Reconfigurable Carry Look-Ahead Adder Trading Accuracy for Energy Efficiency

The modern portable devices exhibiting multimedia applications demand higher energy efficient signal processing due to limited battery size. The approximate adders have shown a remarkable energy-efficiency over the accurate adders for error tolerant applications. In this paper, three novel approximate carry look-ahead adder (ACLA) architectures are proposed. These approximate ACLAs are achieved by simplifying the Boolean expression of carry generation logic such that the probability of error is small while eliminating number of logic gates. Further, a novel accuracy reconfigurable CLA (Re-CLA) that provides desired quality/accuracy in the given energy budget is proposed. The post layout synthesis results using Synopsys IC Compiler of the proposed adders are computed and analysed against the existing adders. These results demonstrate 37.67%, 18.21%, 18.14% and 15.92% reduction in energy consumption by the proposed 8-bit ACLA-I, -II, -III and -IV adders respectively over the existing approximate adder. Further, the proposed 8-bit and 16-bit Re-CLAs require only 1.92% and 7.08% more energy over the existing CLA for achieving accuracy reconfigurability. Finally, the synthesis results of the Gaussian smoothing filters embedded with the proposed adders show higher energy efficiency with acceptable image quality over the state-of-the-art adder architectures.

     

Compact four bit carry look-ahead CMOS adder in multi-output DCVSlogic

A four-bit carry look-ahead (CLA) CMOS adder based on transistor sharing in a multi-output differential cascode voltage switch (MODCVS) logic is presented. This adder uses a new enhanced CLA unit, which enables the generation of all output carries in one single compact gate structure. Simulation results using HSPICE with CMOS 1.0 μm technology designs show that the four-bit adder proposed has 15.7% less transistors, 27.2% less silicon area, ~14% speed improvement, and a 29.1% reduction in average power consumption compared to a standard DCVS implementation     

基于流水线的自检测进位相关和加法器设计

文章提出了一种基于流水线设计的具有自检测功能的进位相关和加法器。该加法器包括四个8位进位相关和加法器（CDSA）．一个4位超前进位单元（BLCU）和一个奇偶校验器。与普通的行波进位加法器相比，文章设计的加法器硬件实现面积仅增加3．85％，而在关键路径的延时上，该加法器要减少39．2％。     

Faster parallel multiplier

Realization of a parallel multiplier has been considered in a paper by Dadda [1] who has proposed various schemes to get the product using (3, 2), (2, 2) counters, and carry look-ahead adders. The complexity of the carry look-ahead adder in terms of number of two-input gates increases with the length of the adder which in effect reduces the speed. This letter presents an approach that reduces the length of carry look-ahead adder, thus increasing the computation speed with a reduction in logic complexity.     

Ultrafast compact 32-bit CMOS adders in multiple-output dominologic

A dynamic CMOS logic style, called multioutput domino logic (MODL), has been developed. In this logic style, single logic gates produce multiple functions, and a circuit's device count can be reduced by a factor of more than 2, depending on the degree of recurrence in the circuit. In addition, MODL circuits are, by construction, considerably more stable than other dynamic circuits including conventional domino. A 32-bit carry lookahead (CLA) structure which reduces the adder's worst-case path by two logic stages has also been devised. This CLA structure has been developed to effectively utilize the advantages of MODL. Taken together, these developments have resulted in two 32-bit CMOS adders, providing area and speed improvements of 1.5× and 1.7× over the combination of the domino and conventional CLA techniques. Both adders have been fabricated in a standard 0.9-μm two-level metal CMOS technology, and measured results show that the straight adder has achieved 32-bit addition times of less than 3.1 ns at 25°C with V_DD+5.0 V     

Low-power design techniques for high-performance CMOS adders

A high-performance adder is one of the most critical components of a processor which determines its throughput, as it is used in the ALU, the floating-point unit, and for address generation in case of cache or memory access. In this paper, low-power design techniques for various digital circuit families are studied for implementing high-performance adders, with the objective to optimize performance per watt or energy efficiency as well as silicon area efficiency. While the investigation is done using 100 MHz, 32 b carry lookahead (CLA) adders in a 0.6 μm CMOS technology, most techniques presented here can also be applied to other parallel adder algorithms such as carry-select adders (CSA) and other energy efficient CMOS circuits. Among the techniques presented here, the double pass-transistor logic (DPL) is found to be the most energy efficient while the single-rail domino and complementary pass-transistor logic (CPL) result in the best performance and the most area efficient adders, respectively. The impact of transistor threshold voltage scaling on energy efficiency is also examined when the supply voltage is scaled from 3.5 V down to 1.0 V     

