A Definition of JTAG
JTAG is a standard for testing the interconnections between integrated circuit chips on a printed circuit board. The standard does not require direct physical contact with the interconnects themselves. It is a widely used quality-control method in modern electronic manufacturing and has made it possible to test increasingly miniaturized circuit boards.
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Testing prior to JTAG
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Prior to the adoption of the JTAG standard, integrated circuit interconnections had to be tested with devices that made direct electrical connections with the circuit board itself. Typical of these was the so-called "bed of nails" tester, which included many tiny probes which were positioned to make contact with specific printed interconnections. As printed circuits became smaller and more complex, however, the cost of building these testing devices increased dramatically, while at the same time they became more difficult to use.
Birth of JTAG
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In the 1980s the electronics industry formed the Joint Task Action Group (thus the acronym, JTAG) to design a standard language and architecture for testing integrated circuits using a technique called "boundary scanning." Boundary scanning involves the use of testing ports integrated into the circuit itself, through which signals can be passed to both sides of a chip interconnect, and includes comparison of the results. This allows the inference of shorted or open tracings between chips without requiring direct contact with the interconnects. The JTAG architecture became an international standard when it was incorporated into IEEE Std.-1149.1 in 1990.
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How JTAG Works
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The JTAG architecture specifies the use of an interface integrated into the printed circuit, which uses a four-wire serial connection called a TAP, or Test Access Port. This interface includes a Test Data In and Test Data Out wire, as well as Test Clock and Test Mode State wires that connect to a TAP controller included in the circuit. The TAP allows the connection of boundary scanning tools that compare input and output signals from the circuit and diagnose interconnection problems.
JTAG Applications
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JTAG was originally designed for use in quality control during the printed circuit manufacturing process. But its wide adoption has made it possible to use JTAG testing equipment for debugging of installed firmware, as well as the testing of circuits during the manufacture of final products. Electronics maintenance and repair staff are also increasingly using JTAG testing equipment to diagnose hardware and software problems at the end user's site.
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