Full custom and Semi custom design

Introduction

In the field of VLSI circuits and systems, Full custom and Semi custom design play a crucial role in the development of integrated circuits. These design methodologies allow engineers to create complex electronic systems with high performance and efficiency. In this article, we will explore the fundamentals of Full custom and Semi custom design, their advantages and disadvantages, the step-by-step design processes, and real-world applications.

Full custom design

Full custom design refers to the process of designing integrated circuits from scratch, where every transistor and interconnect is individually placed and sized. This design approach offers the highest level of customization and optimization, allowing designers to achieve the desired circuit performance. However, it requires a significant amount of time and expertise.

Advantages of Full custom design

• High performance: Full custom design allows for fine-tuning of circuit parameters, resulting in high-performance integrated circuits.
• Low power consumption: By customizing each transistor and interconnect, power consumption can be minimized.
• Small area: Full custom design enables the creation of compact integrated circuits, reducing the overall chip area.

Disadvantages of Full custom design

• Time-consuming: Full custom design involves a lengthy design process, including transistor sizing and placement, which can be time-consuming.
• High cost: Due to the complexity and customization involved, Full custom design can be expensive.
• Limited scalability: Full custom designs are not easily scalable to different process technologies.

Step-by-step walkthrough of Full custom design process

1. Circuit specification and requirements: The first step in Full custom design is to define the circuit specifications and requirements, such as functionality, performance, and power consumption.
2. Schematic design: The circuit is designed at the transistor level using a schematic editor, specifying the interconnections between transistors.
3. Layout design: The layout design involves placing and sizing individual transistors and interconnects to meet the circuit specifications.
4. Verification and testing: The designed circuit is verified using simulation tools to ensure its functionality and performance.

Real-world applications and examples of Full custom design

• Analog circuits: Full custom design is commonly used in the design of analog circuits, such as amplifiers, filters, and data converters.
• Mixed-signal circuits: Integrated circuits that combine both analog and digital components often require Full custom design to achieve the desired performance.

Semi custom design

Semi custom design, also known as structured or standard cell design, is a design methodology that combines pre-designed standard cells with custom-designed circuitry. This approach offers a balance between customization and design efficiency, making it suitable for a wide range of applications.

Advantages of Semi custom design

• Reduced design time: Semi custom design leverages pre-designed standard cells, reducing the overall design time.
• Cost-effective: Compared to Full custom design, Semi custom design is more cost-effective due to the reuse of standard cells.
• Scalability: Semi custom designs can be easily ported to different process technologies, allowing for scalability.

Disadvantages of Semi custom design

• Limited customization: While Semi custom design offers some level of customization, it is not as flexible as Full custom design.
• Moderate performance: The performance of Semi custom designs may not be as high as Full custom designs due to the use of standard cells.

Step-by-step walkthrough of Semi custom design process

1. Standard cell design and cell libraries: Standard cells are pre-designed circuit modules that can be used to build complex integrated circuits. These cells are stored in cell libraries.
2. Circuit specification and requirements: Similar to Full custom design, the circuit specifications and requirements are defined.
3. Selection of standard cells from libraries: The designer selects appropriate standard cells from the cell libraries based on the circuit requirements.
4. Placement and routing: The selected standard cells are placed and interconnected to form the desired circuit layout.
5. Verification and testing: The designed circuit is verified using simulation tools to ensure its functionality and performance.

Real-world applications and examples of Semi custom design

• Digital circuits: Semi custom design is widely used in the design of digital circuits, such as microprocessors, memory chips, and application-specific integrated circuits (ASICs).
• System-on-Chip (SoC): SoCs, which integrate multiple components on a single chip, often utilize Semi custom design to optimize performance and area.

Standard cell design and cell libraries

Standard cell design is a key component of Semi custom design. It involves the creation of pre-designed circuit modules, known as standard cells, which can be reused in different integrated circuit designs.

Importance and benefits of using standard cells

• Design efficiency: Standard cells enable designers to quickly build complex circuits by reusing pre-designed modules.
• Performance optimization: Standard cells are optimized for performance, allowing designers to achieve high-speed operation.
• Area efficiency: By using standard cells, the overall chip area can be minimized, resulting in cost savings.

Cell libraries and their role in Semi custom design

Cell libraries are collections of standard cells that are stored in a database. These libraries contain information about the characteristics and behavior of each standard cell, such as transistor sizes, power consumption, and delay.

Real-world examples of standard cell design and cell libraries

• OpenROAD: OpenROAD is an open-source project that provides a complete flow for designing integrated circuits using standard cells.
• SkyWater Open Source PDK: The SkyWater Open Source Process Design Kit (PDK) includes a library of standard cells for designing integrated circuits.

Conclusion

Full custom and Semi custom design are two important methodologies in VLSI circuits and systems. Full custom design offers the highest level of customization and optimization but requires significant time and expertise. On the other hand, Semi custom design provides a balance between customization and design efficiency, making it suitable for a wide range of applications. Understanding and implementing these design methodologies are crucial for achieving high-performance integrated circuits.

Summary

Full custom and Semi custom design are two important methodologies in VLSI circuits and systems. Full custom design offers the highest level of customization and optimization but requires significant time and expertise. On the other hand, Semi custom design provides a balance between customization and design efficiency, making it suitable for a wide range of applications. Understanding and implementing these design methodologies are crucial for achieving high-performance integrated circuits.

Analogy

Imagine you want to build a house. Full custom design would involve designing every aspect of the house from scratch, including the layout, materials, and finishes. This allows for complete customization but requires a lot of time and expertise. On the other hand, Semi custom design would involve using pre-designed modules, such as standard room layouts and pre-fabricated materials, to build the house. This approach offers a balance between customization and efficiency, making the construction process faster and more cost-effective.