Mutual test and diagnosis: architectures and algorithms for spacecraft avionics

We explain how the advantages of built-in self-test (BIST) can be surpassed by mutual test and diagnosis (MTAD). By applying techniques from system level diagnosis, MTAD substantially decreases (1) the percentage of faulty circuits misdiagnosed as good, (2) the area required to analyze the response of a circuit under test, and (3) the probability of aliasing. In addition, MTAD drives down the cost of manufacturing spacecraft avionics. The overhead of BIST hardware has been reported as low as 3.5% and as high as 15% of the total circuit area. MTAD reduces this overhead to a near-negligible fraction. Instead of an architecture whose nodes test themselves, MTAD uses an architecture whose nodes compare outputs. This approach runs at about the same speed as BIST, but with less overhead and demonstrably greater accuracy. MTAD is particularly useful for spacecraft avionics multiprocessors, wherein the assumption of a master fault-free node is questionable.