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More Capabilities
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A PCA is tested, both in production (to verify correct manufacture) and in field service (to detect and locate faults), using automatic test equipment. ATE supplies test signals to, and collects test responses from, some or all of the pins of individual chips.

The aim of the test structures described in dot4 is to provide test access to individual IC pins from an edge-connector. The idea is to reduce or eliminate the need for the ATE to make direct physical contact via mechanical probes.

The extent to which this aim can be satisfied will depend on the make-up of the PCA: the pins of those chips that conform either to dot4 or to dot1 will be accessible from the edge-connector. This will provide at least partial access to the pins of non-conformant components. If this is inadequate for testing purposes, additional probe access would need to be used. But, even in the worst case, the amount of probe access required would be reduced compared with a PCA with no boundary-scan chips.

Three classes of tests on a mixed-signal PCA are provided for on a dot4 chip: (1) interconnect test, (2) parametric test, and (3) internal test. Let's look at each, starting with interconnect test.

Dot1 provides an effective mechanism for interconnect testing of a PCA populated with digital components. Dot4 provides a similar capability for an assembly populated with analog, digital, and mixed-signal components, together with networks of discrete elements.

The fundamental requirement for interconnect testing is to detect open circuits in the interconnections between ICs, and to detect and diagnose bridging faults anywhere in the interconnect—regardless of whether they normally carry digital or analog signals.

The structure provided by dot4 permits testing in full compatibility with dot1, permitting mixed-signal interconnect testing to take place at the same time, and using the same procedures, as digital interconnect testing. The dot4 structures also provide for differential interconnects testing, whether digital or analog.

Parametric Test
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The second class of test supported by dot4, parametric test, permits analog measurements using analog stimulus and response, permitting, for example, impedances of discrete components to be computed.

By modifying the boundary modules attached to digital pins so that they include elements of the dot4 structure, components such as pull-ups associated with digital pins can also be measured.

Internal Test
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The third capability of dot4, internal test, relates to the ability to test the internal circuitry of the chip, either in isolation or while mounted on a substrate. Comprehensive internal testing may require the incorporation of internal test structures whose impact on the cost and performance of the IC may be deemed unacceptable.

The provision of facilities for internal test, as well as the incorporation of any amount of ad hoc testability features within the internal core structure, is, therefore, optional. The defined structure, however, will support the inclusion of any number of designer-defined test functions.

REFERENCES

[1] J. Baston, "In-Circuit Testing," Van Nostrand Reinhold, New York, 1985

[2] K. P. Parker, "The Boundary-Scan Handbook, 2nd Edition, Analog and Digital," Kluwer Academic Publishers, Boston, 1998

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