Anapplication-specific integrated circuit (ASIC/ˈeɪsɪk/) is anintegrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in adigital voice recorder or a high-efficiencyvideo codec.[1]Application-specific standard product chips are intermediate between ASICs and industry standard integrated circuits like the7400 series or the4000 series.[2] ASIC chips are typicallyfabricated usingmetal–oxide–semiconductor (MOS) technology, asMOS integrated circuit chips.[3]
As feature sizes have shrunk andchip design tools improved over the years, the maximum complexity (and hence functionality) possible in an ASIC has grown from 5,000logic gates to over 100 million. Modern ASICs often include entiremicroprocessors,memory blocks includingROM,RAM,EEPROM,flash memory and other large building blocks. Such an ASIC is often termed a SoC (system-on-chip). Designers of digital ASICs often use ahardware description language (HDL), such asVerilog orVHDL, to describe the functionality of ASICs.[2]
Field-programmable gate arrays (FPGA) are the modern-day technology improvement onbreadboards, meaning that they are not made to be application-specific as opposed to ASICs. Programmablelogic blocks and programmable interconnects allow the same FPGA to be used in many different applications. For smaller designs or lower production volumes, FPGAs may be more cost-effective than an ASIC design, even in production. Thenon-recurring engineering (NRE) cost of an ASIC can run into the millions of dollars. Therefore, device manufacturers typically prefer FPGAsfor prototyping and devices with low production volume and ASICs forvery large production volumes where NRE costs can beamortized across many devices.[4]
Early ASICs usedgate array technology. By 1967,Ferranti and Interdesign were manufacturing earlybipolar gate arrays. In 1967,Fairchild Semiconductor introduced the Micromatrix family of bipolardiode–transistor logic (DTL) andtransistor–transistor logic (TTL) arrays.[3]
Complementary metal–oxide–semiconductor (CMOS) technology opened the door to the broad commercialization of gate arrays. The first CMOS gate arrays were developed by Robert Lipp,[5][6] in 1974 for International Microcircuits, Inc. (IMI).[3]
Metal–oxide–semiconductor (MOS)standard-cell technology was introduced by Fairchild andMotorola, under the trade names Micromosaic and Polycell, in the 1970s. This technology was later successfully commercialized byVLSI Technology (founded 1979) andLSI Logic (1981).[3]
A successful commercial application ofgate array circuitry was found in the low-end 8-bitZX81 andZX Spectrumpersonal computers, introduced in 1981 and 1982. These were used bySinclair Research (UK) essentially as a low-costI/O solution aimed at handling thecomputer's graphics.
Customization occurred by varying a metal interconnect mask. Gate arrays had complexities of up to a few thousand gates; this is now calledmid-scale integration. Later versions became more generalized, with differentbase dies customized by both metal andpolysilicon layers. Some base dies also includerandom-access memory (RAM) elements.
In the mid-1980s, a designer would choose an ASIC manufacturer and implement their design using the design tools available from the manufacturer. While third-party design tools were available, there was not an effective link from the third-party design tools to thelayout and actual semiconductor process performance characteristics of the various ASIC manufacturers. Most designers used factory-specific tools to complete the implementation of their designs. A solution to this problem, which also yielded a much higher density device, was the implementation ofstandard cells.[7] Every ASIC manufacturer could create functional blocks with known electrical characteristics, such aspropagation delay, capacitance and inductance, that could also be represented in third-party tools. Standard-cell design is the utilization of these functional blocks to achieve very high gate density and good electrical performance. Standard-cell design is intermediate between§ Gate-array and semi-custom design and§ Full-custom design in terms of its non-recurring engineering and recurring component costs as well as performance and speed of development (includingtime to market).
