Concurrent system-level error detection techniques using a watchdog processor are surveyed. A watchdog processor is a small and simple coprocessor that detects errors by monitoring the behavior of a system. Like replication, it does not depend on any fault model for error detection. However, it requires less hardware than replication. It is shown that a large number of errors can be detected by monitoring the control flow and memory-access behavior. Two techniques for control-flow checking are discussed and compared with current error-detection techniques. A scheme for memory-access checking based on capability-based addressing is described. The design of a watchdog for performing reasonable checks on the output of a main processor by executing assertions is discussed. >

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Concurrent error detection using watchdog processors-a survey

The design of totally self-checking check circuits for m-out-of-n codes is described. Totally self-checking m-out-of-n checkers provide an error indication whenever the input is not an m-out-of-n code or whenever a fault occurs within the checker itself. Since the checker checks itself, there is no need for additional maintenance access or periodic exercise of the checker to verify its ability to detect errors. The basic structure of the checker relies on the use of majority detection circuits. Various gate level implementations for the majority detection circuits are also presented, although the self-checking capability of the checker does not depend on their particular implementation since they are exhaustively tested by code inputs. The self-testing checkers for k-out-of-2k codes are discussed in the most detail since the totally self-checking checkers for 1-out-of-n and arbitrary m-out-of-n codes are constructed by first translating the code to a k-out-of-2k code via a totally self-checking translator.

Design of Totally Self-Checking Check Circuits for m-Out-of-n Codes

Self-checking circuits can detect the presence of both transient and permanent faults. A self-checking circuit consists of a functional circuit that produces encoded output vectors and a checker that checks the output vectors. The checker has the ability to expose its own faults as well. The functional circuit can be either combinational or sequential. A self-checking system consists of an interconnection of self-checking circuits. The advantage of such a system is that errors can be caught as soon as they occur; thus, data contamination is prevented. Methods for the cost-effective design of combinational and sequential self-checking functional circuits and checkers are examined. The area overhead for all proposed design alternatives is studied in detail. >

Design and synthesis of self-checking VLSI circuits

The theory and design of systematic t-unidirectional error-detecting codes are developed. Optimal systematic codes capable of detecting 2, 3, and 6 unidirectional errors using 2, 3, and 4 check bits, respectively, are given. For r ≥5 where r is the number of check bits, the systematic codes described here can detect up to 5· 2r-4 + r -4 unidirectional errors. Encoding/ decoding methods for these codes are also investigated.

Systematic Unidirectional Error-Detecting Codes

: Some basic theory on unidirectional error (i.e. all bits fail in the same direction) correction/detection are presented for binary block codes. Then a new class of codes are constructed which corrects single errors and detects any number of multiple unidirectional errors. Some codes are shown which hither-to-fore known to possess only symmetric error detection/correction properties which can be modified to make them suitable for unidirectional error correction/detection.

On the Theory of Unidirectional Error Correcting/Detecting Codes.

In designing a self-checking processor, it is essential to recognize the types of failures that are most probable. Matching the checking techniques with the type of faults that are expected to occur should yield the best result with the least amount of hardware. The microprogram control will consist of integrated circuits: large-scale integration (LSI) for the memory and small-scale integration (SSI) for the associated control logic. Because of the density of chips on a plug-in package and the physical proximity of the devices on an integrated circuit, multiple faults within a single circuit are highly probable. The types of faults within a circuit have been analyzed and found to be of the type which would tend to affect the bits in a unidirectional manner. Also the failed bits would probably be adjacent rather than randomly dispersed throughout-the microprogram store word.

T.F. Storey

Papers

Design of a Self-Checking Microprogram Control