FULL CUSTOM, STANDARD CELLS - VLSI Design Styles.pptx

VLSI Design Styles

1. FULL CUSTOM, STANDARD CELLS VLSI DESIGN STYLES

2. FULL CUSTOM

3. What is Full Custom? Full-custom design in VLSI is a methodology that specifies the layout of each transistor and the interconnections between them in an integrated circuit (IC). It gives the designer complete control over the circuit architecture and layout. This method can improve chip performance while reducing its size, but it can be time-consuming to achieve.

4. Aspects of Full Custom Transistor-Level Design : Full custom design involves designing each transistor in the circuit individually. Designers specify the exact dimensions, doping profiles, and other parameters for each transistor to meet the desired performance and functionality requirements. Layout Design : In full custom design, the layout of the transistors and interconnects is created from scratch based on the transistor-level schematic. This involves placing transistors, resistors, capacitors, and interconnects in specific locations on the chip to achieve the desired functionality and performance. Optimization : Full custom design allows for fine-grained optimization at the transistor level. Designers can tweak transistor sizes, layout geometries, and interconnect routing to optimize for performance, power consumption, area, or other design metrics. High Performance : Full custom design often results in circuits that are highly optimized for performance, as designers have full control over the implementation details and can fine-tune the design to meet specific speed targets.

5. Area Efficiency : By customizing the layout of each transistor and interconnect, full custom design can achieve high area efficiency, meaning that the resulting circuits occupy minimal chip area. Complexity and Time-Consuming : Full custom design is typically more complex and time-consuming compared to semi-custom approaches. Designers need to have a deep understanding of device physics, layout design techniques, and fabrication processes to effectively design full custom circuits. Customizability : Full custom design offers the highest level of customizability, allowing designers to tailor the circuit to meet specific requirements and constraints. Full custom design is commonly used in applications where performance, power efficiency, and area are critical, such as high-performance processors, memory circuits, and analog/mixed-signal ICs. However, due to its complexity and resource-intensive nature, full custom design is often reserved for designs where the benefits outweigh the additional design effort and cost.

6. Advantages Optimized performance : Full custom designs can achieve higher performance levels compared to semi-custom designs. Area efficiency : By tailoring every component, full custom designs can minimize chip area. Power optimization : Fine-grained control over individual components enables power optimization.

7. Disadvantages Longer design and production time : Designing and verifying each component from scratch can be time-consuming. Not cost-efficient for small-scale projects : The high upfront investment in design and fabrication makes full custom design less suitable for low-volume productions.

8. STANDARD CELLS

9. What is Standard Cells? Standard cells are pre-designed building blocks used in very-large-scale integration (VLSI) to implement logic functions . They have fixed shapes, sizes, and layouts, and are stored in libraries that can be accessed by automated tools. Standard cells are pre-verified, pre-characterized, and perform specific functions, such as logic gates, arithmetic circuits, and memory elements.

10. Aspects of Standard Cells Pre-designed Logic Functions: Standard cells encapsulate common logic functions such as AND gates, OR gates, NAND gates, XOR gates, flip-flops, and other basic building blocks of digital circuits. Characterization and Library: Standard cells are characterized for various performance parameters such as propagation delay, power consumption, and area usage. These characteristics are stored in libraries, often referred to as standard cell libraries, which are provided by semiconductor foundries or third-party IP vendors. Reusable Blocks: Standard cells are designed to be easily reusable across different designs and projects. Designers can instantiate multiple instances of the same standard cell or combine different standard cells to create complex digital circuits.

11. Interconnection: Standard cells are designed to have standardized dimensions and pin configurations, which facilitate easy interconnection with other standard cells and external signals. Design Flexibility: While standard cells offer a level of design flexibility, they do not provide the same level of customization as full custom design. However, designers can still achieve a wide range of functionalities by combining different standard cells and configuring them appropriately . Design Automation: The use of standard cells enables the use of automated design tools such as place-and-route and synthesis tools. These tools can efficiently place and connect standard cells to meet design constraints and optimize performance metrics such as timing, area, and power . Trade-offs : Standard cell-based design involves trade-offs between design flexibility and design efficiency. While it may not offer the same level of optimization as full custom design, it provides a good balance between design effort and performance for many digital IC designs .

12. Advantages Design Productivity: Standard cell-based design enables faster development cycles by providing pre-designed and pre-characterized building blocks. Designers can focus on higher-level architectural decisions rather than low-level transistor-level implementations. Reusability: Standard cells are reusable across different projects and designs. Once characterized and verified, standard cells can be readily instantiated in new designs, saving time and effort. Design Consistency: By using standardized building blocks, standard cell-based design promotes design consistency and reduces the likelihood of errors or inconsistencies in the design. Design Automation: Standard cell-based design integrates well with automated design tools such as synthesis, place-and-route, and timing analysis tools. These tools help automate many aspects of the design process, improving efficiency and reducing manual effort. Performance Optimization: Standard cells are often optimized for performance metrics such as speed, power consumption, and area usage. Designers can select standard cells from libraries tailored to their specific performance requirements.

13. Disadvantages Limited Customization: Standard cells offer less customization compared to full custom design. Designers have limited control over the internal details of the standard cells, which may restrict the ability to optimize the design for specific requirements. Area Overhead: Standard cell-based designs may have higher area overhead compared to full custom designs, as standard cells often include additional circuitry for versatility and ease of use. Design Constraints: Standard cells impose certain design constraints such as grid-based placement and routing, which may limit design flexibility and optimization opportunities. Library Dependency: Standard cell-based design relies heavily on the availability and quality of standard cell libraries. Designers need access to comprehensive and well-characterized libraries to effectively utilize standard cell-based design methodologies. Power Consumption: While standard cells are optimized for performance, they may not always provide the best power efficiency. Designers need to carefully select and configure standard cells to minimize power consumption in their designs.