Digital System Design: CAD, HDLs, and VLSI Fundamentals

Posted on Aug 5, 2025 in Electronic Systems Engineering

Evolution of CAD Tools & HDLs in Digital System Design

The journey of digital circuits began with fundamental components, evolving significantly over time:

Early Circuits: Digital circuits initially relied on vacuum tubes and transistors.
Small-Scale Integration (SSI): The first integrated circuits (ICs) were SSI, containing only a few logic gates.
Medium-Scale Integration (MSI): This era introduced chips with hundreds of gates.
Large-Scale Integration (LSI): Thousands of gates could be integrated onto a single chip.
Electronic Design Automation (EDA) Emergence: As chip complexity grew, EDA tools were developed to assist in circuit design and testing.
Simulation’s Role: Designers began using simulation tools to verify the functionality of smaller circuit blocks (e.g., 100 transistors).
Manual Layouts: Early circuit layouts were drawn by hand or with basic computer graphics.
Very Large-Scale Integration (VLSI) Revolution: With VLSI, over 100,000 transistors could fit on a chip, making manual testing impractical.
Automatic Tools: Designers adopted automatic layout tools for efficient placement and routing of circuits.
Logic Simulators: Logic simulation became crucial for identifying errors before chip fabrication.

Typical VLSI IC Design Flow

The design of Very Large-Scale Integration (VLSI) integrated circuits follows a structured flow:

Specifications: The process begins with defining the circuit’s function and structure through detailed specifications.
Behavioral Description: Hardware Description Languages (HDLs) are used to create a high-level behavioral description, allowing for early functional and performance testing.
RTL Conversion: The behavioral code is then manually converted into Register Transfer Level (RTL) code using an HDL.
RTL Design: RTL code, which describes data flow and control, is a critical phase where most manual optimization effort is concentrated.
Logic Synthesis: Logic synthesis tools automatically convert the RTL description into a gate-level netlist.
Physical Layout: Tools utilize the gate-level netlist to generate the physical layout of the chip.
Verification & Fabrication: The layout undergoes thorough checks before being sent for chip fabrication.
Behavioral Synthesis (Emerging): Modern behavioral synthesis tools can automate the conversion of high-level algorithms directly into RTL.
Designer Control: Despite automation, designers maintain control; incorrect inputs (Garbage In, Garbage Out – GIGO) can still lead to flawed designs.

Current Trends in Hardware Description Languages

Hardware Description Languages (HDLs) are continually evolving to meet the demands of complex digital designs:

Higher Abstraction Levels: Design is increasingly moving towards higher abstraction levels to manage complexity and achieve faster development cycles.
EDA Tool Dominance: EDA tools now handle most of the implementation details, with designers providing high-level guidance.
RTL Design with HDLs: Register Transfer Level (RTL) design using HDLs like Verilog remains the most popular and practical method in the industry.
Behavioral Synthesis: While behavioral synthesis (designing with algorithms) exists, its widespread adoption is still limited.
Formal Verification: Mathematical methods are employed in formal verification to rigorously check the correctness of designs by comparing RTL and gate-level netlists.
Assertion Checkers: Assertion checkers are embedded within RTL code to verify critical aspects of the design during simulation.
New Verification Languages: Emerging verification languages (often combining HDL and C++ styles) aid in automating testing but do not replace core HDLs like Verilog for design.
Gate-Level Tweaks: For high-speed circuits, manual gate-level adjustments are sometimes necessary to achieve timing performance beyond what automated tools can provide.
Mixed Design Methodologies: Hybrid approaches combine RTL design with pre-built intellectual property (IP) blocks to accelerate simulation and reduce development costs. For example, in a system with a CPU, graphics, I/O, and a system bus, the CPU might be designed in RTL, while other components use pre-verified models or vendor blocks for faster simulation.