By the late 1990s,logic synthesis tools became available. Such tools could compileHDL descriptions into a gate-levelnetlist. Standard-cellintegrated circuits (ICs) are designed in the following conceptual stages referred to aselectronics design flow, although these stages overlap significantly in practice:
These steps, implemented with a level of skill common in the industry, almost always produce a final device that correctly implements the original design, unless flaws are later introduced by the physical fabrication process.[8]
The design steps also calleddesign flow, are also common to standard product design. The significant difference is that standard-cell design uses the manufacturer's cell libraries that have been used in potentially hundreds of other design implementations and therefore are of much lower risk than a full custom design. Standard cells produce adesign density that is cost-effective, and they can also integrateIP cores andstatic random-access memory (SRAM) effectively, unlike gate arrays.
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Gate array design is a manufacturing method in which diffused layers,[9] each consisting oftransistors and otheractive devices, are predefined andelectronics wafers containing such devices are "held in stock" or unconnected prior to themetallization stage of thefabrication process. Thephysical design process defines the interconnections of these layers for the final device. For most ASIC manufacturers, this consists of between two and nine metal layers with each layer running perpendicular to the one below it. Non-recurring engineering costs are much lower than full custom designs, asphotolithographic masks are required only for the metal layers. Production cycles are much shorter, as metallization is a comparatively quick process; thereby acceleratingtime to market.
Gate-array ASICs are always a compromise between rapid design andperformance as mapping a given design onto what a manufacturer held as a stock wafer never gives 100%circuit utilization. Often difficulties inrouting the interconnect require migration onto a larger array device with a consequent increase in the piece part price. These difficulties are often a result of the layoutEDA software used to develop the interconnect.
Pure, logic-only gate-array design is rarely implemented by circuit designers today, having been almost entirely replaced byfield-programmable devices. The most prominent of such devices arefield-programmable gate arrays (FPGAs) which can be programmed by the user and thus offer minimal tooling charges, non-recurring engineering, only marginally increased piece part cost, and comparable performance.
Today, gate arrays are evolving intostructured ASICs that consist of a largeIP core like aCPU,digital signal processor units,peripherals, standardinterfaces, integratedmemories,SRAM, and a block ofreconfigurable, uncommitted logic. This shift is largely because ASIC devices are capable of integrating large blocks ofsystem functionality, andsystems on a chip (SoCs) requireglue logic,communications subsystems (such asnetworks on chip),peripherals, and other components rather than onlyfunctional units and basic interconnection.
In their frequent usages in the field, the terms "gate array" and "semi-custom" are synonymous when referring to ASICs.Process engineers more commonly use the term "semi-custom", while "gate-array" is more commonly used by logic (or gate-level) designers.
By contrast, full-custom ASIC design defines all the photolithographic layers of the device.[7] Full-custom design is used for both ASIC design and for standard product design.
The benefits of full-custom design include reduced area (and therefore recurring component cost),performance improvements, and also the ability to integrateanalog components and otherpre-designed—and thus fully verified—components, such asmicroprocessor cores, that form asystem on a chip.
The disadvantages of full-custom design can include increased manufacturing and design time, increased non-recurring engineering costs, more complexity in thecomputer-aided design (CAD) andelectronic design automation systems, and a much higher skill requirement on the part of the design team.
For digital-only designs, however, "standard-cell" cell libraries, together with modern CAD systems, can offer considerable performance/cost benefits with low risk. Automated layout tools are quick and easy to use and also offer the possibility to "hand-tweak" or manually optimize any performance-limiting aspect of the design.
This is designed by using basic logic gates, circuits or layout specially for a design.
Structured ASIC design (also referred to as "platform ASIC design") is a relatively new trend in the semiconductor industry, resulting in some variation in its definition. However, the basic premise of a structured ASIC is that both manufacturing cycle time and design cycle time are reduced compared to cell-based ASIC, by virtue of there being pre-defined metal layers (thus reducing manufacturing time) and pre-characterization of what is on the silicon (thus reducing design cycle time).
