Application specific integrated circuit (ASIC)

Saturday, 13 January 2007

An application specific integrated circuit (ASIC) is a microelectronic integrated circuit chip designed specifically for an application or product, such as a computer system. Integrated circuits have widespread applications in electronic systems of every variety. An integrated circuit is comprised of thousands of transistors fabricated on a monolithic crystalline lattice typically comprised of silicon or other semiconductor material. The transistors are selectively interconnected through the use of one or more conductive interconnect layers to achieve a particular functionality. For some well-known applications such as the more popular microprocessors and memory devices, a single device is suitable for a large number of applications and a large number of customers. Such devices are commonly called commodity integrated circuits. Because commodity products can be sold in large quantities to a large number of customers, their relatively large one time or non-recurring engineering costs associated with the dedicated mask sets is typically justified by the extremely large number of units over which these non-recurring costs can be spread. In contrast to the commodity products, application specific or customer specific integrated circuits refer to devices that are customized for the needs of a particular application or a particular customer. Application or customer specific integrated circuits are typically limited in the number of customers or the quantity in which the devices will be sold. The integrated circuit business, however, includes a significant market for producing custom integrated circuits for various applications. The demand for custom integrated circuits is rapidly increasing because of a dramatic growth in the demand for highly specific consumer electronics and a trend towards increased product functionality. Also, the use of custom integrated circuits is advantageous because they reduce system complexity and, therefore, lower manufacturing costs, increase reliability and increase system performance. Integrated circuits designed for a specific application are typically designed to perform a single function and cannot perform diverse functions. There are numerous types of custom integrated circuits. Application specific integrated circuits (ASICs) and programmable logic devices (PLDs) represent two distinct types of integrated circuits (ICs) that perform specific logic operations. The programmable logic devices (PLDs) include field programmable gate arrays (FPGAs). FPGAs are designed to be programmed by the end user using special-purpose equipment. FPGAs are field programmable devices that employ programmable gates to allow various configurations and can be reprogrammed in the field. This provides the ability to determine and correct any errors which may not have been detectable prior to use. Additionally, the FPGAs may be reprogrammed to accommodate new devices connected thereto. The ability to program has the advantage of providing design flexibility and faster implementation during the system development effort. Programmable logic devices are, however, undesirable for many applications because they operate at relatively slow speeds, have relatively low capacity, and have relatively high cost per chip. Although FPGAs are flexible, allow for faster implementation, and have low unit costs, there are limits to the function complexity and the I/O capability.

Application specific integrated circuits (ASIC) is another commonly used type of custom integrated circuits. In particular, ASICs are customizable integrated circuits that are customized to implement a circuit specified by a design engineer. The term "ASIC" actually refers to a variety of integrated circuit (IC) styles that vary in degree of customizability, including standard cells, gate arrays, and FPGAs. The advantages of high density integrated circuit technology were initially unavailable for use in designs that had low production runs. The introduction of application specific integrated circuits (ASICs) and gate arrays lowered the number of devices a single design had to produce to be economical. This was accomplished by standardizing cells of transistors and logic gates and constraining users to define the required interconnections necessary to realize the desired functionality. Application specific integrated circuits provide electronic designers with the ability to customize circuits by integrating complex functionality and input/output (I/O) on a single integrated circuit (IC). ASICs are distinguished from PLDs in that ASICs are customized during the design and fabrication process to implement a user's logic function, while PLDs are provided with programmable circuitry that is configured (programmed) by a user after the PLD is fabricated. Use of ASICs has become widespread in the semiconductor industry as giving circuit design engineers a relatively high amount of functionality in a relatively small package. ASICs are used extensively throughout digital computers and other types of electronic circuits. In the semiconductor industry, ASIC memories have gained enormous popularity in integrated circuits (ICs) designs. ASICs allow custom or semicustom design of ICs in shorter turn-around time while reducing the total component count and manufacturing costs. ASICs also interface between a digital computer system's bus and a peripheral device, such as a disk drive, a printer, a scanner, a tape drive, a CD ROM drive or an optical storage drive. Application specific integrated circuits are used in computer display controller cards, video on demand set-top boxes, communication network systems such as Ethernet hubs, switches and routers, industrial embedded controllers such as those used in automobiles, process monitoring and control, portable and cellular telephones, games and household appliances, as well as special purpose systems used to access the Internet. ASIC chips are regarded in the industry as being very fast network interface devices, providing a high performance guarantee, as compared to FPGAs. ASIC system logic controller (SLC) ICs perform various functions essential to a digital computer's operation including, in many instances, interfacing between a high-speed CPU bus and a slower speed Input/output (I/O) bus. Similarly, an ASIC super I/O IC, which is coupled to a digital computer's slower speed I/O bus, provides one or more serial ports, one or more parallel ports, a floppy diskette drive controller, and an interface for an integrated drive electronics (IDE) hard disk drive. Microcontroller based application specific integrated circuits are used in power delivery components such as circuit breakers. In circuit breaker applications, the ASICs are programmed to control opening and closing of a power switch, or relay, electrically connected to designated branches of the power delivery network.

