JTAG, boundary scan is now a well established technology which is widely used in many areas 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 boundary scan technology was introduced and the JTAG spec or JTAG standard is now firmly established.
With the increase in complexity of electronics items in recent years, the JTAG spec has become the accepted test format for testing compact and complicated electronics units. In fact JTAG is often the only practicable method that can be used in many instances as access to many circuit nodes is not possible.
What is JTAG?
JTAG technology uses a technique that does not require the test system to have direct access to each node. The JTAG standard is able access the state of nodes within the circuit by passing a serial data stream into the item under test, and then reading the status of the stream as it exits the board.
To achieve this, the JTAG specification defines a shift register latch cell that is built into each external connection of every boundary scan or JTAG compatible device. Under normal operating conditions, i.e. when not being tested the cell remains transparent and does not affect the operation of the device. When used for boundary scan, JTAG testing the shift register is set to a mode where it can transfer data along to the next cell in the device. There are defined entry and exit points for the data to enter and exit the device, and it is therefore possible to chain several devices together. In this way boundary scan, JTAG can test individual ICs or complete boards (provided there are sufficient JTAG, boundary scan compatible devices on the board.)
When being used in JTAG, boundary scan test mode, it permits a serial data stream (test vector) to be passed from one shift register latch cell to the next. The boundary-scan cells in the devices capture data from integrated circuit line, or force data onto them. In this way a test system that can input a data stream to the shift register chain can set up states on the board, and also monitor data. By setting up one serial data stream, latching this into place, and then monitor the returning data stream, it is possible to gain access to the circuits on the board and check that a returning data stream is what is expected.
JTAG spec development
The development of boundary scan, JTAG technology started in 1985 when a group known as the Joint Test Action Group was set up. The initials for this group were soon shortened to JTAG and the name has since remained to describe this technology which is now firmly established.
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.
Obtaining JTAG specification or spec
In view of the importance of the JTAG spec, and its use by many manufacturers, test equipment vendors and others the specification has been adopted by the IEEE, Institute of Electrical and Electronic Engineers based in the USA.
If copies of the IEEE 1149.1 standard are required, they may be obtained from the following source:
IEEE Standards Department
445 Hoes Lane
P.O. Box 1331
Piscataway, NJ 08855-1331
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 committee 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 1149.6: The IEEE 1149.6 standard was approved in March 2003 and it extends the capability of IEEE 1149 to include AC coupled and differential nets.
- IEEE 1149.7: This defines the next generation Test Access Port, TAP.7.
- 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.
The JTAG specification as defined under the IEEE 1149.1 standard is the one that is used by the electronics test industry. It is widely used because it enables a much greater test coverage to be achieved than any other test technology, especially for assemblies where access to nodes is not possible. As a result the JTAG specification is used for testing items from individual integrated circuits up to complex electronics assemblies. In view of the fact that there are no other viable test technologies available for these circumstances, the JTAG specification will be in evidence for many years to come