Low-power content addressable memory design using two-layer P-N match-line control and sensing

摘要

Content addressable memory (CAM) is a specialized search engine mostly used for speeding memory lookup in network devices. Despite fast searching, activation of all comparison circuits in every clock cycle costs huge power. Power dissipation is more severe in high capacitive NOR match-line (ML) because of higher precharge activity and multiple transitions in ML. This paper proposes a two-layer ML scheme to reduce power due to frequent ML switching between precharge and evaluation phases. The complementary charging property of P and N matching circuits of NOR cells are utilized with the help of a ML precharge and sensing (MLPS) block to charge up only the matched entry while the mismatched entries are held at pre-discharged levels. Also, charging up the first layer due to mismatch limits the discharge levels of the mismatched second layer. These techniques reduce precharge activity besides lessening evaluate-power. Based on a 45-nm CMOS technology, post-layout analysis of the 64 x 32-bit proposed CAM at 1-V supply shows 56% and 24% reductions in precharge-power over a conventional CAM and a gated-power ML sensing CAM, respectively. In addition, the total ML power saving of approximately 2x is achieved when compared to a high-performance master-slave ML and a local-NOR global-NAND ML based CAMs besides decreased macro area. With the help of a charge-hold and charge-up sensing scheme, the proposed design achieves a match function in only 223.52 ps and dissipates 1.42 fJ/bit/search favouring it to be an efficient energy-delay design among the compared designs.