System Test with Boundary Scan (JTAG)

How is system test implemented with boundary scan?

Four primary methods are available for implementing boundary-scan-based system tests. Three of these methods involve connecting an external boundary-scan test tool to the system and the fourth involves embedding boundary-scan test into the system.

The first external method connects all of the boundary scan paths on all of the boards in the system to a single scan path with one point of access for the boundary scan test system (Figure 1). This method requires that the configuration of the system must remain exactly the same in every assembled product. Also, any break in the single scan path will disable all boundary-scan test access. One very long scan path can also result in slow access times.

Figure 1 - Single Scan Path

The second external method is to link each scan path in the system to external connectors where the boundary scan tool will have access to all scan paths (Figure 2). With this method, physical access to each JTAG connector must be available when the system is operating. Additional test system hardware is also needed to connect the test system and the system under test. However, with this method, the configuration of the system can change and the test system is able to manage test access for each new or different configuration of the system. Also, systems that may not have been designed with system-level testing in mind may be tested with this method.

Figure 2 - Multiple Scan Paths/Multiple Access Points

The third method is to design a method for controlling access to the scan paths on the individual boards from one external JTAG port (Figure 3). This is most commonly accomplished with multi-drop gateway devices that control the board for the purposes of testing the system. These gateway devices typically are controlled by the test system. They can be mounted on each board or on the backplane where the boards are installed. If installed on-board, gateway devices will require board real estate. In any event, they will add to the cost of the system. Also, distinct tests must be developed for each configuration of the system.

Figure 3 - Multiple Paths/One-Access Point

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