An Exploration of Microprocessor Self-Test Optimisation Based On Safe Faults

Microprocessor software test libraries (STLs) must provide maximum fault coverage with minimum overhead. Pruning safe faults, which cannot cause errors in the output of the processor, from the fault list can increase fault coverage without adding test overhead. Applying more application-specific constraints can lead to the identification of more safe faults, and some such constraints are yet to be explored. This work explores the use of signal combination-based constraints alongside well-known constant signal-based constraints for identifying safe faults. Also, for the first time, information on safe faults is utilised during test compaction in order to further minimise test overhead. Results for an OpenRISC processor design show up to 2.33% improvement in fault coverage with the use of the proposed constraints. In one test program, a code segment contributing only to the coverage of safe faults is identified, with its removal providing a 1.09 % code size reduction on top of existing compaction techniques. The results may vary for a larger and more complex commercial design with greater scope for redundant logic. This work explores the use of signal combination-based constraints alongside well-known constant signal-based constraints for identifying safe faults. Also, for the first time, information on safe faults is utilised during test compaction in order to further minimise test overhead. Results for an OpenRISC processor design show up to 2.33% improvement in fault coverage with the use of the proposed constraints. In one test program, a code segment contributing only to the coverage of safe faults is identified, with its removal providing a 1.09 % code size reduction on top of existing compaction techniques. The results may vary for a larger and more complex commercial design with greater scope for redundant logic.