[7:4];
    assign uio_oe[7:0] = {8{~en}};

    assign uo_out[3:0] = default_mode_n ? {4{Qp_bypassed}} : {4{ui_inb[0]}};
    assign uo_out[4] = default_mode_n ? Qn_bypassed : ui_inb[0];
    assign uo_out[5] = default_mode_n ? Qj : ui_inb[0]; 
    assign uo_out[6] = ui_inb[0];  // same as tt05-loopback

    // connectivity for all inputs
    // technically ena should always be high, so force en=0 then toggle rst_n
    // this should make an ok RTT for rst_n possible

    assign uo_out[7] = rst_n_c ? en : ena; 

    // This structure does its best to keep in-tile skew the same.
    // I can't account for PnR on TT but the buffers should dominate the delay
    // and we ensure there are the same numbers and sizes of those.
    // As this tile is mostly empty I anticapte the flow should do ok at PnR. 

    mux8 mux8_d( .I(ui_ina[7:0]), .S(uio_out[2:0]), .Z(D));   
    mux8 mux8_c( .I(ui_inc[7:0]), .S(uio_out[5:3]), .Z(mCLK)); 
    
    // This is the phase measurement
    //   Start with Q=0, 
    //   Then put a rising edge on D and CLK on the outside of the IC
    //   If Q=0 then mCLK arrived before D, if Q=1 then D arrived before mCLK
    //   This allows racing inputs vs one another. 
    //   The amount of delay adjust needed on outside of the IC to make 
    //   Q toggle is the amount of skew in path from IC input pin to tile input pin. 

    dff dffp( .D(D), .CLK(mCLK), .Q(Qp));  // test arrival of posedge
    dff dffn( .D(D), .CLK(~mCLK), .Q(Qn)); // test arrival of negedge

    assign Qp_bypassed = bypass ? D : Qp;
    assign Qn_bypassed = bypass ? D : Qn;

endmodule // tt_um_loopback


Reference solution:
rtl/generated.v: `ifdef BEHAV
  assign `LB_DELAY Z = S[2] ? 
      ( S[1] ? ( S[0] ? I[7] : I[6] ) : ( S[0] ? I[5] : I[4] ) ): 
      ( S[1] ? ( S[0] ? I[3] : I[2] ) : ( S[0] ? I[1] : I[0] ) );
`elsif TECH_SKY130
  wire [1:0] w;
        sky130_fd_sc_hd__mux4_2 sky130_fd_sc_hd__mux4_2_0(
                .S1(S[1]),.S0(S[0]),
                .A0(I[0]),.A1(I[1]),.A2(I[2]),.A3(I[3]),
                .X(w[0]));
        sky130_fd_sc_hd__mux4_2 sky130_fd_sc_hd__mux4_2_1(
                .S1(S[1]),.S0(S[0]),
                .A0(I[4]),.A1(I[5]),.A2(I[6]),.A3(I[7]),
                .X(w[1]));
        sky130_fd_sc_hd__mux2_2 sky130_fd_sc_hd__mux2_2(
                .S(S[2]),
                .A0(w[0]),.A1(w[1]),
                .X(Z));
`elsif TECH_IHP130
  wire [1:0] w; // no drive level 2 for mux4
        sg13g2_mux4_1 sg13g2_mux4_2_0(
                .S1(S[1]),.S0(S[0]),
                .A0(I[0]),.A1(I[1]),.A2(I[2]),.A3(I[3]),
                .X(w[0]));
        sg13g2_mux4_1 sg13g2_mux4_2_1(
                .S1(S[1]),.S0(S[0]),
                .A0(I[4]),.A1(I[5]),.A2(I[6]),.A3(I[7]),
                .X(w[1]));
        sg13g2_mux2_2 sg13g2_mux2_2(
                .S(S[2]),
                .A0(w[0]),.A1(w[1]),
                .X(Z));
  sg13g2_buf_2 sg13g2_buf_2(.A(I),.X(Z));
`else
  $error("TECH_xxxxxx");
`endif