JTAG | one TAP, many non-intrusive apps

The origins of JTAG are inextricably bound up with boundary scan. Yet, it provides many capabilities to many purposes. Often, as part of a point tool, whether for debug, programming, test or validation, it takes on just one capability. But, no matter the purpose, as the thread common to all, the standard Test Access Port (TAP) opens the way for many applications. In the test domain, several of these unite under the banner Non-intrusive Board Test (NBT).


Formally, ‘JTAG’ names the Joint Test Action Group, the group that authored the original boundary-scan specification draft that seeded IEEE 1149.1.

Less formally, JTAG is often used in the industry to refer to:

  • the specification as originated by the Joint Test Action Group
  • IEEE 1149.1, the working group and/or the standard (see our IEEE 1149.1 Tutorial)
  • the manifold progeny of IEEE 1149.1 (such as 1149.6 and 1532)
  • the standard Test Access Port in isolation and/or in association with functionality other than boundary scan

To this last point, consider that the TAP comprises a low pin-count interface that, at its essence, scans serial registers through their capture, shift, and update states. While nominally for test, the open nature of the TAP provides non-intrusive means to access any device mode (even those related to device missions) and to exchange data with such modes.

As a result, many applications or ‘apps’ serve many purposes, including validation, test, debug, diagnosis, monitoring, characterization, configuration, and functional use. The one TAP defined by JTAG and standardized per IEEE 1149.1 enables them all.

Surveying the test domain, four such apps comprise the core capabilities of NBT. The simple icons shown here represent each of these. Their names and key attributes are:

  • Boundary-Scan Test (BST)
    • Structural test with diagnostics (opens/shorts/1149.1/1149.6)
    • Cluster test for quasi-static functional diagnostics
    • In-system programming (PLDs/PROMs/Flash/1532)
  • Processor-Controlled Test (PCT)
    • At-speed test of processor and peripheral interfaces with structural diagnostics
  • FPGA-Controlled Test (FCT)
    • At-speed test of FPGA and peripheral interfaces with structural diagnostics
  • High Speed I/O (HSIO)
    • Validation of signal integrity and performance test for high-speed buses

Future posts in this blog space will bring more visibility to these apps…

Of course, you may also wish to give a look at many previous posts gathered under category Non-intrusive Board Test. And you’ll find related eBooks in our eResources library, under heading Manufacturing Test.

Adam Ley