ElectronicsWeekly.com, UK - Mar 28, 2008

For many the term ‘JTAG’ is still a point of confusion, as well it might since, for some engineers, it is a programming port whilst for others it is there to plug-in an emulator or debugger. In fact it was devised for neither of these purposes.

The acronym JTAG is actually short for the Joint Test Action Group and its initial aims were just that, to provide an aid to circuit board testing.

The ‘group’ met over a period of some five years, from 1985 to 1990, all the time devising and then finalising a scheme to embed test circuitry into digital devices to assist in the structural test of the PCBs onto which they would be placed. So far, so good.

Since then the structure of JTAG as a test port has been well documented and understood. It has been a recognised IEEE standard (reference 1149.1) since 1990 and many articles have been published extolling the virtues of the system as a fault-finding tool, especially when harnessed to one of the growing number of test software systems available in the market.

In the meantime, a number of alternative uses for ‘JTAG‘ have been developed, such as its use for debugging and programming. The ‘hi-jacking’ of the JTAG port for debugging goes way back to the early 90s when TI and the fledgling ARM Ltd. figured that this neat bundle of wires and a state-machine could help them access internal (debug) registers, in addition to the IEEE Std. 1149.1 test registers defined by JTAG committee.

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