Gate array
Encyclopedia
A gate array or uncommitted logic array (ULA) is an approach to the design and manufacture of application-specific integrated circuit
s (ASICs). A gate array circuit is a prefabricated silicon chip circuit with no particular function in which transistor
s, standard NAND
or NOR
logic gate
s, and other active devices are placed at regular predefined positions and manufactured on a wafer
, usually called a master slice. Creation of a circuit with a specified function is accomplished by adding a final surface layer or layers of metal interconnects to the chips on the master slice late in the manufacturing process, joining these elements to allow the function of the chip to be customized as desired. This layer is analogous to the copper layer(s) of a printed circuit board
(PCB).
Gate array master slices are usually prefabricated and stockpiled in large quantities regardless of customer orders. The design and fabrication according to the individual customer specifications may be finished in a shorter time compared with standard cell
or full custom
design. The gate array approach reduces the mask
costs since fewer custom masks need to be produced. In addition manufacturing test tooling lead time and costs are reduced since the same test fixtures may be used for all gate array products manufactured on the same die
size. Gate arrays were the predecessor of the more advanced structured ASIC; unlike gate arrays, structured ASICs tend to include predefined or configurable memories and/or analog blocks. Structured ASICs are still sold by companies such as ChipX, Inc.
An application circuit must be built on a gate array that has enough gates, wiring and I/O pins. Since requirements vary, gate arrays usually come in families, with larger members having more of all resources, but correspondingly more expensive. While the designer can fairly easily count how many gates and I/Os pins are needed, the amount of routing tracks needed may vary considerably even among designs with the same amount of logic. (For example, a crossbar switch
requires much more routing than a systolic array
with the same gate count.) Since unused routing tracks increase the cost (and decrease the performance) of the part without providing any benefit, gate array manufacturers try to provide just enough tracks so that most designs that will fit in terms of gates and I/O pins can be routed. This is determined by estimates such as those derived from Rent's rule
or by experiments with existing designs.
The main drawbacks of gate arrays are their somewhat lower density and performance compared with other approaches to ASIC design. However this style is often a viable approach for low production volumes.
Sinclair Research ported an enhanced ZX80
design to a ULA chip for the ZX81
, and later used a ULA in the ZX Spectrum
. A compatible chip was made in Russia as T34VG1. Acorn Computers
used several ULA chips in the BBC Micro
, and later managed to compress almost all of that machine's logic into a single ULA for the Acorn Electron
. Many other manufacturers from the time of the home computer
boom period used ULAs in their machines. Ferranti
in the UK pioneered ULA technology, then later abandoned this lead in semi-custom chips. The IBM PC
took over much of the personal computer market, and the sales volumes made full-custom chips more economical.
Designers still wished for a way to create their own complex chips without the expense of full-custom design, and eventually this wish was granted with the arrival of the field-programmable gate array
(FPGA), complex programmable logic device (CPLD), and structured ASIC. Whereas a ULA required a semiconductor wafer foundry to deposit and etch the interconnections, the FPGA and CPLD had programmable interconnections.
Application-specific integrated circuit
An application-specific integrated circuit is an integrated circuit 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...
s (ASICs). A gate array circuit is a prefabricated silicon chip circuit with no particular function in which transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s, standard NAND
Nand
NAND may stand for:*Nand , an Indian classical raga.*Logical NAND , a binary operation in logic.**NAND gate, an electronic gate that implements a logical NAND....
or NOR
Nor
Nor may refer to:*In grammar, nor is a coordinating conjunction*Nór, the eponymous founder-king of Norway in Norse mythology*Nor , a character in the book in Wicked...
logic gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...
s, and other active devices are placed at regular predefined positions and manufactured on a wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
, usually called a master slice. Creation of a circuit with a specified function is accomplished by adding a final surface layer or layers of metal interconnects to the chips on the master slice late in the manufacturing process, joining these elements to allow the function of the chip to be customized as desired. This layer is analogous to the copper layer(s) of a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
(PCB).
