Implementation and Performance Evaluation of Prefix Adders uing FPGAs

Parallel Prefix Adders have been established as the most efficient circuits for binary addition. The binary adder is the critical element in most digital circuit designs including digital signal processors and microprocessor data path units. The final carry is generated ahead to the generation of the sum which leads extensive research focused on reduction in circuit complexity and power consumption of the adder. In VLSI implementation, parallel-prefix adders are known to have the best performance. This paper investigates four types of carry-tree adders (the Kogge-Stone, sparse Kogge-Stone, spanning tree, Brent kung Adder) and compare them to the simple Ripple Carry Adder and Carry Skip Adder. These designs of varied bit-widths are simulated using model-sim simulator of 6.4 version and implemented on a Xilinx 10.1 version Spartan 3E FPGA. These fast carry-chain carry-tree adders support the bit width up to 256. We report on the area requirements and reduction in circuit complexity for a variety of classical parallel prefix adder structures. Keywords-FPGA, Prefix Adder, ALU, Xilinx, VLSI, CLA, Simulation, Synthesis