39.5 Boundary Scan Testing

Boundary Scan Testing (BST) is the method of controlling and observing the boundary pins of the JTAG compliant device, like those of the PIC18-Q84 family, utilizing software control. BST can be used to test connectivity between devices by daisy-chaining JTAG compliant devices to form a single scan chain. Several scan chains can exist on a PCB to form multiple scan chains. These multiple scan chains can then be driven simultaneously to test many components in parallel. Scan chains can contain both JTAG compliant devices and non-JTAG compliant devices.

A key advantage of BST is that it can be implemented without physical test probes; all that is needed is a 4-wire or 5-wire interface and an appropriate test platform. Since JTAG boundary scan has been available for many years, many software tools exist for testing scan chains without the need for extensive physical probing. The main drawback to BST is that it can only evaluate digital signals and circuit continuity; it cannot measure input or output voltage levels or currents.