An application-specific integrated circuit (ASIC) is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC.
In contrast, the 7400 series and 4000 series integrated circuits are logic building blocks that can be wired together for use in many different applications. Intermediate between ASICs and standard products are application specific standard products (ASSPs).
As feature sizes have shrunk and design tools improved over the years, the maximum complexity (and hence functionality) possible in an ASIC has grown from 5,000 gates to over 100 million. Modern ASICs often include entire 32-bit processors, memory blocks including ROM, RAM, EEPROM, Flash and other large building blocks. Such an ASIC is often termed a SoC (System-on-a-chip). Designers of digital ASICs use a hardware description language (HDL), such as Verilog or VHDL, to describe the functionality of ASICs.
Field-programmable gate arrays (FPGA) are the modern-day technology for building a breadboard or prototype from standard parts; programmable logic blocks and programmable interconnects allow the same FPGA to be used in many different applications. For smaller designs and/or lower production volumes, FPGAs may be more cost effective than an ASIC design even in production. The non-recurring engineering cost of an ASIC can run into the millions of dollars.
The initial ASICs used gate array technology. Ferranti produced perhaps the first gate-array, the ULA (Uncommitted Logic Array), around 1980. A successful commercial example is the ULA found in the 8 bit personal computer ZX Spectrum from SINCLAIR RESEARCH (UK) essentially aimed at handling the computer's graphics. Customization occurred by varying the metal interconnect mask. ULAs had complexities of up to a few thousand gates. Later versions became more generalized, with different base dies customised by both metal and polysilicon layers. Some base dies include 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 the layout and actual semiconductor process performance characteristics of the various ASIC manufacturers. Most designers ended up using factory specific tools to complete the implementation of their designs. A solution to this problem that also yielded a much higher density device was the implementation of Standard Cells. Every ASIC manufacturer could create functional blocks with known electrical characteristics, such as propagation 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 fits between Gate Array and Full Custom design in terms of both its NRE (Non-Recurring Engineering) and recurring component cost.
By the late 1980s, logic synthesis tools became available. Such tools could compile HDL descriptions into a gate-level netlist. This enabled a style of design called standard-cell design. Standard-cell Integrated Circuits (ICs) are designed in the following conceptual stages, 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.
These design steps (or 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 full custom design. Standard Cells produce a design density that is cost effective, and they can also integrate IP cores and SRAM (Static Random Access Memory) effectively, unlike Gate Arrays.
Gate array ASICs are always a compromise as mapping a given design onto what a manufacturer held as a stock wafer never gives 100% utilization. Often difficulties in routing the interconnect require migration onto a larger array device with consequent increase in the piece part price. These difficulties are often a result of the layout software used to develop the interconnect.
Pure, logic-only gate array design is rarely implemented by circuit designers today, replaced almost entirely by field-programmable devices, such as field-programmable gate arrays (FPGAs), which can be programmed by the user and thus offer minimal tooling charges (non-recurring engineering (NRE)), marginally increased piece part cost and comparable performance. Today gate arrays are evolving into structured ASICs that consist of a large IP core like a CPU, DSP unit, peripherals, standard interfaces, integrated memories SRAM, and a block of reconfigurable uncommited logic. This shift is largely because ASIC devices are capable of integrating such large blocks of system functionality and "system on a chip" requires far more than just logic blocks.
In their frequent usages in the field, the terms "gate array" and "semi-custom" are synonymous. Process engineer 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 photo lithographic layers of the device. Full-custom design is used for both ASIC design and for standard product design.
The benefits of full-custom design usually include reduced area (and therefore recurring component cost), performance improvements, and also the ability to integrate analog components and other pre-designed (and thus fully verified) components such as microprocessor cores that form a system-on-chip.
The disadvantages of full-custom design can include increased manufacturing and design time, increased non-recurring engineering costs, more complexity in the computer-aided design (CAD) system and a much higher skill requirement on the part of the design team.
However for digital-only designs, "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 optimise any performance-limiting aspect of the design.
This is effectively the same definition as a gate array.
What makes a structured/platform ASIC different from a gate array is that in a gate array the predefined metal layers serve to make manufacturing turnaround faster. In a structured/platform ASIC the predefined metallization is primarily to reduce cost of the mask sets and is also used to make the design cycle time significantly shorter as well. For example, in a cell-based or gate-array design the user often must design power, clock, and test structures themselves; these are predefined in most Structured/Platform ASICs and therefore can save time and expense for the designer compared to gate-array. Likewise, the design tools used for structured/Pptform ASIC can be substantially lower cost and easier (faster) to use than cell-based tools, because the tools do not have to perform all the functions that cell-based tools do. In some cases, the structured/platform ASIC vendor requires that customized tools for their device (for example, custom physical synthesis) be used, also allowing for the design to be brought into manufacturing more quickly. ChipX, Inc. eAsic, and Triad Semiconductor are examples of vendors offering this kind of structured ASIC.
One other important aspect about structured/platform ASIC is that it allows IP that is common to certain applications or industry segments to be "built in", rather than "designed in". By building the IP directly into the architecture the designer can again save both time and money compared to designing IP into a cell-based ASIC.
The Altera technique of producing a structured cell ASIC where the cells are the same design as the FPGA, but the programmable routing is replaced with fixed wire interconnect is called HardCopy. These devices then do not need re-programming and cannot be re-programmed as an FPGA.
The Xilinx technique of producing a customer specific FPGA, that is 30% - 70% less expensive than a standard FPGA and where the cells are the same as the FPGA but the programmable capability is removed, is called EasyPath.
What most engineers understand as "intellectual property" are IP cores, designs purchased from a third party as sub-components of a larger ASIC. They may be provided as an HDL description (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 IP, and larger organizations may have an entire department or division to produce such IP for the rest of the organization. For example, one can purchase CPUs, ethernet, USB or telephone interfaces. Indeed, the wide range of functions now available is a significant factor in the phenomenal increase in electronics in the late 1990s and early 2000s; as intellectual property takes a lot of time and investment to create, its re-use and further development cuts product cycle times dramatically and creates better products.
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.
There are two different types of ASIC suppliers, IDM and fabless. An IDM supplier's ASIC product is based in large part on proprietary technology such as design tools, IP, packaging, and usually although not necessarily the process technology. Fabless ASIC suppliers rely almost exclusively on outside suppliers for their technology. The classfication can be confusing since several IDM's are also fabless semiconductor companies.