22-10-2012, 03:14 PM
Design Of 128 Bit Low Power and Area Efficient Carry Select Adder
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Abstract—
Design of area- and power-efficient high-speed data path logic systems forms the largest areas of research in VLSI system design. In digital adders, the speed of addition is limited by the time required to transmit a carry through the adder. Carry Select Adder (CSLA) is one of the fastest adders used in many data-processing processors to perform fast arithmetic functions. From the structure of the CSLA, it is clear that there is span for reducing the area and power consumption in the CSLA. This work uses a simple and efficient gate-level modification to drastically reduce the area and power of the CSLA. Based on this modification 8, 16, 32, 64 and 128-bit square-root CSLA (SQRT CSLA) architectures have been developed and compared with the regular SQRT CSLA architecture. The proposed design has reduced area and power as compared with the regular SQRT CSLA with only a minor increase in the delay. This work estimates the performance of the proposed designs in terms of delay, area, power, and their products are implemented in Xilinx FPGA. The results analysis shows that the proposed CSLA structure is better than the regular SQRT CSLA. The proposed design is applied to the FIR filter structure in the adder part.
INTRODUCTION
Area and power reduction in data path logic systems are the main area of research in VLSI system design. High-speed addition and multiplication has always been a fundamental requirement of high-performance processors and systems. In digital adders, the speed of addition is limited by the time required to propagate a carry through the adder. The sum for each bit position in an elementary adder is generated sequentially only after the previous bit position has been summed and a carry propagated into the next position. The major speed limitation in any adder is in the production of carries and many authors have considered the addition problem.
The CSLA is used in many computational systems to moderate the problem of carry propagation delay by independently generating multiple carries and then select a carry to generate the sum. However, the CSLA is not area efficient because it uses multiple pairs of Ripple Carry Adders (RCA) to generate partial sum and carry by considering carry input and then the final sum and carry are selected by the multiplexers (mux).
BASIC STRUCTURE OF REGULAR 16-BIT SQRT CSLA
A 16-bit carry select has two types of block size namely uniform block size and variable block size. A 16-bit carry select adder with a uniform block size has the delay of four full adder delays and three MUX delays. While a 16-bit carry select adder with variable block size has the delay of two full adder delays, and four mux delays. Therefore we use 16-bit carry select adder with variable block size. Ripple-carry adders are the simplest and most compact full adders, but their performance is limited by a carry that must ripple from the least-significant to the most-significant bit. A carry-select adder achieves speeds 40% to 90% faster by performing additions in parallel and reducing the maximum carry path.
IMPLEMENTATION RESULTS
The design proposed in this paper has been developed using Verilog-HDL and synthesized in Xilinx ISE 9.1i. For each word size of the adder, the same value changed dump (VCD) file is generated for all possible input conditions and imported the same to Xilinx ISE 9.1i Power Analysis to perform the power simulations. The similar design flow is followed for both the regular and modified SQRT CSLA. Table 5 exhibits the simulation results of both the CSLA fir filter structures in terms of delay, area and power.