Definition from Foundations of Embedded Systems states that:[10]
In a "structured ASIC" design, the logic mask-layers of a device are predefined by the ASIC vendor (or in some cases by a third party). Design differentiation and customization is achieved by creating custom metal layers that create custom connections between predefined lower-layer logic elements. "Structured ASIC" technology is seen as bridging the gap between field-programmable gate arrays and "standard-cell" ASIC designs. Because only a small number of chip layers must be custom-produced, "structured ASIC" designs have much smaller non-recurring expenditures (NRE) than "standard-cell" or "full-custom" chips, which require that a full mask set be produced for every design.
— Foundations of Embedded Systems
This is effectively the same definition as a gate array. What distinguishes a structured ASIC from a gate array is that in a gate array, the predefined metal layers serve to make manufacturing turnaround faster. In a structured ASIC, the use of predefined metallization is primarily to reduce cost of the mask sets as well as making the design cycle time significantly shorter.
For example, in a cell-based or gate-array design the user must often design power, clock, and test structures themselves. By contrast, these are predefined in most structured ASICs and therefore can save time and expense for the designer compared to gate-array based designs. Likewise, the design tools used for structured ASIC can be substantially lower cost and easier (faster) to use than cell-based tools, because they do not have to perform all the functions that cell-based tools do. In some cases, the structured ASIC vendor requires customized tools for their device (e.g., custom physical synthesis) be used, also allowing for the design to be brought into manufacturing more quickly.
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Cell libraries of logical primitives are usually provided by the device manufacturer as part of the service. Although they will incur no additional cost, their release will be covered by the terms of anon-disclosure agreement (NDA) and they will be regarded as intellectual property by the manufacturer. Usually, their physical design will be pre-defined so they could be termed "hard macros".
What most engineers understand as "intellectual property" areIP cores, designs purchased from a third-party as sub-components of a larger ASIC. They may be provided in the form of ahardware description language (often termed a "soft macro"), or as a fully routed design that could be printed directly onto an ASIC's mask (often termed a "hard macro"). Many organizations now sell such pre-designed cores – CPUs, Ethernet, USB or telephone interfaces – and larger organizations may have an entire department or division to produce cores for the rest of the organization. The companyARMonly sells IP cores, making it afabless manufacturer.
Indeed, the wide range of functions now available in structured ASIC design is a result of the phenomenal improvement in electronics in the late 1990s and early 2000s; as a core takes a lot of time and investment to create, itsre-use and further development cuts product cycle times dramatically and creates better products. Additionally,open-source hardware organizations such asOpenCores are collecting free IP cores, paralleling theopen-source software movement in hardware design.
Soft macros are often process-independent (i.e. they can be fabricated on a wide range of manufacturing processes and different manufacturers). Hard macros are process-limited and usually further design effort must be invested to migrate (port) to a different process or manufacturer.
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Some manufacturers and IC design houses offermulti-project wafer service (MPW) as a method of obtaining low cost prototypes. Often called shuttles, these MPWs, containing several designs, run at regular, scheduled intervals on a "cut and go" basis, usually with limited liability on the part of the manufacturer. The contract involves delivery of bare dies or the assembly and packaging of a handful of devices. The service usually involves the supply of a physical design database (i.e. masking information or pattern generation (PG) tape). The manufacturer is often referred to as a "silicon foundry" due to the low involvement it has in the process.
Anapplication-specific standard product orASSP is anintegrated circuit that implements a specificfunction that appeals to a wide market. As opposed to ASICs that combine a collection of functions and are designed by or for onecustomer, ASSPs are available as off-the-shelf components. ASSPs are used in all industries, from automotive to communications.[11]
For example, two ICs that might or might not be considered ASICs are a controller chip for a PC and a chip for amodem. Both of these examples are specific to an application (which is typical of an ASIC) but are sold to many different system vendors (which is typical of standard parts). ASICs such as these are sometimes called application-specific standard products (ASSPs).
Examples of ASSPs are encoding/decoding chip, Ethernet network interface controller chip and flash memory controller chip.[12][irrelevant citation]
{{cite book}}:|website= ignored (help)In the UK, Ferranti, with their bipolar collector diffused isolation (CDI) arrays, pioneered the commercial use of gate arrays and for many years this was by far the most widely used technology.
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