Application-specific integrated circuit

A tray of application-specific integrated circuit (ASIC) chips

An application-specific integrated circuit (ASIC) /ˈeɪsɪk/, is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed to run in a digital voice recorder or a high-efficiency Bitcoin miner is an ASIC. Application-specific standard products (ASSPs) are intermediate between ASICs and industry standard integrated circuits like the 7400 or the 4000 series.

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 microprocessors, memory blocks including ROM, 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 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 or lower production volumes, FPGAs may be more cost effective than an ASIC design even in production. The non-recurring engineering (NRE) cost of an ASIC can run into the millions of dollars.

History

The initial ASICs used gate array technology. An early successful commercial application was the gate array circuitry found in the 8-bit ZX81 and ZX Spectrum low-end personal computers, introduced in 1981 and 1982. These were used by Sinclair Research (UK) essentially as a low-cost I/O solution aimed at handling the computer's graphics.

Customization occurred by varying the metal interconnect mask. Gate arrays 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.

Standard-cell designs

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, which 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 non-recurring engineering and recurring component cost.

By the late 1990s, logic synthesis tools became available. Such tools could compile HDL descriptions into a gate-level netlist. Standard-cell integrated circuits (ICs) are designed in the following conceptual stages, although these stages overlap significantly in practice:

  1. A team of design engineers starts with a non-formal understanding of the required functions for a new ASIC, usually derived from requirements analysis.
  2. The design team constructs a description of an ASIC (application specific integrated circuits) to achieve these goals using an HDL. This process is analogous to writing a computer program in a high-level language. This is usually called the RTL (register-transfer level) design.
  3. Suitability for purpose is verified by functional verification. This may include such techniques as logic simulation, formal verification, emulation, or creating an equivalent pure software model (see Simics, for example). Each technique has advantages and disadvantages, and often several methods are used.
  4. Logic synthesis transforms the RTL design into a large collection of lower-level constructs called standard cells. These constructs are taken from a standard-cell library consisting of pre-characterized collections of gates (such as 2 input nor, 2 input nand, inverters, etc.). The standard cells are typically specific to the planned manufacturer of the ASIC. The resulting collection of standard cells, plus the needed electrical connections between them, is called a gate-level netlist.
  5. The gate-level netlist is next processed by a placement tool which places the standard cells onto a region representing the final ASIC. It attempts to find a placement of the standard cells, subject to a variety of specified constraints.
  6. The routing tool takes the physical placement of the standard cells and uses the netlist to create the electrical connections between them. Since the search space is large, this process will produce a “sufficient” rather than “globally optimal” solution. The output is a file which can be used to create a set of photomasks enabling a semiconductor fabrication facility (commonly called a 'fab') to produce physical ICs.
  7. Given the final layout, circuit extraction computes the parasitic resistances and capacitances. In the case of a digital circuit, this will then be further mapped into delay information, from which the circuit performance can be estimated, usually by static timing analysis. This, and other final tests such as design rule checking and power analysis (collectively called signoff) are intended to ensure that the device will function correctly over all extremes of the process, voltage and temperature. When this testing is complete the photomask information is released for chip fabrication.

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.

The 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 design

Microscope photograph of a gate-array ASIC showing the predefined logic cells and custom interconnections. This particular design uses less than 20% of available logic gates.

Gate-array design is a manufacturing method in which the diffused layers, i.e. transistors and other active devices, are predefined and wafers containing such devices are held in stock prior to metallization—in other words, unconnected. The physical design process then defines the interconnections of the final device. For most ASIC manufacturers, this consists of from two to as many as nine metal layers, each metal layer running perpendicular to the one below it. Non-recurring engineering costs are much lower, as photolithographic masks are required only for the metal layers, and production cycles are much shorter, as metallization is a comparatively quick process.

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, having been 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, only 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 engineers more commonly use the term "semi-custom", while "gate-array" is more commonly used by logic (or gate-level) designers.

Full-custom design

Microscope photograph of custom ASIC (486 chipset) showing gate-based design on top and custom circuitry on bottom

By contrast, full-custom ASIC design defines all the photolithographic 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.

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 design

Structured ASIC design (also referred to as "platform ASIC design"), is a relatively new term in the 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). One definition states that

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.

This is effectively the same definition as a gate array. What makes a structured ASIC different 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; these are predefined in most structured ASICs and therefore can save time and expense for the designer compared to gate-array. 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 that customized tools for their device (e.g., custom physical synthesis) be used, also allowing for the design to be brought into manufacturing more quickly.

Cell libraries, IP-based design, hard and soft macros

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 a non-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" 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 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. Indeed, the wide range of functions now available 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, its re-use and further development cuts product cycle times dramatically and creates better products. Additionally, organizations such as OpenCores are collecting free IP cores, paralleling the open 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.

Multi-project wafers

Some manufacturers offer multi-project wafers (MPW) as a method of obtaining low cost prototypes. Often called shuttles, these MPW, containing several designs, run at regular, scheduled intervals on a "cut and go" basis, usually with very little liability on the part of the manufacturer. The contract involves 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.

See also

References

    Sources

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