Data Type: Nets

Nets model physical connections between drivers. All net types are inherently four-state - each bit can be 0, 1, X (unknown) or Z (high-impedance). This is due to several reasons.

Nets have to have more than 2 states (binary values) because they allow multiple drivers. They must allow X and Z states to model un-driven or partially-driven connections. When there are multiple sources (like bus signals or tri-state buffers), there might be a conflict. When there is a conflict the net will have X value and Z if there is no drivers.

Floating (Z)

The z state specifically refers to a floating or un-driven condition—the net is not being actively driven by any source.

When there are multiple drivers:

• If one driver puts z and another puts 1, the result is 1.
• If one driver puts z and another puts z, the result is z.
• If one driver puts 1, another puts 0, result is x (conflict).

Contention (X)

Since multiple drivers can “pushes” a value to the same port, there would be some conflict. In that case, simulation resolves it to X to flag the conflict.

module A(output wire a); assign a = 1; endmodule
module B(output wire b); assign b = 0; endmodule

wire foo;
A u1(.a(foo));
B u2(.b(foo));

// foo sees 1 from A and 0 from B → foo becomes X (conflict)

Contention Resolution

Whenever there is a conflict, simulation uses a table shown below to resolve the conflict:

Net Types

1. Basic Nets Wires require one driver or resolves them using wired logic. It simply represents a physical connection.

• wire (4-state): the most common net
• wire [N-1:0] bus is simply an N-bit vector of wires. It is N individual 1-bit nets all bundled under one name. It tells the compiler “I want N separate wires, numbered 0 through N-1". On an FPGA or ASIC, each bit of that vector becomes its own metal interconnect (or FPGA routing wire). When you hook a 32-bit bus between two modules, the placer & router will physically lay out 32 parallel tracks.

2. Pulled Nets: It is an alias for wire, but used when we expect there to be multiple drivers. It has pull up, which means a default state when there are no drivers.

• tri0/tri1: when undriven, defaults to 0 or 1.
• trireg: - like wire, but with an implicit pull-down resistor.

3. Wired-AND / Wired-OR They are resolved net types. Instead of returning x on multiple conflicting drivers, wand and wor resolve the multiple drivers using logic rules:

• wand: Open-drain: any driver pulling low causes low output. All must be high (or unconnected) for output to be high.
• wor: Open-source: any driver pulling high causes high output. All must be low (or unconnected) for output to be low.

4. Supply Nets Supply nets have constant voltage tied to it. It is either logical high or low. Think of them as your power supply/ground for your circuit.

• supply1 models a net tied directly to Vcc (logic high).
• supply0 models a net tied directly to GND (logic low).

Summary Table