Design for diagnosability guidelines

The concept of design for diagnosability (DfD) is introduced as a different concept than design for testability (DfT). Separating design for diagnosability guidelines from those of the more general design for testability guidelines enables circuit designers to distinguish between those guidelines that separate good circuits from faulty ones and guidelines that are aimed at improving the repair process. Comprehensive fault detection does not result in cost effective repair without unambiguous fault isolation. A model is used to demonstrate the distinctions between test and diagnostic goals. Diagnoses related terminology is introduced and diagnostic complications are discussed. Hidden faults, cannot duplicates (CNDs), retest OK, fault isolation ambiguity, false remedies and false alarms are explored in great detail. A Venn diagram model is used to further explain Diagnosability and the diagnostic complications. The term Distinguishability is added to our vocabulary as the positive dual of ambiguity. The paper probes the possible causes of the various diagnostic issues and provides guidelines designers can incorporate to eliminate them. The paper offers specific design guidelines to achieve a more diagnosable or diagnostic-friendly circuit design. In doing so, diagnostic complications can be reduced, resulting in lower cost repairs and maintenance.