VERILOG : 5. Gate-Level Modeling in Verilog

When we think of Verilog, most engineers picture RTL (Register Transfer Level) coding – writing always blocks, designing with if-else or case statements, and abstracting digital logic into behavioral or dataflow descriptions.

However, Verilog also provides a lower-level way of modeling hardware, called Gate-Level Modeling. This is where you describe your circuits directly in terms of logic gates, switches, and transistor-level primitives.

Even though gate-level coding is rarely used in RTL design, it plays a vital role after synthesis in ASIC and FPGA design flows. Let’s break it down.

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🔑 Why Gate-Level Modeling Matters

• RTL Coding → Synthesis → Gate-Level Netlist
  In real chip design, RTL code is synthesized into logic gates. The synthesis tool generates a Verilog netlist made of gate primitives and standard cells.

• Gate-Level Simulation (GLS)
  This netlist is then simulated (often with back-annotated timing from an SDF file) to check how the design behaves with real-world delays.

• ASIC/FPGA Cells
  Library cells like IO buffers, flip-flops, and synchronizers are typically instantiated using gate-level primitives.

👉 In short: You might not write gate-level Verilog daily, but the EDA tools you use definitely do!

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⚡ Categories of Verilog Primitives

Verilog offers three main groups of gate-level primitives:

1. Gate Primitives
   (AND, OR, XOR, NOT, etc.)

2. Transmission Gate Primitives
   (Buffers, tri-state buffers, inverters with enable)

3. Switch Primitives
   (nMOS, pMOS, CMOS, and resistive switches)

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1️⃣ Gate Primitives

These represent basic logic gates.

📌 Syntax:

gate_type instance_name (output, input1, input2, ...);

• First terminal = output

• Remaining terminals = inputs

Supported Gates

Gate Description and N-input AND gate nand N-input NAND gate or N-input OR gate nor N-input NOR gate xor N-input XOR gate xnor N-input XNOR gate not Inverter

🧩 Example

module gates();
  wire out0, out1, out2;
  reg in1, in2, in3, in4;

  not U1(out0, in1);                  // Inverter
  and U2(out1, in1, in2, in3, in4);   // 4-input AND
  xor U3(out2, in1, in2, in3);        // 3-input XOR

  initial begin
    $monitor("in1=%b in2=%b in3=%b in4=%b | out0=%b out1=%b out2=%b",
              in1, in2, in3, in4, out0, out1, out2);
    in1 = 0; in2 = 0; in3 = 0; in4 = 0;
    #1 in1 = 1;
    #1 in2 = 1;
    #1 in3 = 1;
    #1 in4 = 1;
    #1 $finish;
  end
endmodule

✅ This example simulates how inputs propagate through different gates.

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2️⃣ Transmission Gate Primitives

Transmission gates are bi-directional elements that pass or block signals depending on enable controls.

📌 Syntax:

primitive_name instance_name (output, input, control);

Common Transmission Gates

Gate	Description
buf	Buffer
not	Inverter
bufif0	Tri-state buffer (active-low enable)
bufif1	Tri-state buffer (active-high enable)
notif0	Tri-state inverter (active-low enable)
notif1	Tri-state inverter (active-high enable)

🧩 Example

module transmission_gates();
  reg data_enable_low, in;
  wire data_bus, out1, out2;

  bufif0 U1(data_bus, in, data_enable_low);  // Tri-state buffer
  buf    U2(out1, in);                       // Buffer
  not    U3(out2, in);                       // Inverter

  initial begin
    $monitor("@%g in=%b en=%b out1=%b out2=%b bus=%b", 
              $time, in, data_enable_low, out1, out2, data_bus);
    data_enable_low = 0; in = 0;
    #4 data_enable_low = 1;
    #8 $finish;
  end

  always #2 in = ~in;
endmodule

✅ Observe how the data bus floats (Z) when the tri-state buffer is disabled.

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3️⃣ Switch Primitives

Switch primitives model transistor-level behavior. These are used when you need to simulate MOS-level connectivity.

📌 Syntax:

keyword unique_name (drain, source, gate);

Types of Switches

Primitive	Description
nmos/pmos	Uni-directional switches
rnmos/rpmos	Resistive NMOS/PMOS
cmos/rcmos	CMOS switches (2 control signals)
tran/tranif0/tranif1	Bi-directional switches
pullup/pulldown	Weak resistors to force signals

🧩 Example

module switch_primitives();
  wire net1, net2, net3;
  wire net4, net5, net6;

  tranif0 my_gate1 (net1, net2, net3);   // Controlled transistor
  rtranif1 my_gate2 (net4, net5, net6);  // Resistive transistor
endmodule

✅ Used in transistor-level modeling and IO pad simulations.

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🔢 Logic Values in Verilog

Unlike real hardware, Verilog allows multi-valued logic for accurate simulation.

Value	Meaning
0	Logic low
1	Logic high
z/Z	High impedance (floating)
x/X	Unknown/uninitialized/conflict

This is important for GLS (Gate-Level Simulation) where multiple drivers may compete for a net.

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💡 Signal Strengths in Verilog

Not all signals are equal! Verilog allows modeling of signal strengths to resolve conflicts.

Strength Keyword 7 (Strongest) supply0, supply1 6 strong0, strong1 5 pull0, pull1 4 large 3 weak0, weak1 2 medium 1 small 0 (Weakest) highz0, highz1

Example of Strength Resolution

• A supply0 (strong 0) will override a pull1 (weaker 1).

• Similarly, supply1 will override a large1.

This ensures realistic conflict resolution in circuits.

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🎯 Conclusion

Gate-level modeling in Verilog is not just about drawing AND/OR gates — it is the foundation of the post-synthesis world.

• RTL engineers design at a higher abstraction.

• Synthesis tools translate RTL into gate-level netlists.

• Verification engineers simulate these gate-level Verilog files with back-annotated timing (SDF).

Even if you don’t manually code in gates often, understanding gate-level primitives helps you debug and verify synthesized designs effectively.

👉 So, the next time you run a GLS simulation, remember: you’re working directly with Verilog’s gate-level modeling power.