Evaluation of two-summand adders implemented in ECDL CMOSdifferential logic

Four different adders were implemented using a CMOS differential logic, enable/disable differential CMOS logic (ECDL). The authors discuss the design and implementation of several common addition algorithms using ECDL. These adders have the self-timed characteristic. Comparisons are made among these algorithms in terms of delay and area. The actual implementation was done with MOSIS 3-μm scalable process. Evaluations are performed in terms of area and delay. One conclusion that can be made is that the carry-skip adder seems to have the best speed/area combined performance. A first-order modeling method is used to estimate the area and speed of different implementations     

High performance 8 bit cascaded carry look ahead adder with precise power consumption

Multiple bit adders like ripple carry adder make the propagation of carry bit very slow and this is the reason why it must be replaced with fast adders as carry‐look‐ahead adder (CLA). Power consumption in digital circuits depends on the number of metal–oxide–semiconductor field‐effect transistor employed and various other parameters. If number of metal–oxide–semiconductor field‐effect transistor is reduced the power consumption would definitely be reduced. Conventional CLAs would consume significant amount of power that still needs to be improved. The paper here deals with the implementation of 8 bit CLA with the aim of reducing the size and to precise the power consumption within nanowatt range, by improving the fundamental components of the circuit. All the parameters have been calculated by using Cadence Virtuoso tool at 45 nm technology. Copyright © 2014 John Wiley & Sons, Ltd.     

A review of 0.18-/spl mu/m full adder performances for tree structured arithmetic circuits

The general objective of our work is to investigate the area and power-delay performances of low-voltage full adder cells in different CMOS logic styles for the predominating tree structured arithmetic circuits. A new hybrid style full adder circuit is also presented. The sum and carry generation circuits of the proposed full adder are designed with hybrid logic styles. To operate at ultra-low supply voltage, the pass logic circuit that cogenerates the intermediate XOR and XNOR outputs has been improved to overcome the switching delay problem. As full adders are frequently employed in a tree structured configuration for high-performance arithmetic circuits, a cascaded simulation structure is introduced to evaluate the full adders in a realistic application environment. A systematic and elegant procedure to scale the transistor for minimal power-delay product is proposed. The circuits being studied are optimized for energy efficiency at 0.18-/spl mu/m CMOS process technology. With the proposed simulation environment, it is shown that some survival cells in stand alone operation at low voltage may fail when cascaded in a larger circuit, either due to the lack of drivability or unsatisfactory speed of operation. The proposed hybrid full adder exhibits not only the full swing logic and balanced outputs but also strong output drivability. The increase in the transistor count of its complementary CMOS output stage is compensated by its area efficient layout. Therefore, it remains one of the best contenders for designing large tree structured arithmetic circuits with reduced energy consumption while keeping the increase in area to a minimum.     

有效的模(2n+1)乘算法及其VLSI设计

模乘运算在剩余数值系统、数字信号处理系统及其它领域都具有广泛的应用,模乘法器的硬件实现具有重要的作用。提出了一种改进的模(2~n 1)余数乘法器的算法及其硬件结构,其输入为通常的二进制表示,因此无需另外的输人数据转换电路而可直接用于数字信号处理应用。通过利用模(2~n 1)运算的周期性简化其乘积项并重组求和项,以及采用改进的进位存储加法器和超前进位加法器优化结构以减少路径延时和硬件复杂度。比较其它同类设计,新的结构具有较好的面积、延时性能。     

Variants of an improved m-valued carry look-ahead adder

An improved m-valued carry look-ahead adder has been described by Manzoul et al. It is based on Ling's type of binary adder. Ling's adder is faster and less expensive than the conventional carry look-ahead binary adder. In this paper, the concept of Ling's approach is explained with a map-method first. Then a few improved m-valued carry look-ahead adders are proposed.     

A BiCMOS wired-OR logic

This paper proposes a BiCMOS wired-OR logic for high-speed multiple input logic gates. The logic utilizes the bipolar wired-OR to circumvent the use of a series connection of MOS transistors. The BiCMOS wired-OR logic was found to be the fastest compared with such conventional gates as CMOS NOR, BiCMOS multiemitter logic and CMOS wired-NOR logic, when the number of inputs was more than four and the supply voltage was 3.3 V. The BiCMOS wired-OR logic was also determined to be the fastest of the four when the fan-out number was below 20 and the number of inputs was eight. In addition, the speed was more than twice as faster when the fan-out number was less than 10. The BiCMOS wired-OR logic was applied to a 64-b 2-stage carry look-ahead adder, and was fabricated with a 0.5-μm BiCMOS process technology. A critical path delay time of 3.1 ns from an input to a sum output was obtained at the supply voltage of 3.3 V. This is 35% faster than that of conventional BiCMOS adders     

