1. 1 Analyzing Reconvergent Fanouts in Gate Delay Fault Simulation Dept. of ECE, Auburn University Auburn, AL 36849 Hillary Grimes & Vishwani D. Agrawal

2. 2 Outline Problem Statement Reconvergent Fanout Analysis  Ambiguity Lists Fault Detection  Detection Threshold  Detection Gap Results Conclusion

3. 3 Definitions Gate Delay Fault Model  Assume that a delay fault is lumped at a single faulty gate Detection Threshold  Minimum size delay fault that is guaranteed to be detected by the test Detection Gap  Relates the detection threshold to the slack at the fault site

4. 4 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

5. 5 Bounded Delay Simulation 1,3 1,2 3,4 1 3 2 5 3 5 5 9 4 11 0 1 1

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

7. 7 Ambiguity Lists Ambiguity Lists generated at fanout points contain  originating fanout name  ambiguity interval – min and max delays from fanout to gate Ambiguity list propagation is similar to fault list propagation in concurrent fault simulation

8. 10/25/2007 ITC-07 Paper 26.38 Ambiguity List Propagation Ambiguity lists at the inputs of a reconvergent gate help determine its output  If signal correlations are such that no hazard can occur, the hazard is suppressed  Otherwise, the ambiguity lists are propagated to the gate’s output, and ambiguity intervals are updated

9. 10/25/2007 ITC-07 Paper 26.39 Ambiguity List Propagation Bounded Delay Simulation  Ambiguity lists propagated through every gate Detection Threshold Evaluation  Ambiguity Lists propagated through downcone of the fault site

10. 10 Detection Threshold 1,3 1,2 3,4 1 3 2 5 5 9 0 1 1 3 5 4 6 11 Corrected Det. Threshold = 6 Ts = 12 Det. Threshold = 8

11. 11 Detection Gap for a Gate Ideal gate delay test should activate longest path p1, detection threshold = slack, gap = 0 A test that activates path p2, p2 < p1, gap = detection threshold – slack Smaller the gap, better is the test PI PO p1 - longest delay path through gate p2 Gate Ts p1 delay p2 delay gap DT(p2) slack t

12. 12 Results ISCAS85 benchmark circuits simulated with 10,000 random vectors Simple wireload model  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

13. 13 Results: Fault-Free Simulation Circuit Without Reconvergent Fanout Analysis With Reconvergent Fanout Analysis Largest EALargest LSLargest EALargest LS c354096.0204.0121.6196.8 C531576.8204.091.2194.4 C6288158.4576.0236.8504.0 C755291.2204.0104.0201.6 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

14. 14 Results: Fault Simulation Average detection gap and fault coverage of faults detected with gap ≤ 3.5 recorded For fault coverage, faults are counted as detected if they are detected:  Through the longest path through the gate  Through a path which is shorter than longest path by only one gate delay

15. 15 Results: Fault Simulation Circuit Without Reconvergent Fanout Analysis With Reconvergent Fanout Analysis Average Detection Gap Faults Detected with Gap ≤ 3.5 Average Detection Gap Faults Detected with Gap ≤ 3.5 c432110.47.35%108.97.08% c49951.74.91%44.012.85% c88016.448.41%12.948.86% c135550.84.80%42.213.62% c190855.221.70%47.125.10% c267041.831.25%36.036.54% c354050.432.60%44.033.19% c531521.755.72%6.157.31% c755239.413.43%22.522.83%

16. 16 Conclusion When reconvergent fanout analysis is used, gate delay fault simulation results are less pessimistic During simulation, ambiguity lists can grow quite large  Efficiency in list propagation needs to be improved This min-max delay simulator has found application in hazard-free delay test generation