Current Sensing Completion Detection for high speed and area efficient arithmetic

Providing carry completion signaling in low cost ripple carry adders can allow the control logic to schedule the next addition as soon as an earlier one is complete, thereby achieving the average case, rather than worst case addition delay over a set of computations. Earlier attempts at using current sensing for such carry completion signaling suffered from serious limitations. In this paper we present a new approach for the design of a ripple carry adder with a current sensing capability which observes late settling carry signal nodes in the circuit and indicates when they reach a quiescent state. Simulations show better than 50% speedup, on average, with less than 10% area overhead. To demonstrate a potential application of such an approach, we incorporate our carry completion adder into a Booth multiplier design and study the performance gain over a traditional ripple carry adder based design. Simulation results show that a 32-bit Booth Multiplier using the new completion signaling circuits can outperform a 32-bit Booth Multiplier with ripple carry adder (RCA) by 20-30%, while requiring less than 2% additional silicon area, This is comparable to the gains from the best carry look ahead adder designs at a fraction of the area overhead costs.