Reconvergent Fanout Analysis of Bounded Gate Delay Faults

1. Reconvergent Fanout Analysis of Bounded Gate Delay Faults Master’s Defense Hillary Grimes Thesis Advisor: Dr. Vishwani D. Agrawal Thesis Committee: Dr. Victor P. Nelson and Dr. Charles E. Stroud Dept. of ECE, Auburn University Auburn, AL 36849

2. Outline • Background • Problem Statement • Ambiguity Lists • Fault-Free Circuit Simulation • Detection Threshold Evaluation • Experimental Setup • Results and Discussion • Conclusions

3. Delay Testing • Delay testing ensures a manufactured design meets it’s timing specifications • Gate Delay Fault Model • Assume that a delay fault is lumped at a faulty gate • All other gates have their delays within the specified (min, max) range.

4. A Gate Delay Test • A delay test consists of a vector pair • First vector (V1) initializes the circuit • Second vector (V2) produces the required transitions • For a slow-to-rise gate delay fault: • V1 – places a logic 0 at the fault site • V2 – stuck-at-0 test for the same fault site

5. Fault-Free Circuit Simulation IV – initial value for V1-V2 transition – stable logic value at gate after V1 is applied FV – final value for V1-V2 transition – stable logic value at gate after V2 is applied EA – earliest arrival time for gate output after V2 is applied LS – latest stabilization time for gate output after V2 is applied

6. Fault-Free Circuit Simulation 0 1 3 3 5 1,3 1,3 1,3 1,2 4 11 EA = 0 LS = 0 1,2 1,2 2 5 IV = 1 1 1 FV = 0 3,4 1,3 1,3 1,3 5 9 EA = ∞ LS = -∞ EA = 5 LS = 9

7. Faulty Waveforms • Faulty Propagating Value - FPV • Signal’s logic value in the presence of a stuck-at-IV fault at the fault site • Propagates the fault’s logic effect through the circuit • Used to determine whether or not a delay fault of any size (transition fault) is detected

8. Fault Propagating Values 0 1 3 3 5 1,3 1,3 1,3 1,2 4 11 Slow-To Fall 1,2 1,2 2 5 1 1 3,4 1,3 1,3 1,3 5 9

9. Fault Propagating Values 0 FPV = 1 1 3 3 5 1,3 1,3 1,3 1,2 FPV = 1 4 11 FPV = 0 FPV = 0 1,2 1,2 2 5 1 1 FPV = 1 FPV ≠ FV FPV = 1 3,4 1,3 1,3 1,3 Assuming the delay fault size is large enough, it is detected 5 9

10. Detection Threshold • For gate delay faults, we also need to know the size (δ) of faults detected • Detection Threshold – minimum size delay fault detectable by the test • Requires timing information about faulty waveforms to be propagated along with FPVs • RTa & RTb – signal is at FPV between the times RTa to RTb+δ

11. Detection Threshold Evaluation Ts = 12 RTa = -∞ RTb = 3 0 FPV = 1 1 3 1,3 1,3 1,3 3 5 1,2 FPV = 1 RTa = -∞ RTb = 1 4 11 2 5 FPV = 0 1,2 1,2 RTa = -∞ RTb = 4 FPV = 0 RTa = -∞ RTb = 2 RTa = -∞ RTb = 5 1 1 FPV = 1 3,4 FPV = 1 1,3 1,3 1,3 RTa = -∞ RTb = ∞ Ts and RTb at the output determine detection threshold 5 9

12. Detection Threshold Evaluation FPV = 0 RTa = -∞ RTb = 4 Ts = 12 4 11 Detection Threshold = 8 • The output signal is at FPV between times RTa  (RTb+δ): -∞  (4+δ) • Detection Threshold is Ts – RTb: (12-4)=8 • Fault detected if it’s size is greater than 8: δ > 8

13. Detection Gap • Calculating the “detection gap” provides a way to relate the detection threshold of a detected gate delay fault to the slack at the fault site • Detection gap is: DT(G) – slack(G) • DT(G)  detection threshold at fault site G • slack(G)  sum of all minimum gate delays along the longest delay path through G

14. Detection Gap • The smaller the detection gaps are for a set of vectors, the better quality that set provides for detecting gate delay faults • If a test detects a fault with gap = 0: • The smallest possible gate delay fault has been detected • If detection gap > 0: • There is a possibility a better test exists to detect the fault with a smaller threshold

15. An Illustration of Detection Gap p1 - longest delay path through gate PI Ts PO p1 delay slack Gate gap p2 delay DT(p2) p2 A test that detects the fault through path p1 would be better than a test that detects the fault through path p2

16. Problem Statement • When signals produced by a common fanout point reconverge, the inputs to the reconvergent gate are correlated • Conventional simulation ignores this correlation when bounded gate delays are used • Produces pessimistic results in both bounded delay simulation and gate delay fault simulation

17. Reconvergent Fanout Analysis Fall occurs at time ‘x’ Hazard cannot occur 0 1 x 3 3 5 1,3 1,3 1,3 1,2 4 6 11 1,2 1,2 x+1 5 1 1 3,4 1,3 1,3 1,3 Output rises at least 1 unit after ‘x’ 5 9

