Wiggler (JTAG) - Overview

Overview

JTAG was an industry group formed in 1985 to develop a method to test populated circuit boards after manufacture. At the time, multi-layer boards and non-lead-frame ICs were becoming standard and connections were being made between ICs which were not available to probes. The majority of manufacturing and field faults in circuit boards were due to solder joints on the boards, imperfections in board connections, or the bonds and bond wires from IC pads to pin lead frames. JTAG was meant to provide a pins-out view from one IC pad to another so all these faults could be discovered.

The industry standard finally became an IEEE standard in 1990 as IEEE Std. 1149.1-1990 after many years of initial use. That same year Intel released the first processor with JTAG — the 80486 — which led to quicker industry adoption by all manufacturers. In 1994, a supplement that contains a description of the boundary scan description language (BSDL) was added. Further refinements regarding the use of all-zeros for EXTEST, separating the use of SAMPLE from PRELOAD and better implementation for OBSERVE_ONLY cells were made and released in 2001. Since 1990, this standard has been adopted by electronics companies all over the world. Boundary-scan is now mostly synonymous with JTAG, but JTAG has essential uses beyond such manufacturing applications.