ECE 269 VLSI System Testing Krish Chakrabarty Lecture 20 Bridging Faults ECE 269 Krish Chakrabarty 1 Outline Some perspective Non feedback bridging faults Feedback bridging faults Test generation ECE 269 Krish Chakrabarty 2 1 Perspectives and Trends In the past bridging faults were seldom considered in practice Too many Strength properties ill defined May introduce feedback and oscillation Usually considered as an add on to SSL faults Today submicron technology makes it necessary to consider bridging faults directly ECE 269 Krish Chakrabarty 3 Types of Bridging Faults Non feedback bridging fault does not create feedback relatively easy to handle Feedback bridging fault FBF transforms combinational circuit to become sequential FBF may even cause oscillations ECE 269 Krish Chakrabarty 4 2 AND Non Feedback BF Theorem Test t detects the AND NFBF between x and y iff t detects x 0 and sets y to 0 or vice versa Application NFBF test generation can be done using ATPG for SSL faults Proof Sufficiency Suppose t detects NFBF between x and y Then t makes x y 1 0 or x y 0 1 Moreover t creates a sensitized path to the primary output If t makes x y 1 0 then it detects x 0 and places 0 on y In the other case Necessity Suppose t does not detect the NFBF Then either it does not activate the fault places 00 or 11 on xy or does not create a sensitized path Extend to OR by interchanging 1 and 0 ECE 269 Krish Chakrabarty 5 AND NFBFs Corollary Let x and y be inputs to a N OR gate and let x and y not both fan out Then the AND NFBF between x and y dominates x 0 and y 0 Conjecture Friedman 1974 The above result holds when both x and y fan out ECE 269 Krish Chakrabarty 6 3 AND Feedback Bridging Faults f b Theorem If t detects f 0 and sets b to 0 then t detects F b f Theorem If t detects b 0 and sets f 0 without sensitizing the path b f then t detects F b f Krish Chakrabarty ECE 269 7 Test Generation for BFs 1 Create workaround circuit containing an SSL equivalent to the BF x z y z x a y z z x y a 0 Problems Circuit complexity is greatly increased Unnecessary and meaningless SSL faults are introduced ECE 269 Krish Chakrabarty 8 4 Test Generation for BFs 2 Simulation implicit using preceding relations between BFs and SSL faults Experimental Data Abramovici and Menon 1985 pre submicron era Good SSL test sets 95 fault coverage detect most 83 BFs Some bad cases exist High percentage 75 of potentially oscillating BFs were detected Overhead for simulating BFs is modest Nowadays explicit testing of BFs is necessary IDDQ etc ECE 269 Krish Chakrabarty 9 5
View Full Document
Unlocking...