IEEE 1149.6: AC coupled JTAG

- a summary, overview or tutorial of the basics of IEEE 1149.6, the boundary scan JTAG test system that can accommodate AC coupled signals.

Boundary Scan JTAG Includes:
What is Boundary Scan / JTAG     Boundary scan description language, BSDL     Design for test with boundary scan     JTAG Spec & IEEE 1149 Standard     JTAG TAP & connector     IEEE 1149.6 (AC coupled JTAG)     Compact JTAG cJTAG IEEE 1149.7     IJTAG, IEEE 1687    

The IEEE 1149.1, JTAG standard for boundary scan testing has been in existence for many years and it is now well established. Boundary scn testing ahs revolutionished However there are some limitations to this form of testing. In particular IEEE 1149.1 does not address AC coupled signals or differential nets. In order to address these shortfalls, a new committee was set up to develop a new standard to address these problems. Known as IEEE 1149.6, the new specification adds additional functionality to the boundary scan test technique, allowing it to be used in additional circumstances.

Drivers for IEEE 1149.6 development

In their brief the IEEE indicated they were well aware that the existing boundary scan IEEE 1149.1 or JTAG standard did not address some of the newer digital network topologies such as AC coupled differential connections on very high speed digital paths (i.e. more than 1 GBps). The original IEEE 1149.1 boundary scan structures and methods were intended to test DC coupled single ended networks since the AC coupling blocks any static signals.

In addition to this, differential networks are also inadequately tested. To achieve the testing of differential networks it is necessary to insert boundary cells between the differential driver or receiver and the chip pads, or insert boundary cells before the differential driver or after a differential receiver. Neither of these solutions is particularly acceptable because it may degrade the performance or the testing.

In addition to this the IEEE 1149.4 methods that are intended for testing analogue circuits do not naturally lend themselves to testing the very high speeds encountered in the high speed AC coupled differential networks. Often the methods required for analogue testing are too intrusive for these digital networks and it can have an impact on the pin count.

Accordingly the aim of IEEE 1149.6 was to define a standard that was robust, and provided a greater test and diagnostic capability than previous methods yet required minimally intrusive structures and test methods. The project was aimed at addressing the physical interface as well as the protocols and any changes to software and BSDL.

The IEEE 1149.6 standard was initially released in March 2003, and its use has grown since then as a result of the capabilities it offers.

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