Variable Latency Speculative Han-Carlson Adder

Abstract

Variable latency adders have been recently proposed in literature. A variable latency adder employs speculation: the exact arithmetic function is replaced with an approximated one that is faster and gives the correct result most of the time, but not always. The approximated adder is augmented with an error detection network that asserts an error signal when speculation fails. Speculative variable latency adders have attracted strong interest thanks to their capability to reduce average delay compared to traditional architectures. This paper proposes a novel variable latency speculative adder based on Han-Carlson parallel-prefix topology that resulted more effective than variable latency Kogge-Stone topology. The paper describes the stages in which variable latency speculative prefix adders can be subdivided and presents a novel error detection network that reduces error probability compared to previous approaches. Several variable latency speculative adders, for various operand lengths, using both Han-Carlson and Kogge-Stone topology, have been synthesized using the UMC 65 nm library. Obtained results show that proposed variable latency Han-Carlson adder outperforms both previously proposed speculative Kogge-Stone architectures and non-speculative adders, when high-speed is required. It is also shown that non-speculative adders remain the best choice when the speed constraint is relaxed.