JTAG specification

- an overview of the JTAG specification and the IEEE 1149 standards that define JTAG, Boundary Scan test technology.

JTAG, boundary scan is now a well established technology which is widely sued for a variety of aspects of test within the electronics industry. The use of JTAG technology arose out of the need to be able to provide sufficient test access for every more complex boards while test access was reducing. As a result of this need JTAG technology arose and is now a firmly established technique.

The development of boundary scan, JTAG technology started in 1985 when the Joint Test Action Group (JTAG). The resulting solution devised by JTAG was the boundary scan technology for test. The resulting JTAG specification has since been widely used by many areas of the electronics industry, becoming a standard technique used by many manufacturers

Boundary scan, JTAG technology relies on using VLSI integrated circuits that have a boundary scan capability. As a result there is a need for standardisation across the electronics industry. In order to ensure this occurred, boundary scan was adopted by the Institute or Electrical and Electronics Engineers, IEEE in the USA as IEEE1149. The first issue of the boundary scan standard was released in 1990 and its stated purpose was to test the interconnections between integrated circuits mounted on boards, modules, hybrids and other substrates. Since then further revisions of the JTAG specification have taken place.

JTAG specification

In order to adopt the boundary scan, or JTAG test solution, the IEEE set up a number of committees or working groups to address the different aspects of the technology, and the resulting standards bear their numbers. In fact, Joint Test Action Group or JTAG is the usual name used for the IEEE 1149.1 standard entitled Standard Test Access Port and Boundary-Scan Architecture. The IEEE 1149 standard numbers are the ones that are quoted as the definitions for JTAG technology.

The committee numbers are given below:

• IEEE 1149.1:     this group addresses test for digital assemblies. This number is the one normally seen as that used for the JTAG specification.



• IEEE 1149.2:     the group has merged with IEEE 1149.1 group and is now obsolete.



• IEEE 1149.3:     group has become obsolete.



• IEEE 1149.4:     this group addresses test for mixed signal and analogue assemblies.



• IEEE 1149.5:     addresses system level test.



• IEEE 1532:     this is a derivative standard for in-system programming of digital devices.

Although IEEE 1149.1 is the most commonly used standard, i.e. the JTAG specification and it is often seen quoted in the literature, the others are also important in their own areas. As can be seen from the list, some groups working on different or allied aspects of the JTAG specification have completed their tasks and they have been merged or made obsolete.

JTAG specification summary

The JTAG specification as defined under IEEE 1149.1 is the one that is used by the electronics test industry. It is widely used and provides an excellent method of testing items from integrated circuits through to complete equipments that have limited test access.