Gate array master slices are usually prefabricated and stockpiled in large quantities regardless of customer orders. The design and fabrication according to the individual customer specifications may be finished in a shorter time compared with standard cell
Standard cell
In semiconductor design, standard cell methodology is a method of designing application-specific integrated circuits with mostly digital-logic features. Standard cell methodology is an example of design abstraction, whereby a low-level very-large-scale integration layout is encapsulated into an...
or full custom
Full custom
Full-custom design is a methodology for designing integrated circuits by specifying the layout of each individual transistor and the interconnections between them...
design. The gate array approach reduces the mask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
costs since fewer custom masks need to be produced. In addition manufacturing test tooling lead time and costs are reduced since the same test fixtures may be used for all gate array products manufactured on the same die
Die (integrated circuit)
A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...
size. Gate arrays were the predecessor of the more advanced structured ASIC; unlike gate arrays, structured ASICs tend to include predefined or configurable memories and/or analog blocks. Structured ASICs are still sold by companies such as ChipX, Inc.
An application circuit must be built on a gate array that has enough gates, wiring and I/O pins. Since requirements vary, gate arrays usually come in families, with larger members having more of all resources, but correspondingly more expensive. While the designer can fairly easily count how many gates and I/Os pins are needed, the amount of routing tracks needed may vary considerably even among designs with the same amount of logic. (For example, a crossbar switch
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
requires much more routing than a systolic array
Systolic array
In computer architecture, a systolic array is a pipe network arrangement of processing units called cells. It is a specialized form of parallel computing, where cells , compute data and store it independently of each other.thumb|240px...
with the same gate count.) Since unused routing tracks increase the cost (and decrease the performance) of the part without providing any benefit, gate array manufacturers try to provide just enough tracks so that most designs that will fit in terms of gates and I/O pins can be routed. This is determined by estimates such as those derived from Rent's rule
Rent's Rule
Rent's rule pertains to the organization of computing logic, specifically the relationship between the number of external signal connections to a logic block with the number of logic gates in the logic block, and has been applied to circuits ranging from small digital circuits to mainframe...
or by experiments with existing designs.
The main drawbacks of gate arrays are their somewhat lower density and performance compared with other approaches to ASIC design. However this style is often a viable approach for low production volumes.
Sinclair Research ported an enhanced ZX80
Sinclair ZX80
The Sinclair ZX80 is a home computer brought to market in 1980 by Science of Cambridge Ltd. . It is notable for being the first computer available in the United Kingdom for less than a hundred pounds...
design to a ULA chip for the ZX81
Sinclair ZX81
The ZX81 was a home computer produced by Sinclair Research and manufactured in Scotland by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and was designed to be a low-cost introduction to home computing for the general public...
, and later used a ULA in the ZX Spectrum
ZX Spectrum
The ZX Spectrum is an 8-bit personal home computer released in the United Kingdom in 1982 by Sinclair Research Ltd...
. A compatible chip was made in Russia as T34VG1. Acorn Computers
Acorn Computers
Acorn Computers Ltd. was a British computer company established in Cambridge, England, in 1978. The company produced a number of computers which were especially popular in the UK. These included the Acorn Electron, the BBC Micro, and the Acorn Archimedes...
used several ULA chips in the BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
, and later managed to compress almost all of that machine's logic into a single ULA for the Acorn Electron
Acorn Electron
The Acorn Electron is a budget version of the BBC Micro educational/home computer made by Acorn Computers Ltd. It has 32 kilobytes of RAM, and its ROM includes BBC BASIC along with its operating system....
. Many other manufacturers from the time of the home computer
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
boom period used ULAs in their machines. Ferranti
Ferranti
Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. Known primarily for defence electronics, the Company was once a constituent of the FTSE 100 Index but ceased trading in 1993.The...
in the UK pioneered ULA technology, then later abandoned this lead in semi-custom chips. The IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
took over much of the personal computer market, and the sales volumes made full-custom chips more economical.
Designers still wished for a way to create their own complex chips without the expense of full-custom design, and eventually this wish was granted with the arrival of the field-programmable gate array
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
(FPGA), complex programmable logic device (CPLD), and structured ASIC. Whereas a ULA required a semiconductor wafer foundry to deposit and etch the interconnections, the FPGA and CPLD had programmable interconnections.
See also
- Field programmable gate array (FPGA)
- Complex programmable logic device (CPLD)
- Structured ASIC