Match sensing using match-line stability in content-addressable memories (CAM)

This paper presents a match-line (ML) sensing scheme that distinguishes a match from a miss by first shunting every ML with a fixed negative resistance, then exciting the MLs with an initial charge, and subsequently observing their voltage developments. It is shown that the voltage on the matched ML will grow to V-DD, as in an unstable system, whereas the voltage on a missed ML will decay to zero, as in a stable system. Since the initial excitation charge on the ML's can be as low as the noise level in the system, this scheme can approach the minimum possible energy consumption level for match-line sensing. We have implemented, in 0.18 mu m CMOS, a 144 x 144 ternary CAM array that includes the stability-based sensing scheme along with two previously-reported sensing schemes. The measured results confirm the power savings of the proposed sensing scheme. In addition, the CAM includes a pipelined search-line (SL) architecture that can reduce the SL portion of CAM power by up to 50%.