Single-Driver Data Line using uwire

January 11, 2026

Solving Approach

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Solving Approach

Use a uwire to enforce a single-driver rule at compile time. Connect only the permitted input (d1) to the output, ensuring no other signals drive the line. This guarantees that the data line has exactly one valid driver and prevents accidental multiple-driver conflicts.

Code

module single_driver_line(
  input  wire d1, d2,
  output uwire y            // uwire: unresolved wire
);
  // LEGAL: single driver, assign d1 to output (y) here
  assign y = d1;

  // ILLEGAL (assign d2 to output to see compile-time error due to multiple drivers on uwire):
  // assign y = d2;
endmodule

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Testbench Code

`timescale 1ns/1ps
module tb_single_driver_line;
  reg  d1, d2;
  wire y;
  wire expected_y;
  wire mismatch;

  single_driver_line dut(.d1(d1), .d2(d2), .y(y));

  assign expected_y = d1;         // only d1 legally drives 'y'
  assign mismatch   = (y !== expected_y);

  integer TOTAL_TEST_CASES, TOTAL_PASSED_TEST_CASES, TOTAL_FAILED_TEST_CASES;
  reg [8*24-1:0] tc_name;

  initial begin
    $dumpfile("tb_single_driver_line.vcd");
    $dumpvars(0, d1, d2, y, expected_y, mismatch);
  end

  task run_case;
    input id1, id2;
    input [8*24-1:0] name;
    begin
      d1 = id1; d2 = id2; tc_name = name; #1;
      $display("CASE=%s : d1=%0b d2=%0b | y=%0b expected_y=%0b %s",
               tc_name, d1, d2, y, expected_y,
               mismatch ? "MISMATCH" : "OK");
      TOTAL_TEST_CASES = TOTAL_TEST_CASES + 1;
      if (!mismatch) TOTAL_PASSED_TEST_CASES = TOTAL_PASSED_TEST_CASES + 1;
      else begin
        TOTAL_FAILED_TEST_CASES = TOTAL_FAILED_TEST_CASES + 1;
        $display("  FAILED INPUTS: d1=%0b d2=%0b  EXPECTED: expected_y=%0b",
                 d1, d2, expected_y);
      end
      #4;
    end
  endtask

  initial begin
    TOTAL_TEST_CASES = 0; TOTAL_PASSED_TEST_CASES = 0; TOTAL_FAILED_TEST_CASES = 0;

    run_case(0,0,"both_zero");
    run_case(1,0,"d1_drives");
    run_case(0,1,"d2_only (ignored)");
    run_case(1,1,"both_one (only d1 matters)");

    $display("TOTAL_TEST_CASES=%0d", TOTAL_TEST_CASES);
    $display("TOTAL_PASSED_TEST_CASES=%0d", TOTAL_PASSED_TEST_CASES);
    $display("TOTAL_FAILED_TEST_CASES=%0d", TOTAL_FAILED_TEST_CASES);
    $display("ALL_TEST_CASES_PASSED=%s", (TOTAL_FAILED_TEST_CASES==0) ? "true" : "false");

    $finish;
  end
endmodule