The advantages of full custom design are primarily complete flexibility and a high degree of optimization in performance and area. This method offers unparalleled control over every aspect of the integrated circuit's layout and functionality.
Here's a breakdown of these advantages:
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Complete Flexibility: Full custom design gives engineers the power to tailor every transistor and wire to meet specific requirements. Unlike standard cell or gate array approaches, there are no pre-defined building blocks limiting the design. This flexibility is crucial for specialized applications.
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High Degree of Optimization:
- Performance: Because the designer controls every detail, they can optimize the circuit for speed, power consumption, and other performance metrics far beyond what's achievable with automated design flows. This optimization can include carefully sizing transistors, minimizing wire lengths, and implementing advanced circuit techniques.
- Area: By precisely packing components and routing connections, full custom design can minimize the overall chip area. This is especially important for cost-sensitive applications and for integrating a large number of functions on a single chip.
Examples of when full custom design is preferred:
- High-performance microprocessors: The most advanced CPUs rely heavily on full custom or semi-custom design to squeeze every last bit of performance out of the silicon.
- Analog and mixed-signal circuits: Analog design, in particular, benefits immensely from the precision and control offered by full custom techniques.
- Memory chips: Optimizing memory cell size and performance often requires full custom layout.
- Specialized ASICs (Application-Specific Integrated Circuits): When an application demands very specific performance characteristics, full custom design can provide the optimal solution.
While full custom design offers these significant advantages, it's important to acknowledge its drawbacks:
- High Design Effort: It's a very labor-intensive process, requiring skilled engineers and sophisticated design tools.
- Longer Design Cycle: The increased complexity translates to a significantly longer time to market compared to other design methodologies.
- High Development Costs: The specialized expertise and increased design time translate to higher development costs.
- Increased risk: Any issue in the circuit can cause a total loss of design time and cost.
- Complexity: Testing becomes complex.
Because of these disadvantages, full custom design is typically reserved for critical sections of a design or for products where performance or area are paramount. Otherwise, methodologies like standard cell design are more commonly employed to balance performance with design effort and time-to-market.
