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3. Multiple Drivers

module top_module(input a, input b, output y);
  assign y = a;
  assign y = b;
endmodule

 

⚠️ Important Note: Multiple Drivers on a Wire

In this example, the wire y is driven by two continuous assignments:

assign y = a; assign y = b;

🔍 What happens?

  • If both drivers set the same value → y takes that value.
  • If drivers set different values → simulator shows x (unknown).
  • In real hardware, this is electrical contention (two outputs fighting).
    • Can cause high current, heat, or even damage to devices.

✅ Best Practices for Real Designs

  • One driver per net in RTL.
  • If you need both signals:
    • Combine with logic → assign y = a & b; or assign y = a | b;
    • Use a multiplexer → assign y = sel ? a : b;
  • For shared buses:
    • Use tri-state (z) carefully with arbitration.

👉 Takeaway: In clean designs, each wire should have exactly one driver. Multiple drivers are for special cases only (like testbenches or controlled tri-state buses).