Verilog HDL’s Popularity in Digital Circuit Design

Verilog HDL has become a dominant language in digital circuit design due to several key advantages:

Ease of Learning: Verilog’s syntax is similar to the C programming language, making it relatively easy for software programmers to learn and adopt.
Multi-Level Abstraction Support: Designers can write Verilog code at various abstraction levels, including switch-level, gate-level, Register Transfer Level (RTL), and behavioral models, all within a single design.
Widespread Tool Support: Verilog is well-supported by most logic synthesis tools and is the preferred language for a vast number of digital designers.
Vendor Compatibility: Nearly all chip fabrication vendors provide support for Verilog and offer extensive libraries for simulation and synthesis.
Programming Language Interface (PLI): The PLI feature allows designers to integrate C code with Verilog simulators, enabling customization for specific simulation and verification needs.

Advantages of HDLs Over Traditional Schematic-Based Design

Hardware Description Languages (HDLs) offer significant benefits compared to older schematic-based design methods:

Higher Abstraction: HDLs allow designers to describe circuit behavior at a higher level, rather than drawing every individual gate.
Faster Design: Writing HDL code is generally much quicker than manually drawing complex schematics.
Modularity and Reusability: HDLs facilitate the creation of reusable design blocks or modules, promoting efficient design practices.
Easy Simulation: Designs can be easily tested and verified using testbenches, enabling early error detection.
Tool Automation: HDLs seamlessly integrate with Electronic Design Automation (EDA) tools for automated synthesis and physical layout.
Scalability: HDLs are well-suited for designing large and complex circuits, such as modern processors.
Text-Based: Being text-based, HDL code is easy to manage with version control systems like Git.
Consistent Results: HDLs reduce human error often associated with manual schematic drawing, leading to more consistent and reliable designs.
Design Iterations: Modifying and updating designs is significantly easier and faster with HDL code.

Hierarchical Digital System Design

Hierarchical Design Methodology is a powerful technique for managing the complexity of digital systems. It involves breaking down a large, intricate system into smaller, more manageable modules or blocks. These modules can be designed, tested, and reused independently before being interconnected to form the complete system. This approach significantly enhances design clarity, simplifies debugging, and promotes modularity.

Steps in Hierarchical Design:

Top-Level Design (System Specification): Define the overall system functionality, including its inputs, outputs, and intended operations.
Divide into Submodules: Decompose the system into smaller, distinct functional units or modules.
Design Each Module: Each submodule is independently described, designed, and verified to ensure its correctness.
Instantiate Modules in Top-Level Design: The verified submodules are then integrated by instantiating them within the top-level module.
Verify Entire System: After integration, the complete design undergoes comprehensive simulation and testing to ensure it meets all specifications.

Advantages of Hierarchical Design:

Easily manages large and complex systems.
Encourages design reuse, saving time and effort.
Simplifies testing and debugging processes.
Allows different team members to develop modules in parallel, accelerating development.

Verilog for 4-Bit Ripple Carry Counter (T-Flip-Flop Example)

While a full 4-bit ripple carry counter would typically involve cascading multiple T-flip-flops, here is the Verilog HDL program for a single T-flip-flop, which is a fundamental building block for counters:

module t_flip_flop (
    input T,
    input clk,
    input reset,
    output reg Q
);
    always @(posedge clk or posedge reset) begin
        if (reset)
            Q

Verilog HDL: Modules and Instances Explained

In Verilog HDL, two fundamental concepts for structuring designs are modules and instances:

Module:

A module in Verilog serves as a blueprint or template that describes the functionality of a digital component. It can represent anything from a simple logic gate to a complex processor. Within a module, you define its inputs, outputs, and the internal logic that dictates its behavior.

module and_gate (
    input a,
    input b,
    output y
);
    assign y = a & b;
endmodule

Instance:

An instance is created when you use or call a module within another module. This allows for the reuse of predefined modules multiple times within a larger design, similar to how you might use multiple copies of a specific component in a physical circuit.

module top_module;
    wire x, y, z;
    and_gate my_gate (.a(x), .b(y), .y(z));
endmodule

Verilog HDL Description Styles

Verilog HDL supports various description styles, each offering a different level of abstraction for designing digital circuits:

Behavioral Description
This style describes what the circuit does using high-level programming constructs such as if, case, and for loops. The focus is on the functionality of the design rather than its specific hardware implementation. Behavioral descriptions are typically written using always blocks and procedural statements.
```
always @(posedge clk)
    if (reset)
        q 
```
Register Transfer Level (RTL) Description
RTL describes how data moves between registers and how operations are performed on that data. It is the most common design style used in the industry because it is synthesizable, meaning it can be directly converted into a gate-level netlist by logic synthesis tools.
```
always @(posedge clk or posedge reset)
    if (reset)
        count 
```
Gate-Level Description
This style describes the circuit using primitive logic gates (e.g., AND, OR, NOT). It represents a lower level of abstraction and is typically generated by synthesis tools from RTL code. Gate-level descriptions are useful for post-synthesis simulation and timing analysis.
```
and (y, a, b); // y = a & b
or  (z, a, b); // z = a | b
```
Switch-Level Description
The switch-level description models the circuit at the transistor (MOSFET) level. This is the lowest level of abstraction and is primarily used for modeling very low-level custom cells or for detailed analog/mixed-signal simulations. It is not commonly used in modern digital design flows for large systems.
```
nmos (out, in, control); // NMOS transistor
pmos (out, in, control); // PMOS transistor
```

Simulation, Synthesis, and Stimulus in Digital Design

Understanding the roles of simulation, synthesis, and stimulus is crucial in the digital design workflow:

Simulation – 5 Key Points

Verifies the functional correctness of the design before any hardware is physically built.
Produces waveforms that illustrate how signals change over time within the circuit.
Utilizes testbenches to apply input signals (stimulus) and observe the corresponding outputs.
Can simulate designs at various abstraction levels, including behavioral, RTL, or gate-level.
Helps in detecting logical errors early in the design process, saving significant time and cost.

Synthesis – 5 Key Points

Converts Hardware Description Language (HDL) code, typically at the Register Transfer Level (RTL), into a physical hardware representation (a gate-level netlist).
Targets specific FPGA (Field-Programmable Gate Array) or ASIC (Application-Specific Integrated Circuit) technologies.
Ignores non-synthesizable constructs found in HDLs, such as explicit delays or display functions.
Optimizes the design for various constraints, including area, power consumption, and timing performance.
Is used to generate the physical hardware implementation from the verified RTL code.

Stimulus – 5 Key Points

Stimulus is the input signals applied during simulation.
It is written in a testbench module.
Helps in checking how the design responds to different inputs.
Can simulate clock pulses, resets, data inputs, etc.
Essential for verifying correctness of the HDL code

Write test bench to test the 4-bit ripple carry counter.

module tb_ripple_counter;
reg clk;
reg reset;
wire [3:0] count;
ripple_counter uut (
.clk(clk),
.reset(reset),
.count(count)
);
initial begin
clk = 0;
forever #5 clk = ~clk;
end
initial begin
reset = 1;
#10;
reset = 0;
#100;
reset = 1;
#10;
reset = 0;
#50;
$stop;
end
endmodule

14. Write a test bench to test the 4-bit ripple carry adder

module tb_ripple_carry_adder;
reg [3:0] A, B;
reg Cin;
wire [3:0] Sum;
wire Cout;
ripple_carry_adder uut (
.A(A),
.B(B),
.Cin(Cin),
.Sum(Sum),
.Cout(Cout)
);
initial begin
$display(“Time\tA\tB\tCin\t|\tSum\tCout”);
$monitor(“%0t\t%b\t%b\t%b\t|\t%b\t%b”, $time, A, B, Cin, Sum, Cout);
A = 4’b0000; B = 4’b0000; Cin = 0; #10;
A = 4’b0101; B = 4’b0011; Cin = 0; #10;
A = 4’b1100; B = 4’b1010; Cin = 0; #10;
A = 4’b1111; B = 4’b0001; Cin = 1; #10;
A = 4’b0111; B = 4’b0111; Cin = 0; #10;
A = 4’b1111; B = 4’b1111; Cin = 1; #10;
$finish;
end
endmodule