High-performance electronic circuits would incorporate self-checking features for detection of radiation-induced single-event latchups (SELs), according to a proposal. The basic SEL-detection scheme calls for redundant circuitry and a current-voting scheme similar to voting schemes that have been used to reveal malfunctions in other redundant systems. The redundancy and voting scheme could also be combined with other fault-tolerance features [e.g., for detection of single-event upsets (SEUs)].
As in some older schemes for detecting SEL and other anomalies, the proposed current-voting scheme would involve detection of operating current outside the normal range for a circuit to be protected. However, unlike in some older methods for detecting SEL, no attempt would be made to establish precise limits of normal operating current - limits that could be difficult if not impossible to establish for a complex circuit that normally operates over a wide dynamic range of current and/or is subject to radiation or to variations in temperature. Instead, one would build a duplicate of the circuit to be protected and would operate both circuits concurrently under the same nominal conditions, using comparator circuitry to detect differences between the currents drawn by the two circuits (see left side of figure). Each of the duplicate circuits would serve as a high-fidelity model of "normal" behavior for the other. "Normal" behavior would be defined ratiometrically; that is, in terms of a range, α , of allowable fractional difference between the currents (or corresponding voltages) in the two duplicate circuits. Any excursion from the allowable range would be detected by the comparator circuitry, which would respond by triggering an alarm, shutdown, reset, or other appropriate corrective signal.
The current-voting scheme could be implemented, for example, by the current-comparison and threshold-logic circuitry shown on the right side of the figure. Potentials V1 and V2 are voltages representative of the currents flowing from a power supply (at potential VCC) to each of two duplicate circuits. The values of R1 and R2 would be chosen so that R1/(R1+R2) = α . Thus, the left voltage divider (R1, R2) would provide comparison voltages V1 and V1(1 - α ), while the right voltage divider (R1, R2) would provide comparison voltages V2 and V2(1 - α ). Then the output of the upper comparator would go high if V2 were less than V1(1 - α ), whereas the output of the lower comparator would go high if V1 were less than V2(1 - α ). It is noted that in this scheme, it would not matter which voltage (V1 or V2) was the "normal" voltage; instead, if either voltage deviated from the other by a fraction > α , the behavior would be deemed to be abnormal, causing the circuit to generate an "out-of-bounds" signal.
A Duplicate of the Circuit To Be Protected would be operated concurrently, under the same conditions. The currents drawn by the protected circuit and its duplicate would be indicated by V1 and V2. If either of V1 or V 2 differed from the other by a fraction greater than R1/(R1+ R2) = α , then the circuit would generate an "out-of-bounds" signal.
This work was done by Douglas W. Caldwell of Caltech forNASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20143, volume and number of this NASA Tech Briefs issue, and the page number.
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Self-checking circuitry for detecting single-event latchups
(reference NPO20143) is currently available for download from the TSP library.
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