Practically the entire electronics world is familiar with the venerable JTAG, or IEEE 1149.1 standard. It was developed during an era when disco music and CB radios were fading from the scene and the age of microelectronics was dawning. In the early 1980's most electronics assemblies consisted of multilayer circuit boards with through-hole, dual in-line package (DIP) devices that were soldered into place. Testing was performed by means of "bed of nails" testers that placed a tester node on each net of the circuit board.
The first wave of miniaturization was in the form of surface mount technology (SMT) packages which were soldered on both sides of the board, prohibiting the bed-of-nails approach for direct testing. Test engineers scratched their head at how to get access to all the test points without physical access. Semiconductor companies banded together to form a joint test action group to study this problem and devise a solution. The solution they eventually arrived at became familiarly known as JTAG, and formally known as 1149.1 after ratification of the approach as an IEEE standard.
JTAG was simple. Based on an easy to understand 4-bit state machine, every engineering student since 1985 could code up a JTAG controller for an interview test. As an IEEE standard, it became ubiquitous and a feature on every chip to allow board testing in ever shrinking and congested boards. Over the years, clever engineers figured new and creative ways to use the JTAG standard for purposes other than board test. For example, it became the defacto interface for chip level debug and for connecting the trace interfaces of microprocessors to debuggers. This made a lot of sense because JTAG was expensive for chips in that it cost 4-5 pins. Over time JTAG became even more overloaded by adding "side band" signals to allow for functions that could not be incorporated in standard JTAG protocol which caused the IEEE standard aspects of JTAG to be weakened.
The electronics community has been busily working for the last few years under the wraps of a new 1149 working group to modernize JTAG into a new standard-- 1149.7, or informally known as CJTAG (compact JTAG).
CJTAG is a major step improvement to the original standard, preserving all the functionality of original JTAG standard and extending it to cover all the current test, debug, and trace capabilities while reducing the pin count to 2 and making changes to the protocol to allow new capabilities to be added without the need for adding non-standard side-band signals.
For those of you interested in reading more about CJTAG, there are a lot of web resources available for you:
- The IEEE news release and access to the standard can be found here
- Information from TI about 1149.7 in their products can be found here
- Information about debug and test equipment from Lauterbach and Sophia Systems
- Information about verification IP from Globetech here
- Information about IP cores co-developed with TI and available from IPextreme can be be found here
The whole semiconductor industry is looking forward to extending the legacy of JTAG another 25 years.
