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
1. 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.
2. 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.
3. 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.
4. 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. 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.
6. 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.
7. 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.
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
1.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.
2.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.
3.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. 5. Interconnection: Standard cells are designed to have standardized dimensions and pin
configurations, which facilitate easy interconnection with other standard cells and
external signals.
5. 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.
6. 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.
7. 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.
