On the p-untestability of combinational faults

We describe the p-untestability of faults in combinational circuits. They are similar to redundant faults, but are defined probabilistically. P-untestable fault is a fault that is not detectable after N random pattern simulation or a fault, FAN either proves to be redundant or aborts after K backtracks. We chose N to be about 1000000 and K to be about 1000. We provide a p-untestability detectability algorithm that works in about 85% of the cases, with average of about 14% false negatives. The algorithm is a simple hack to FAN and uses structural information and can be easily implemented. The algorithm does not prove redundancy completely but establishes a fault as a probabilistically redundant, meaning a fault with low probability of detection or no detection.