3. Multiple Drivers

Every design in Verilog starts with a module. A module is like a “box” with inputs and outputs that describes how signals are connected or processed.

Structure:

module module_name (
	input a, // Port declarations
	input b,
	output y
);

   // Internal signals (optional)
   wire w;
   
   // Logic description
   assign y = a & b;   // continuous assignment
   
endmodule

• module … endmodule → defines the boundaries of your design.
• Inputs: signals coming into the design.
• Outputs: signals produced by the design.
• Internal wires: connections inside the module.
• assign: continuous assignment → drives wires automatically.

Equivalent verilog code:

// Sample top_module (and gate) in verilog
module top_module(
	input a, // for all input/output default type is wire
	input b,
    output y 
);
	assign y = a & b;
endmodule

• A wire represents a physical connection between components in hardware (just like a real copper wire on a circuit board
• It does not store a value → it only reflects whatever is being driven onto it
• If no one drives it, the wire’s value defaults to High-Impedance (z).

How to Use wire

• You must drive a wire using:
  1. A continuous assignment (assign)
  2. Or by connecting it to a module’s output port

Characteristics of wire

• Direction of flow: values flow into the wire from its driver(s).
• Updates instantly: whenever the right-hand side changes, the wire updates.
• No memory: unlike reg, it cannot “remember” values across time.

Multiple Drivers

• In real circuits, you can connect multiple outputs to a single wire → in Verilog too, a wire can have multiple drivers.
• If drivers conflict:
  • 0 vs 1 → results in x (unknown)
  • Pull-ups/pull-downs (supply0, supply1) can resolve defaults

👉 Beginners don’t usually need this immediately, but it explains why fan-out (1 source → many loads) is fine, but fan-in (many sources → 1 wire) needs caution.

💡Analogy for Beginners

Think of a wire as a hose:

• Water (signal) flows through it from a tap (driver).
• If nothing is connected to the tap, the hose is empty (undefined).
• If two taps push different flows into the same hose, the result is chaos (x).
• The hose itself never “remembers” water — it only shows what’s flowing now.

Bitwise NOT (~)

Flips every bit. Directly maps to NOT gate.

Example:
  assign y = ~a;   // Inverter

Truth table:

Bitwise AND (&)

The result is 1 if both inputs are 1. Directly maps to AND gate.

Example:
  assign y = a & b;   // AND gate

Bitwise OR (|)

The result is 1 if at least one input is 1. Directly maps to OR gate.

Example:
  assign y = a | b;   // OR gate

Bitwise XOR (^)

The result is 1 if inputs are different. Directly maps to XOR gate.

Example:
  assign y = a ^ b;   // XOR gate