Basic Verilog Module Structure

A Verilog module is the fundamental building block for digital circuit design. The typical structure is:

module ModuleName(params); // Parameters usually include inputs and outputs

// Port Declarations
input  [3:0] A, B;   // 4-bit inputs
input        C_in;   // Single-bit input
output [3:0] Sum;    // 4-bit output
output       C_out;  // Single-bit output

// Note: If parameters have different widths, declare them on separate lines.

// Internal Signal Declarations
wire C_0, C_1, C_2; // Wires connect components within the module

// Main Logic Design
FA FA1 (
    .A(A[0]),
    .B(B[0]),
    .C_in(C_in),
    .Sum(Sum[0]),
    .C_out(C_0) // Corrected naming to C_out
);

endmodule

Key Points

• module ... endmodule: Defines the design block.
• Ports (input, output): Specify external connections.
• wire: Represents internal connections between components.
• Instantiation: Using other modules (e.g., FA for Full Adder) inside the main module.

Circuit Design Abstraction Levels in Verilog

There are four primary abstraction levels for designing digital circuits:

1. Gate Level

• Designs are described using individual logic gates (AND, OR, NOT, etc.).
• Example:

and (Y, A, B);
```

2. Data Flow Level

• Uses Boolean expressions or continuous assignments (assign) to describe data movement.
• Example:

assign Y = (A & B) | (~A & C);
```

3. Behavioural Level

• Describes circuit behaviour using high-level constructs like if-else, case, and loops.
• Example:

always @(*) begin
if (A > B)
Y = 1;
else
Y = 0;
end
```

4. Register-Transfer Level (RTL)

• Combines data flow and behavioural modelling.
• Focuses on data transfer between registers and operations performed on that data.
• Example:

always @(posedge clk) begin
Q <= D;
end
```