On-line IDDQ testing of security circuits

The new at-speed on-line IDDQ testing method is based upon the properties of a special class of security circuits. These circuits implement dual-rail encoding and return-to-spacer protocol, where the spacer is either all-zeroes or all-ones. The alternating spacers of different polarity guarantee that all wires switch once within each clock cycle, thus making energy consumed within a clock cycle independent from data processed. This property earlier used for security purposes is now exploited in order to separate the transient current from the quiescent current, thus making it possible to measure the latter under normal operation. Power signatures are analysed in the frequency domain and the fault signature filtration method is proposed. The proposed method can be used in both production, where it covers all interconnect stuck-at faults in just two clock periods; and on-line testing, where it guarantees the bounded and short period of self-test independent from data. From security point of view, it directly detects a side channel created under a fault injection attack.