1. Enhancing Random Access Scan for Soft Error Tolerance Fan Wang* Vishwani D. Agrawal Department of Electrical and Computer Engineering, Auburn University, AL 36849 *Now with Juniper Networks, Inc. Sunnyvale, CA, 94086 42ndIEEE Southeastern Symposium on System Theory, March, 2010

2. Motivation for This Work • Recent work on random access scan (RAS) has shown its advantages in reducing test time, test volume and test power over serial scan (SS). • The RAS structure can also improve the fault tolerance ability in both normal function mode and test mode.

3. Outline • Background • Review of RAS design • Soft error tolerance of RAS • A new scan-out structure • Further enhancing error tolerance using RAS structure • Conclusion

4. Soft Errors • Soft errors are caused by the operating environment. • They are not due to permanent hardware faults. • Soft errors are intermittent or random, which makes their testing unreliable. • One way to deal with soft errors is to make hardware robust: • Capable of detecting soft errors • Capable of correcting soft errors • Both measures are probabilistic

5. Effect on Digital Circuit Charged Particles Charged Particles Combinational Logic Flip-flops Flip-flops IN OUT CK M. Nicolaidis (Editor), Soft Errors in Modern Electronic Systems, Springer, 2010.

6. Random Access Scan (RAS) • Testing requires that flip-flops be controllable and observable. Two methods are: • Serial scan (SS) using shift register • Random access scan (RAS) using memory-like addressing • RAS reduces test application time and test power, which are otherwise complementary objectives in SS. • Previous and current publications on RAS: • Ando, COMPCON-80 • Wagner, COMPCON-83 • Ito, DAC-90 • Bushnell & Agrawal, textbook, pp. 484-485 • Mudlapur et al., ITC-05 • Saluja et al., VLSI Design-04, ITC-05, ATS-05, VLSIDesign-06, VLSI Design-10.

7. Background Error tolerant computing techniques are characterized by the level of reliability: • Device level error tolerance techniques either increase the device critical charge or decrease the collected charge to reduce SER • Circuit or system level error tolerance techniques include error detection and correction (EDAC) codes and time/space redundancy.

8. BISER Design With C-Element S. Mitra, N. Seifert, M. Zhang, Q. Shi, and K. S. Kim, “Robust System Design with Built-In Soft-Error Resilience,” Computer, vol. 38, no. 2, pp. 43-52, February 2005.

9. The “Toggle” RAS Flip-Flop Combinational Logic Data 1 M S To Output BUS M U X Combinational Logic Data 0 Clock Output BUS Control x y RAS-FF √nff Lines √nff Lines Row Decoder Column Decoder Address (log2nff)

10. Natural Soft Error Tolerance of RAS • For SS, soft error can be induced on each SFF as it transports test data to output. • For RAS, only when selected RAS cell has induced error, will the result be affected.

11. SER Analysis • For a 4- cell RAS structure, the SER is • N · ( A + Δ)· P · α f • For a 4-cell SS structure, the SER is • 4N · A · P · α f • Where • N is the particle flux in #particles · cm-2 · s-1 • A is sensitive area per FF in cm2 • P is probability of SET per strike in a FF • Δ is average area overhead (routing, decoder, etc.) per FF to implement RAS • αf is a temporal derating factor between 0 and 1

12. Fault Tolerant Design Using BISER-RAS Copy R1 R11 R12 R13 RAS FF11 RAS RAS RAS FF12 RAS RAS FF13 RAS FF14 RAS C- x1 ck Copy R2 R21 R22 R23 RAS RAS FF11 RAS FF12 RAS RAS RAS FF13 RAS RAS FF14 C- x2 To Next Level Copy R3 R31 R32 R33 RAS FF11 RAS RAS FF12 RAS RAS FF13 RAS RAS FF14 RAS C- x3 y1 y2 y3 y4

13. Hardware overheads comparison for ISCAS’89 Circuit

14. Conclusion • The RAS design has a natural soft error tolerance capability that is inherited from its unique structural and operation. • In a circuit with N FFs, the SER of RAS can be nearly 1/N that for the SER of SS. • The BISER-RAS can save on average 20.51% hardware over BISER applied to SS, and TMR-RAS saves on average of 179.28% over TMR-SS for ISCAS89 benchmarks.