System Test with Boundary Scan (JTAG)

What is system test and why would you want to do it?

The logical place to begin a discussion of boundary-scan system-level testing is to define what system test means and to distinguish it from board test, which is the more typical arena for boundary scan. A system can be defined as “a group of interrelated, interacting, or interdependent constituents forming a complex whole.” (Webster Dictionary) For the purposes of this discussion, system will mean more than one board connected together. This could be an assembly as simple as a mother/daughterboard combination or an extremely complex backplane-based computer with hundreds of boards.

So, applying this definition, it can be said that an electronic system is a group of interrelated devices and boards that function together to accomplish a task. Although the components that comprise an electronic system may be tested individually without regard to the task of the entire system, the overarching goal of system test is to verify the structural integrity and/or functionality at the level of the complex whole. At a fundamental level, testing a system’s structural integrity would involve verifying that each component is properly connected to all other components that require such connections.

Why System Test?

Most electronic products can be seen as a system. Even if all of the system’s components are fully tested individually, the system still may not function properly. Many problems can arise when systems are assembled. Boundary scan is well suited to finding and diagnosing these problems. For example, a connector may not be making good electrical contact or a connector’s pins might be damaged during assembly. Boards on a backplane may be missing, out of place or simply not functioning properly. Functional tests might identify that the system is not functioning as expected, but isolating and diagnosing the fault with functional tests would be very time consuming and often would require high-level system expertise to troubleshoot the cause.

In addition, the usefulness of boundary scan at the system level extends well beyond what is usually thought of as test functions. Besides structural integrity tests, boundary-scan can also perform on-board programming of flash memory, in-system configuration of programmable logic devices (PLDs), on-board programming with I2C data and emulation-based functional testing through a processor’s JTAG debug port. Beginning with system-level debug and production test, boundary scan provides many other system benefits when it is extended to field support. For example, boundary scan can be used for quick and easy updates of programs stored in flash memory or updates to system functionality in the field through the re-configuration of on-board PLDs.

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