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 mum CMOS, a 144 times 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%.