Data Type: Nets

Nets model physical connections between drivers. When you have multiple 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)

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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)

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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

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

Net Types

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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.

1. 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.

1. 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.

1. Supply Nets

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

Summary Table

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