High Speed Frequency-Mapping-Based Associative Memory Using Compact Multi-Bit Encoders and a Path-Selecting Scheme

Abstract

A compact multi-bit encoding concept for nearest-distance search-speed improvement of scalable and reliable associative-memories utilizing a mapping operation of the distances into frequency space is reported. The distance differences are transformed into signal delays which are finally detected with a time-domain winner-take-all (WTA) circuit. Ring oscillators programmable in discrete frequency steps are used for the distance–frequency mapping. This implementation enables to decrease the effects of process-induced variations, because the step size is a constraint-free design parameter. To further improve the search reliability, frequency dividers are used to enlarge the size of the frequency steps. The multi-bit encoder achieves a substantial search-time reduction by optimizing the basic-ring oscillator delay for distance zero with a path-selecting scheme. The proposed multi-bit encoding concept has been evaluated with two test-chip designs in 180 nm complementary metal oxide semiconductor (CMOS) technology. Search-time reductions by a factor 1.7 in typical search cases and a compact circuit implementation are verified.