Low-power design techniques for high-performance CMOS adders

An optimal high-speed and low-power VLSI architecture requires an efficient arithmetic processing unit that is optimized for speed and power consumption. Adders are one of the widely used in digital integrated circuit and system design. High speed adder is the necessary component in a data path, e.g. Microprocessors and a Digital signal processor. The present paper proposes a novel high-speed adder by combining the advantages of Carry Look Ahead Adder (CLAA) and Carry Select Adder (CSA), devising a hybrid CSA. In the proposed adder, CSA uses CLAA technology to generate the carry bits for each sum bit, which are then used to select the respective multiplexer (MUX) which adds the carry bit to the sum accordingly. The proposed adder has been synthesized with bulk 40 nm standard CMOS library on Synopsys Design Compiler. Analysis has indicated the superiority of proposed adder over CLAA and CSA. As compared to CSA and CLAA, the proposed Carry Select Ahead Adder (CSAA) provides shorter average path and a simpler hardware. This has led to faster processing speed by increasing the complexity of circuit on the chip. The proposed adder finds its applications in various Arithmetic and Logic Units (ALU) of CPUs for faster arithmetic results. Keywords—Carry Look Ahead Adder (CLAA), Carry Select Adder (CSA), Multiplexer (MUX), Carry Select Ahead Adder (CSAA), Arithmetic and Logic Unit (ALU).

Since miniaturization of CMOS has taken place, speed and power are the most important parameters for any circuit, like speed of an adder is a critical parameter in the overall performance of an Arithmetic Logic Unit (ALU). In order to identify the best suitable 2-bit adder in terms of speed, power dissipation, area and leakage current acomparative study of five different kinds of standard 2-bit adder architectures, Ripple Carry (RCA), Carry Look ahead (CLA), Manchester Carry Chain (MCC), Carry Select (CSL) and Kogge Stone (KSA) adders has been done in this paper.The adders are re-designed, implemented and analyzed at supply voltages ranging from 0.6 - 1 V on 28nm CMOS Technology to improve the speed and power. Simulation results for the different adders show that MCC at 0.6V is 32.4% faster than RCA and 15.6% faster than CLA. RCA having minimum transistor count has the least power dissipation and CLA gives minimum leakage current for lower supply voltages, but as the supply voltage increases MCC produces the least leakage current.

In this paper, a new high-speed circuit technique called differential cascode voltage switch with pass-gate (DCVSPG) logic tree is presented. The circuit technique is designed using a pass-gate logic tree in DCVSPG instead of the nMOS logic tree in the conventional DCVS circuit, which eliminates the floating node problem. By eliminating the floating node problem, the DCVSPG becomes a new type of ratioless circuit, and it also provides superior performance with less power dissipation and better silicon area tradeoff. The basic DCVSPG design technique, the methodology for optimization, and synthesis of the pass-gate logic tree are described. The standard cell library development taking advantage of the dual-rail outputs of DCVSPG gates is also discussed. The performance comparisons with other existing pass-gate circuit techniques [complimentary pass-transistor logic (CPL), double pass-transistor logic (DPL), and swing restored pass-transistor logic (SRPL)] are presented. For more robust design, the DCVSPG with inverter buffers is also the best choice. A Viterbi macro design using the DCVSPG circuit technique is demonstrated. The process that the design is based upon is a 0.5-/spl mu/m CMOS technology with 0.25-/spl mu/m effective channel length and five layers of metal. This macro can run up to 500 MHz at the nominal process condition. In comparison with other existing dynamic circuit techniques, the results also clearly show that the dynamic DCVSPG has the superior power-delay performance.

Our objective was to determine the most energy efficient 64 b static CMOS adder architecture, for a range of high-performance delay targets. We examine extensively carry-lookahead (CLA) and carry-select adders with a wide range of tradeoffs in logic levels, fanouts and wiring complexity. We propose sparse CLA adder architectures based on buffering techniques to reduce logic redundancy and improve energy efficiency. All the designs were implemented using an energy-delay layout optimization flow with full RC extraction. Our new 64 b adder designs have a relative delay as low as 9.9 F04 (fanout-offour inverter) delays and promise better scaling for smaller technology nodes. They yield the best energy efficiency for a wide range of delay targets and are 30%, 15% and 7% more energy efficient than full Kogge-Stone, sparse-2 Kogge-Stone and Han-Carlson, respectively, at the fastest points. They consume only about 1/3 the energy of dynamic adders.

