Asic Designer
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Introduction :
ASIC design starts with an initial concept of the required IC part. Early in this product conceptualisation phase, it is important to decide the design style that will be most suitable for the design and validation of the eventual ASIC chip. An important decision for any design team is the design flow that they will accept. A design style refers to a broad method of designing circuits using specific techniques and technologies for the design implementation and validation. In particular a design style determines the specific design steps and the use of library parts for the ASIC part. Design styles are determined, in part by the economic viability of the design, as determined by tradeoffs between performance, pricing and production volume. For some applications, such as defence systems and space applications, although the volume is low, the cost is of no concern due to the time criticality of the application and the requirements of high performance and reliability. For applications such as consumer electronics, the high production volume will offset high production costs.
Designers classify design styles into two namely custom design and semi-custom design. Custom designs as the name suggests, involve the complete design to be handcrafted so as to optimise the circuit for performance, and/or area for a given application. Although this is an expensive design style in terms of effort and cost, it leads to high-quality circuits for which the cost can be amortized over a large volume production.
Semi-custom design styles :
The semi-custom design style adopted by designers in some companies limits the circuit primitives and uses predesigned blocks, which cannot be further fine-tuned. These pre-designed primitive blocks are usually optimised, well -designed and well-characterized, and ultimately help raise the level of abstraction in the design. This design style leads to reduced design times and facilitates easier development of CAD (Computer Aided Design) tools for design and optimisation. These CAD tools allow the designer to choose among the various available primitive blocks and interconnect them to achieve the design functionality and performance. Semi-custom design styles are becoming the norm due to increasing design complexity. At the current level of circuit complexity, the loss in quality by using a semi-custom design style is often very small compared to a custom design style.
Semi-custom designs can be classified into two major classes: cell-based design and array-based design, which can further be subdivided into subclasses. Cell-based designs use libraries of predesigned cells or cell generators, which can synthesize cell layouts given their functional description. The predesigned cells can be characterized and optimised for the various process technologies that the library targets.
Cell based designs :
Cell based designs are based on standard-cell design, in which basic primitive cells are designed once and, thereafter, are available in a library for each process technology or foundry used. Each cell in the library is parameterised in terms of area, delay and power. Designers update these libraries whenever the foundry technology changes. CAD tools can then be used to map the design to the cells available in the library in a step known as technology mapping or library binding. Once the cells are selected, then are placed and wired together.
Another cell-based design style adopted by some designers uses cell generators to synthesize primitive building blocks, which are used for macro cell-based design. These generators were traditionally used for automatic synthesis of memories and programmable logic arrays (PLAs), although recently, module generators have been used to generate complex data path components such as multiplexers. Module generators for macro-cell generation are parameterizable, that is, they can be used to generate different instances of a module such as an 8 x 8 and a 16 x 8 multiplexer.
Array based designs :
In contrast to cell-based designs, array-based designs use a prefabricated matrix of non-connected components known as sites. These sites are wired together to create the circuit required. Array-based circuits can be either pre-diffused or pre-wired, commonly known as mask programmable and field programmable gate arrays respectively (MPGAs and FPGAs). In MPGAs, wafers consisting of arrays of unwired sites are manufactured and then the sites are programmed by connecting them with wires, via different routing layers during the chip fabrication process. There are several types of pre-diffused arrays, such as gate arrays, sea-of-gates and compacted arrays.
Unlike the MPGAs, pre-wired gate arrays or FPGAs are programmed outside the semi conductor foundry. FPGAs consist of programmable arrays of modules implementing generic logic. In the anti-fuse type of FPGAs, wires can be connected by programming the anti-fuses in the array. Anti-fuses are open circuit devices that become a short circuit when an appropriate current is applied to it. In this way, the circuit design required can be achieved by connecting the logic module inputs appropriately by programming the anti-fuses. On the other hand, memory-based FPGAs store the information about the interconnection and configuration of the various generic logic modules in memory elements inside the array.
The use of FPGAs is becoming more and more popular as the capacity of the arrays and their performance are improving. At present designers use them extensively for circuit prototyping and verification. Their relative ease of design and customisation leads to low cost and time overheads. However, FPGA is still an expensive technology since the number of gate arrays required to implement a moderately complex design is large. The cost per gate of prototype design is decreasing due to continuous density and capacity improvement in FPGA technology.
Selecting the right design style :
Several design styles are available to a designer and choosing a particular design rests upon trade-offs relating to cost, time-to-market, performance and reliability. In real-life applications nearly all designs are a mix of custom and semi-custom design styles, particularly those with cell-based styles. Depending on the application, designers adopt an approach of embedding some custom designed roadblocks inside a semi-custom design. This leads to lower overheads since only the critical parts of the design have to be handcrafted. For example, a microprocessor normally will have a custom designed data path and the control logic is synthesized using a standard cell-based technique. Given the complexity of microprocessors, recent efforts in CAD are attempting to automate the design process of data path blocks as well. Prototyping and circuit verification using FPGA-based technologies has become popular due to high costs and time over-runs involved, in case of a faulty design problem emerging once the chip is manufactured using other techniques.
Related topics :
• Front end design • Large chip designs • Standard cell design • Structured design
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