Boundary-Scan Tutorial:

Extest Mode

Using the boundary-scan cells to test the interconnect structure between two devices is called External Test, shortened to Extest – see Figure 15. The use of the cells for Extest is the major application of boundary-scan structures, searching for opens and shorts plus damage to the periphery of the device. In this mode, the boundary-scan cells are often referred to as virtual nails

Intest Mode

It is also possible to use boundary-scan cells to test the internal functionality of a device (Figure 16). This use of the boundary-scan register is called Internal Test, shortened to Intest. Intest is only really used for very limited testing of the internal functionality to identify defects such as the wrong variant of a device, or to detect some gross internal defect.

In the next section, we will take a closer look at the overall architecture of an 1149.1-compliant device and, particularly, the function of the Instruction register.

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