In electronic applications, better performance of the digital systems can be achieved using a faster adder circuit. This paper shows a modified carry-select adder (CSA) architecture which has low power and reduced area compared to the regular CSA. This high speed CSA is achieved by replacing the existing binary to excess-1 converter (BEC-1) with D-latch enabled CSA. Regular square-root CSA (SQRT CSA) architectures have also been developed and compared with the proposed BEC-1 D-latch enabled CSA. This work evaluates the performance of delay, area, power, and their products for existing and proposed CSA designs. This shows that proposed D-latch CSA structure is better than the regular and modified SQRT CSA and existing Ripple CSA. The proposed CSA architecture requires approximately 86 % fewer gates and 77 % shorter delay than the original CSA designs.

This paper analyses various circuits in CMOS and CNTFET technology. CNTFET technology along with Multi valued logic, complementary pass transistors, ternary logic and double pass transistor logic is far better than their CMOS counter parts. Present VLSI circuit designs can be potentially improvised through such techniques and circuits. This paper provides a glimpse of recent advancements in these techniques. Circuits such as Decoders, exact multipliers, inexact multipliers and logic gates, are reviewed in both the CMOS and CNTFET with various Logics such as Ternary, DPL, MVL logic. Exact multiplier’s energy consumption is reduced upto 6 times and energy efficiency is improved upto 35 times in inexact design, studied here. CNTFET showed tremendous improvements.

Computational speed of basic data path elements such as adders plays a crucial role in performance of digital and signal processing systems. Among various adder architectures, carry select adder (CSLA) has better performance in terms of speed in comparison with others. However, this merit is accompanied by its high power dissipation and area occupancy due to redundant computing elements used in each stage hence limiting its application. Power dissipation in these adders can be minimized by various low power implementation techniques such as reversible logic gates, multi-V th approach, and adiabatic logic. Among these low power approaches, adiabatic logic proves to be more efficient in comparison with other low power approaches. In the proposed work, 180 nm CMOS technology in CADENCE environment is used to carry out the design and analysis of both conventional and proposed CSLA architectures. Power dissipation in this proposed 8-bit CSLA architecture reduces by about 82% as compared to conventional 8-bit CSLA adder architecture. Further, the computational delay is also reduced when compared with the conventional 8-bit CSLA architecture. It is noteworthy that delay in adiabatic logic circuits greatly depends on frequency of operation [1].

Carry select adder (CSLA) one of the fastest adders used in complex data processing to perform fast arithmetic functions. Minimum amount of power consumption is a major driving force behind the development of CMOS technologies. From the structure of CSLA it is clear that there is scope for reducing the area & power consumption in the CSLA. In this paper 8T full adder is used as a building block for 8-bit SQRT CSLA. 8T full adder is designed by XNOR-XNOR hybrid CMOS design. Based on this modification, 8-, 16-, 32-, 64-bit SQRT CSLA architecture has been developed & compared with the regular SQRT CSLA. Binary to Excess one converter (BEC) based CSLA are analyzed to study the data dependence & to identify all redundant logic operations. In this paper, all the redundant logic operations in the conventional CSLA are eliminated and a new logic formulation where carry select operation is scheduled before the calculation of final sum is proposed. Gate Diffusion Input (GDI) technique of low power digital combinatorial circuit design is also described. The frequency of operation is 5 MHZ. This work estimates the performance of the proposed design with the regular design in terms of area & power through the logic design & layout in 250nm CMOS process technology & implemented using Tanner (S-Edit) tool. The result analysis shows that the proposed structure is better than the conventional CSLA.

