Error Detection in Hardware

1. Error Detection in Hardware VO Hardware-Software-Codesign Philipp Jahn

2. Error detection • How to detect errors with hardware methods during system operation • Conditions • Coverage (probability that error is detected) • Latency (time between start of error and detection) • Performance Slide from VO „Echtzeitsysteme“, H. Kopetz Error Detection in Hardware

3. Hardware-based error detection • Hardware redundancy • Passive (TMR, majority voting) • Active (duplication and comparison, standby) • Hybrid • Information redundancy • Parity • Checksums • Arithmetic Codes • Time redundancy • Watchdog timers • Checking • Capability Checking • Consistency Checking • Control-Flow Checking Error Detection in Hardware

4. Information redundancy (1) • Detection / Correction • Hamming distance • X = (1001), Y = (0111) • d(X,Y) = 3 • SEC – DED Error Detection in Hardware

5. Information redundancy (2) • Parity • One extra bit (even / odd) • Decoding circuit (set of XOR gates) • Routine checking in busses, memory and registers • Detecting singlebit errors(no stuck-at faults) Error Detection in Hardware

6. Information redundancy (3) • Overlapping parity • m of n codes • Duplication codes • Cycle redundancy checks • Sender and receiver agree upon generator polynom G(x) • Append checksum (k bit) at end of data frame (n-k bit) • Checksum / G(x) = 0  correct • Simple implementation (linear feedback shift register and XOR gates) • Detect single-bit errors, multiple adjacent bit errors affecting fewer than n-k bits, and burst transient errors • High successful in serial transmission (communication channels: Ethernet, Token Ring) Error Detection in Hardware

7. Information redundancy (4) • Checksums Error Detection in Hardware

8. Information redundancy (5) • Arithmetic Codes • Detect errors in arithmetic units (parity would not be preserved) • Separate or nonseparate • Examples • AN codes • Residue codes Error Detection in Hardware

9. Time redundancy (1) • Repetition of computations two or more times and then comparing (detection or correction by majority) • Error detected  maybe retry • Good for detecting transient faults • Not protecting against errors resulting from permanent faults • No extra hardware needed but longer processing time • Non-time-critical applications • Alternate Logic also detects permanent faults (self-checking circuits f(x) = f ‘(x’)) Error Detection in Hardware

10. Time redundancy (2) • Handle permanent faults per encoding the second computation (must not alter calculation) e.g. k-shift • Error in k-1 consecutive bit of arithmetic or logical operation detected • Additional hardware (two shifters, storage register, comparator) Error Detection in Hardware

11. Watchdog timers • Implemented in hardware (external timer) or software (process) • If timer expires  system reset or recover • Detect only very specific type = control-flow error • If error occurs but timer reset  no detection • Difficult to determine runtime • High detection latency Error Detection in Hardware

12. Capability & Consistency Checking • Capability checking limits access to objects (e.g. memory segments) to authorized users (processes) • Implemented in hardware (error traps) or software (firewall) • e.g. checking of address validity by MMU • Consistency checking determines if states or results are reasonable • e.g. range checking, address checking, opcode checking Error Detection in Hardware

13. Control-Flow Checking (1) • Hardware scheme • Divide application program into blocks • Each block has a single entry and exit point • Reference signature represents an encoding of the correct execution • Watchdog processor validates the application program by comparing the runtime with the signature • 70% of transient faults lead to control flow errors • Limitations • Only suitable for processors running single programs (multiple processes or threads) • Reduced coverage if transmission errors on the bus to the watchdog processor occurs Error Detection in Hardware

14. Control-Flow Checking (2) • Signatured Instruction Stream (SIS) • Hardware: Watchdog processor with cyclic code signature generator • Software: Modified assembler and loader • Control Flow Checking using Shadow Processing Error Detection in Hardware

15. Summary • Hardware low error latency • Hardware is more expensive • e.g. Massively parallel multiprocessors • Combining error detection mechanism Error Detection in Hardware

16. References • Ravishankar K. Iyer, Zbigniew Kalbarczyk - Hardware and Software Error Detection - Center for Reliable and High-Performance Computing, University of Illinois at Urbana-Champaign • Real-Time Systems, Design Principles for Distributed Embedded Applications Kopetz, Hermann, 1997, 356 p., Hardcover, ISBN: 978-0-7923-9894-3 • Alireza Vahdatpour, Mahdi Fazeli, Seyed Ghassem Miremadi - Transient Error Detection in Embedded Sysetms Using Reconfigurable Components - IES, October 2006 • M. Dal Chin, W. Hohl, E. Michel, A. Pataricza - Error Detection Mechansims for Massively Parallel Multiprocessors - IEEE Proceedings, 1993 • Evaluation of error detection coverage and fault-tolerance of digital plant protection system in nuclear power plants • http://robotics.ee.uwa.edu.au/courses/faulttolerant/notes/FT2b.pdf • A. Steiniger, C. Scherrer - Identifying Efficient Combinations of Error Detection Mechanisms Based on Results of Fault Injection Experiments - IEEE Transactions on computers, Vol. 51, No. 2, February 2002 Error Detection in Hardware