18. Correct Detection Threshold Ts = 12 FPV = 0 RTa = - ∞ RTb = 6 4 6 11 Detection Threshold = 6 • The output signal is at FPV between times RTa to RTb+δ • In an accurate analysis, RTb=6, not 4 • Fault detected if it’s size is greater than 6: δ > 6

19. Ambiguity Lists • Ambiguity Lists generated at fanout points contain • originating fanout name • ambiguity interval – min and max delays from fanout to gate • Ambiguity lists at the inputs of a reconvergent gate help determine its output

20. Ambiguity Lists List propagation is similar to fault list propagation in concurrent fault simulation For accurate detection threshold evaluation, ambiguity lists are propagated during both fault-free and faulty waveform calculations

21. Ambiguity Lists – Fault-Free Circuit Ambiguity lists propagated through all gates during fault-free circuit simulation If signal correlations are such that no hazard can occur, the hazard is suppressed: (EA = ∞) & (LS = -∞) Otherwise, the ambiguity lists are propagated to the gate’s output, and ambiguity intervals are updated

22. Fault-Free Circuit Simulation EA = ∞ LS = -∞ 0 1 3 1,3 1,3 1,3 3 5 1,2 EA = 1 LS = 3 EA = 0 LS = 0 4 6 11 2 5 1,2 1,2 EA = 6 LS = 11 EA = 2 LS = 5 EA = 5 LS = 9 1 1 3,4 1,3 1,3 1,3 EA = ∞ LS = -∞ EA = ∞ LS = -∞ 5 9

23. Ambiguity Lists – Detection Threshold Evaluation Ambiguity lists propagated through downcone of the fault site If signal correlations are such that no hazard can occur, the hazard is suppressed: (RTa = -∞) & (RTb = ∞) Otherwise, the hazard lists are propagated to the gate’s output, and ambiguity intervals are updated

24. Detection Threshold Evaluation Tc = 12 FPV = 1 RTa = -∞ RTb = ∞ 0 1 3 1,3 1,3 1,3 3 5 FPV = 1 RTa = -∞ RTb = 1 1,2 4 6 11 2 5 FPV = 0 RTa = - ∞ RTb = 6 1,2 1,2 FPV = 0 RTa = - ∞ RTb = 2 FPV = 1 RTa = - ∞ RTb = 5 1 1 3,4 1,3 1,3 1,3 FPV = 1 RTa = -∞ RTb = ∞ Now, RTb at the output is correctly evaluated as 6 5 9

25. Experimental Setup • C program implemented to perform gate delay fault simulation on combinational circuits • Simple wireload model used for gate delays • Bounded delays set to (3.5n ± 14%), where n is the number of fanouts • Program can accept any available gate delay data, which may be normally available from process technology characterization

26. Experimental Setup • Program inputs • Netlist in bench format • Vector file • Program outputs for vector set • Average detection gap of detected gate delay faults • Fault coverage of faults detected with gap ≤ 3.5

27. Experiment A Gate delay fault simulation  1,000 vectors Ambiguity lists propagated during faulty waveform calculations only Average detection gap and fault coverage of faults detected with gap ≤3.5 (nominal gate delay) recorded

28. Experiment A • For fault coverage, faults are counted as detected if they are detected: • though the longest path through the gate • through a path which is less than the longest path by only one gate delay

29. Results: Experiment A

30. Experiment B Bounded delay simulation of the fault-free circuit  10,000 vectors Ambiguity lists propagated through every gate Largest EA and LS values at circuit outputs for all vectors recorded to illustrate difference seen at outputs when ambiguity lists are used

31. Results: Experiment B Using reconvergent fanout analysis generally results in larger EA and smaller LS values at outputs More apparent for circuits that contain a large number of reconvergent fanouts, such as in multiplier circuit c6288

32. Experiment C Gate delay fault simulation  10,000 vectors Ambiguity lists propagated during both fault-free circuit simulation and detection threshold evaluation Average detection gap and fault coverage of faults detected with gap ≤3.5 recorded

33. Results: Experiment C

34. Discussion When reconvergent fanout analysis is used, the average detection gap is smaller and more faults are detected with smaller gaps To accurately evaluate detection thresholds, signal correlations must be considered in both fault-free waveform and faulty waveform calculations

35. Conclusion Propagating ambiguity lists during simulation provides useful information about signal correlations due to reconvergent fanouts The use of this information during both fault-free and faulty waveform calculations produces more accurate results for gate delay fault simulation This min-max delay simulator has found application in hazard-free delay test generation

36. Future Work • During simulation, ambiguity lists can grow quite large • Efficiency in list propagation needs to be improved • Can information provided by propagating ambiguity lists help reduce pessimism in static timing analysis?

37. Publications related to this work S. Bose, H. Grimes and V. D. Agrawal, “Delay Fault Simulation with Bounded Gate Delay Model”, in Proc. IEEE International Test Conference, paper 26.3, 2007. H. Grimes and V. D. Agrawal, “Analyzing Reconvergent Fanouts in Gate Delay Fault Simulation”, in Proc. 17th IEEE North Atlantic Test Workshop, May 2008.

38. Thank You