Application specific integrated circuit (ASIC) chip technology has undergone rapid changes in recent years. Many different ASIC technologies are available. The ASIC technology includes two major categories: array-based and cell-based. Array-based ASICs configure a customer's design at the metal layers, whereas cell-based ASICs are uniquely fabricated at all layers of the silicon process including the diffusion layers. Array-based ASIC products include an array of repeating identical base cells (or gates) in a core region of a semiconductor chip. Each of such base cells contains the same predetermined number and arrangement of transistors. Gate arrays are integrated circuits having a fixed number of identical sites arranged in a regular manner, each site comprising a number of simple circuit elements. The circuit elements are configured in a manner which enables easy implementation of common logic circuits such as transfer gates, inverters, NAND gates, etc., depending upon how they are connected. A gate array can then be used to implement a specific circuit design simply by specifying the interconnection between the otherwise standard circuit elements. The gate array generally includes an array of function blocks, each of which are predesigned and/or prefabricated to include a particular number, arrangement, and type of semiconductor devices, e.g., transistors. To customize a gate array to implement a particular user-designed circuit specified by a design engineer, various connections are made among the semiconductor devices within the function block and/or various connections are made among function blocks. An array based application specific integrated circuit with tens of thousands to hundreds of thousands of transistors on a single chip that may be interconnected through several metallization layers according to customer specifications. Array based ASICs offer high logic density and fast customized implementation of a customer's logic due to the few number of masks required. As such, array based ASICs have moderate costs and turnarounds, especially when design changes and enhancements are so predominant in the electronics industry. Another approach to the design of ASIC integrated circuits is the use of standard cells. Like gate arrays, standard cell designs rely on a set of predefined circuit elements called standard cells with which to implement the design. Standard cells have been stored in a library set and are retrieved from the library set as specified by the netlist of the design. Standard cells comprise a library of functions ranging from primitive functions such as AND and OR gates to more comples functions such as random access memory (RAM). The designer designs the chip by placing and interconnecting the pre-defined library of functions. Cell-based ASICs generally employ many different base cells, as opposed to a single repeating base cell. Typically, these various base cells have different sizes, shapes, transistor arrangements, etc., and the cells can be located anywhere on the die in accordance with a given circuit design. Unlike gate arrays, the complexity of standard cells can range from simple logic gates such as those found in gate arrays to block-level components such as RAMs, ROMs, PLAs and Maga-cells such as CPU cores. Cell-based ASICs use available chip space more efficiently than array-based ASICs. Cell-based ASICs, however, are fabricated at all layers of the silicon process including the diffusion layers, and thus one design is very difficult to modify to perform another function or customer design. The ASIC technology also includes application specific standard products (ASSPs) and embedded arrays. ASSPs are also full custom chips, but they have been designed and defined by the ASSP vendor, rather than by a particular customer. Embedded array technology is a hybrid of array-based and cell-based architectures.

An ASIC forms, in a single, semi-conductor substrate, the equivalent of several different integrated circuits each designed to perform one or more desired operations, such as a microprocessor operation, a memory operation, various interface operations, etc. In general, an application specific integrated circuit (ASIC) device has two main types of circuits, namely, primary input/output circuits and core circuits. The primary input/output circuits enable the application specific integrated circuit to communicate with other electronic components located within an electronic system. The core circuits perform various functions for which the ASIC device is intended, such as data processing, data computations, controls, etc. With the development of sub-micron technology processes, very large functions can be incorporated within a single application specific integrated circuit device. Application specific integrated circuits are typically designed by combining pre-defined, standard functional blocks called core cells from a variety of sources with discrete logic to perform a desired function or group of functions. ASIC operational blocks are typically designed to include thousands of individual logic gates necessary for performing the desired operations. These logic gates are formed in a substrate layer according to known semi-conductor fabrication techniques. The operational logic gates can assume a wide variety of forms, and include varying circuitry configurations. Once formed, the various operational logic gates are subsequently interconnected by wiring routed into metal layers formed on top of the substrate. Application specific integrated circuits commonly contain a multiplicity of registers for the storage of data, which may be message data or control data. It is also common place to provide an addressable multiplexing and demultiplexing system which is addressable by means of, for example, a central processing unit so that data may be written in to or read out of such registers. ASIC chip contains single or multiple configurable memory arrays with row memory lines intersecting column memory lines. A plurality of column memory lines can be grouped together to form an I/O (input/output) memory block. Application specific integrated circuits often have an internally generated clock that activates the internal logic of the ASIC. Internally generated clocks are generated from an external functional clock. Frequently, the frequency of the functional clock is reduced using a frequency divider. The division is performed to accommodate specific internal clock requirements.

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