Gate Level Modelling
Gate-level modeling in Verilog is a low-level representation of a digital circuit that describes the logic gates and their interconnections explicitly. It involves defining the digital design in terms of the basic logic gates (like AND, OR, NOT, NAND, NOR, XOR, XNOR) and the interconnections between them. This type of modeling is often used when a designer needs to represent a circuit very closely to its physical implementation.
Gate-level modeling is useful for understanding the underlying hardware structure and is typically used in the synthesis process or when optimizing a design for specific performance metrics like speed, area, or power consumption.
Basic Syntax for Gate-Level Modeling
Verilog provides predefined primitives for gate-level modeling:
and for AND gate
or for OR gate
not for NOT gate
nand for NAND gate
nor for NOR gate
xor for XOR gate
xnor for XNOR gate
The syntax for using these primitives generally follows this pattern:
Example of Gate-Level Modeling in Verilog
Let’s create a simple Verilog model for a 2-input XOR gate using the basic logic gates.
Design of 2-Input XOR Gate Using Basic Gates
A 2-input XOR gate can be built using the following logic gates:
A XOR B can be expressed as (A AND ~B) OR (~A AND B).
Here’s how we can model this using gate-level Verilog:
// Verilog code for a 2-input XOR gate using gate-level modeling
module xor_gate (output wire Y, input wire A, B);
wire notA, notB;
wire and1, and2;
// NOT gates
not (notA, A); // notA = ~A
not (notB, B); // notB = ~B
// AND gates
and (and1, A, notB); // and1 = A & ~B
and (and2, notA, B); // and2 = ~A & B
// OR gate
or (Y, and1, and2); // Y = (A & ~B) | (~A & B)
endmodule
Explanation of the Code
Module Declaration:
The module keyword is used to define a module named xor_gate with output Y and inputs A and B.
Intermediate Wires:
We declare intermediate wires notA, notB, and1, and and2 to hold the outputs of the logic gates.
NOT Gates:
Two NOT gates are used to generate notA (the complement of A) and notB (the complement of B).
AND Gates:
Two AND gates generate and1 (which is A AND NOT B) and and2 (which is NOT A AND B).
OR Gate:
Finally, an OR gate takes the outputs and1 and and2 to produce the XOR output Y.
Testbench for the 2-Input XOR Gate
To verify the functionality of the XOR gate, we can write a testbench that simulates the gate-level model:
verilog
// Testbench for 2-input XOR gate
module testbench;
// Declare inputs as registers and output as wire
reg A, B;
wire Y;
// Instantiate the xor_gate module
xor_gate uut (.Y(Y), .A(A), .B(B)); // ‘uut’ is the instance name for the XOR gate
// Initial block for simulation
initial begin
// Monitor the output
$monitor(“Time = %0t | A = %b, B = %b, Y (A XOR B) = %b”, $time, A, B, Y);
// Test all input combinations
A = 0; B = 0; #10; // Time delay of 10 units
A = 0; B = 1; #10;
A = 1; B = 0; #10;
A = 1; B = 1; #10;
// End simulation
$finish;
end
endmodule
Explanation of the code
Register and Wire Declarations:
A and B are declared as reg because their values will change over time. Y is declared as wire because it is driven by the xor_gate module.
Instantiation of the xor_gate Module: The xor_gate module is instantiated with the name uut (Unit Under Test) and connected to the inputs A, B and the output Y.
Initial Block:
The initial block is used to provide the test stimulus by setting different combinations of A and B. The $monitor statement prints the values of A, B, and Y whenever they change.
Test Cases:
Different combinations of A and B are applied to verify that the XOR gate behaves correctly. End Simulation:
The $finish command stops the simulation once all test cases have been evaluated.
Simulation Results
When you run this testbench, you should see an output similar to the following:
Time = 0 | A = 0, B = 0, Y (A XOR B) = 0
Time = 10 | A = 0, B = 1, Y (A XOR B) = 1
Time = 20 | A = 1, B = 0, Y (A XOR B) = 1
Time = 30 | A = 1, B = 1, Y (A XOR B) = 0
This confirms the correct behavior of the XOR gate: Y is 1 when A and B are different and 0 when they are the same.
This example illustrates how gate-level modeling in Verilog can be used to build digital circuits from basic logic gates, providing a clear view of the underlying hardware implementation. Let me know if you’d like to explore more examples or details!