A hyper optimal encoding scheme for self-checking circuits

Jien Chung Lo, IEEE


A typical self-checking circuit has an unordered code encoded output. The optimal scheme needs ⌈log (r + 1)⌉ check bits, where r is the number of unique weights in all output patterns. A hyper optimal scheme for self-checking output encoding is proposed in this paper where the number of check bits will be further reduced in some cases. Two algorithms are presented to search for the hidden m-out-of-n code words. The hidden m-out-of-n code words are found when all unique output patterns, specified by the circuit specification, in the n selected output bits have exactly m 1s. The output bits that belong to the hidden m-out-of-n code words are then excluded from further encoding. Typically, the number of added check bits of the proposed technique ranges from 0 to ⌈log (p + 1)⌉, where p ≤ r. When hidden m-out-of-n code words exist, applying the proposed scheme usually results in significant hardware cost and delay time reduction. In the five MCNC FSM benchmark circuits that have been identified with hidden m-out-of-n code words, 10% to 41% hardware reductions are exhibited compared to the theoretically optimal separable code encoding scheme. In addition, 7% to 45% reductions in checking delays are demonstrated for the same circuits compared to the separable code encoding scheme. ©1996 IEEE.