A parallel-serial-combined search scheme, which performs a multi-bit-by-multi-bit parallel-serial search for a single search, is proposed for a magnetic tunnel junction (MTJ)-based high-density and energy-efficient nonvolatile ternary content-addressable memory (TCAM). A two transistor and two MTJ device (2T-2MTJ)-based TCAM cell circuit can be utilized for a bit-parallel search operation up to 4 bits under random variations of MOS and MTJ device characteristics by amplifying the multi-bit cell-array resistance difference owing to the source-degeneration cell structure in combination with the cascode structure of the pre-amplification stage in the word circuit. In the proposed parallel-serial-combined search scheme, the bit length of a parallel operation in a single cycle and the search cycle count are optimized, so that the cell activity is minimized by tuning the trade-off between power consumption and search speed. When the proposed nonvolatile TCAM performs a variable-bit parallel-serial-combined search, the cell activity of the proposed nonvolatile TCAM is reduced to 60% of that of a conventional bit-parallel nonvolatile TCAM with a three-level segmentation scheme, which indicates higher density and higher energy efficiency with acceptable search speed.
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