We present a 64 b Carry Look-ahead (CLA) adder having a 2.6 ns delay time at 3.3 V power supply within 0.27 mm/sup 2/ using a 0.5 /spl mu/m CMOS technology. We derived its structure from considering the tradeoffs between speed and area. This consideration includes not only the gate intrinsic delay but also the wiring delay and the gate capacitance delay. Moreover we introduced a new carry select scheme called Modified Carry Select (MCS). MCS has 20% area advantage over the conventional Carry Select Adder (CSA).

Full adders are one of the most important building blocks in VLSI digital arithmetic. The area, electrical, timing, power consumed and noise generated characteristics of this cell are strongly dependent on the design technique. An exhaustive work taking into account the above parameters is done, and that complete analysis will be of utility for the community of digital designers. Emphasis will be done in power/noise figures, of most important concern in current CMOS mixed-signal design. The full adders considered are those using complementary CMOS, pass-transistor logic, double pass-transistor logic, and two versions based on CMOS transmission gate. Main parameters as area, delay, power consumption and noise generation have been measured by electrical simulation in a 0.35 μm CMOS technology. The main results obtained are, on one hand, the selection of a logic family for a specific application and, on the other hand, the selection of a specific full adder structure for an optimized parameter option -power, noise or speed.

PurposeAdders play a vital role in almost all digital designs, as all four arithmetic operations can be confined within addition. Hence, area and power optimization of the adders will result in overall circuit optimization. Being the fastest adder, the carry select adder (CSLA) gains higher importance among the different adder styles. However, it suffers from the drawback of increased power and area. The implementation of CSLA in digital circuits requires lots of study for optimization. Hence, to overcome this problem, various improvements were made to the CSLA structure to reduce area and, consequently, reduce power. Among these, modified CSLAs show a significant improvement, as they utilize a binary excess-1 code (BEC) to replace the add-one circuit.Design/methodology/approachThis paper presents further enhancement in the modified CSLA by proposing a decision-based CSLA, which activates BEC on demand. This leads to reduced switching activity. The performance of the proposal is done by analyzing and comparing it with different adders. The comparison is done on the basis of three performance parameters: area, speed and power consumption. This is done by implementing the architecture on Xilinx Virtex5 XC5VLX30 in Verilog environment and is synthesized using Cadence® RTL Compiler® using TSMC 180-nm CMOS cell library.FindingsOptimization of power, area and increasing the speed of operation are the three main areas of research in very-large-scale integration (VLSI) design for portable devices. As adders are the most fundamental units for any VLSI design, optimization at the adder level has a huge impact on the overall circuit. The modified CSLA has a BEC which continuously switches irrespective of the previous carry bit generated. The unwanted switching results in excess power consumption while also introducing additional delay. Hence, the author has proposed a decider circuit to avoid this excess switching activity. This allows switching of the BEC only when a previous carry is generated. The modified CSLA is based on the ripple carry adder, while the decider-based CSLA utilizes a carry look-ahead adder. This makes a decider-based CSLA faster while utilizing less area and power consumption when compared to the modified CSLA.Originality/valueThe efficiency of the proposed decider-based CSLA has been verified using Cadence RTL Compiler using TSMC 180-nm CMOS cell library and has been found to have 17 per cent power and 11.57 per cent area optimization when compared to the modified CSLA, while maintaining operating frequency.

Adders are one of the basic arithmetic circuits of any processor, microcomputer, multiplication circuits, etc. Presently, the most substantial areas in the research of VLSI design are area, power, and efficient high-speed circuits. In this paper two new architectures Carry Select Adder (CSLA) using D-latch and CSLA using multiplexers are proposed to reduce the power, delay, and its efficiency is compared with conventional carry select adder (CSLA) and existing literature. Architectural level power reduction is the most important area where it plays a vital role in improving the speed and power of the overall circuit. The existing and proposed CSLAs are synthesized with the Synopsys EDA tool using 32 nm technology node is used for the design and implementation. The values obtained across technology in nm underline the dominance of the proposed adder architectures in terms of delay and energy and area efficiency. For the proposed architectures, the evaluation results show that 60%–75% improvement in delay and 22%–56%in power when compared to other architectures. Proposed architectures shows increment in area but overall are delay product reduces compares to existing designs The proposed CSLA-DLATCH-FIR obtained significant reduction for 16-bits for power (46.4 (µW)), delay (0.66), ADP (359.8 × 10–15), and PDP (30.63 × 10–15). While the proposed CSLA-MUX-FIR has attained substantial performance for power (52.44 (µW)), delay (0.82 ns), ADP (457.392 × 10–15), and PDP (42.9 × 10–15). With the use of this proposed CSLA, a FIR filter was able to significantly reduce its power and delay.

