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182 VLSI Test Principles and Architectures of a for the previous decision, so a is reset to “don’t care.” By assigning b = 0, a conflict is observed. Reversing b also cannot justify all the J -frontier. At this time, backtracking on b leads to no prior decisions. Thus, target fault g/ 1 is declared to be untestable. 4.4.4 PODEM In the D algorithm, the decision space encompasses the entire circuit. In other words, every internal gate could be a decision point. However, noting that the end result of any ATPG algorithm is to derive a solution vector at the primary inputs and that the number of primary inputs generally is much fewer than the total number of gates, it may be possible to arrive at a very different ATPG algorithm that makes decisions only at primary inputs rather than at internal nodes of the circuit. PODEM [Goel 1981] is based on this notion and makes decisions only at the primary inputs. Similar to the D algorithm, a D -frontier is kept. However, because decisions are made at the primary inputs, the
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