This paper evaluates the performance of the gate diffusion input technique (GDI) on self-checking and self-resetting logic (SRL) GDI technique of carry select adder (CSA) and carry look-ahead adder (CLA) for different word lengths. In the proposed design adapting level restoration technique eliminates conductance overlap between n-MOS and p-MOS devices. The various self-resetting logic implementations for self-correcting of carry select adder and carry look-ahead adder can detect all single stuck faults in normal operation mode. Besides, two rail encoding technique is used in carry select adder are used for multiple error detection/correction. The SRLGDI setup consists of a pair of full adders, multiplexers, two rail checking, and self-resetting logic. In this technique, the self-checking carry select adder is used for reducing area and self-resetting improves the performance of the adder. The analysis of CSA and CLA with 180nm technology is simulated by using Cadence tools. The delay reduction is 12.836% for 8-bit CLA using SRL GDI, power reduction is 27.55% for 8-bit CSA using SRL-GDI compared to the gate diffusion input technique. This GDI technique implementation saves the average energy consumption up to 31.14% in CSA and 32.83% in CLA making it suitable for power optimization in high speed VLSI systems.

Multimedia applications are among the most dominant computing workloads driving innovations in high performance and cost effective systems. In this regard, modern general-purpose microprocessors have included multimedia extensions (e.g., MMX, SSE, VIS, MAX, ALTIVEC) to their instruction set architectures to improve the performance of multimedia with little added cost to microprocessors. Whereas prior studies of multimedia extensions have primarily focused on a single processor, this paper quantitatively evaluates the impact of multimedia extensions on system performance and efficiency for different number of processing elements (PEs) within an integrated multiprocessor array. This paper also identifies the optimal PE granularity for the array system and implementation technology in terms of throughput, area efficiency, and energy efficiency using architectural and workload simulation. Experimental results with cycle accurate simulation and technology modeling show that MMX-type instructions (a representative Intel’s multimedia extensions) achieve an average speedup ranging from 1.24( (at a 65,536 PE system) to 5.65( (at a 4 PE system) over the baseline performance. In addition, the MMX-enhanced processor array increases both area and energy efficiency over the baseline for all the configurations and programs. Moreover, the highest area and energy efficiency are achieved at the number of PEs between 256 and 1,024. These evaluation techniques composed of performance simulation and technology modeling can provide solutions to the design challenges in a new class of multiprocessor array systems for multimedia.

A multi-valued logic (MVL) pass gate is an important element in configuring multi-valued logic. Multiple logical levels, which are different from binary pass gates, are required to be discriminated in MVL pass gates. In this paper, we designed the quaternary MIN (QMIN)/negated MIN (QNMIN) gate, and the quaternary MAX (QMAX)/negated MAX (QNMAX) gate using double pass-transistor logic (DPL) with neuron MOS (vMOS) threshold gates. In addition, we designed quaternary truncated sum (QTS) and quaternary truncated difference (QTD) gates using vMOS down literal circuits (DLC). The DPL improved the gate speed without increasing the input capacitance. It has a symmetrical arrangement and double-transmission characteristics. The threshold gates are composed of vMOS DLC. The proposed gates obtain the signal value, to realize various multi threshold voltage circuits. In this paper, these circuits use a 3 V power supply voltage and the parameters of the 0.35 /spl mu/m N-well 2-poly 4-metal CMOS technology. HSPICE simulation results are also presented.