An enhanced high-speed multi-digit BCD adder using quantum-dot cellular automata

The advent of development of high-performance, low-power digital circuits is achieved by a suitable emerging nanodevice called quantum-dot cellular automata(QCA). Even though many efficient arithmetic circuits were designed using QCA, there is still a challenge to implement high-speed circuits in an optimized manner. Among these circuits, one of the essential structures is a parallel multi-digit decimal adder unit with significant speed which is very attractive for future environments. To achieve high speed, a new correction logic formulation method is proposed for single and multi-digit BCD adder. The proposed enhanced single-digit BCD adder(ESDBA) is 26% faster than the carry flow adder(CFA)-based BCD adder. The multi-digit operations are also performed using the proposed ESDBA, which is cascaded innovatively. The enhanced multi-digit BCD adder(EMDBA) performs two 4-digit and two 8-digit BCD addition 50% faster than the CFA-based BCD adder with the nominal overhead of the area. The EMDBA performs two 4-digit BCD addition 24% faster with 23% decrease in the area, similarly for 8-digit operation the EMDBA achieves 36% increase in speed with 21% less area compared to the existing carry look ahead(CLA)-based BCD adder design. The proposed multi-digit adder produces significantly less delay of(N-1)+3.5 clock cycles compared to the N*One digit BCD adder delay required by the conventional BCD adder method. It is observed that as per our knowledge this is the first innovative proposal for multi-digit BCD addition using QCA.     

An area-efficient static CMOS carry-select adder based on a compact carry look-ahead unit

This paper presents a highly area-efficient CMOS carry-select adder (CSA) with a regular and iterative-shared transistor structure very suitable for implementation in VLSI. This adder is based on both a static and compact multi-output carry look-ahead (CLA) circuit and a very simple select circuit. Comparisons with other representative 32-bit CSAs show that the proposed adder reduces the area by between 25 and 16%, the number of transistors by between 43 and 30%, and the dynamic power supply between 35 and 16%, while maintaining a high speed.     

A low-power adder operating on effective dynamic data ranges

To design a power-efficient digital signal processor, this study develops a fundamental arithmetic unit of a low-power adder that operates on effective dynamic data ranges. Before performing an addition operation, the effective dynamic ranges of two input data are determined. Based on a larger effective dynamic range, only selected functional blocks of the adder are activated to generate the desired result while the input bits of the unused functional blocks remain in their previous states. The added result is then recovered to match the required word length. Using this approach to reduce switching operations of noneffective bits allows input data in 2's complement and sign magnitude representations to have similar switching activities. This investigation thus proposes a 2's complement adder with two master-stage and slave-stage flip-flops, a dynamic-range determination unit and a sign-extension unit, owing to the easy implementation of addition and subtraction in such a system. Furthermore, this adder has a minimum number of transistors addressed by carry-in bits and thus is designed to reduce the power consumption of its unused functional blocks. The dynamic range and sign-extension units are explored in detail to minimize their circuit area and power consumption. Experimental results demonstrate that the proposed 32-bit adder can reduce power dissipation of conventional low-power adders for practical multimedia applications. Besides the ripple adder, the proposed approach can be utilized in other adder cells, such as carry lookahead and carry-select adders, to compromise complexity, speed and power consumption for application-specific integrated circuits and digital signal processors.     

用于加法器的功耗延迟积优化混合进位算法

为了实现高性能的加法器,提出了面向功耗延迟积(PDP)优化的混合进位算法。该算法能快速搜索加法器的混合进位,以优化PDP。采用超前进位算法和行波进位算法交替混合,兼具超前进位算法速度快和行波进位算法功耗低的特点。该算法采用C语言实现并编译,结果应用于MCNC Benchmark电路,进行判定测试。与应用三种传统算法的加法器相比,应用该算法的加法器在位数为8位、16位、32位和64位时,PDP改进量分别为40.0%、70.6%、85.6%和92.9%。     

TC~2CLA的混合模块延迟公式及优化序列

为提高长加法器的运算速度,扩展操作位数,提出了一种加法器结构--混合模块顶层进位级联超前进位加法器(TC2CLA).该结构将层数Mi>1的CLA模块底层进位级联改为顶层超前进位单元进位级联.在CLA单元电路优化和门电路标准延迟时间tpd的基础上,由进位关键路径推导出混合模块TC2CLA的模块延迟时间公式,阐明了公式中各项的意义.作为特例,导得了相同模块TC2CLA的模块延迟时间公式.并得出和证明了按模块层数递增级联序列是混合模块TC2CLA各序列中延迟时间最短、资源(面积)占用与功耗不变的速度优化序列.这一结论成为优化设计的一个设计规则.还给出了混合模块级联序列数的公式和应用实例.TC2CLA和CLA的延迟时间公式表明,在相同模块序列和不等待(组)生成、传输信号的条件下,最高位进位延迟时间及最高位和的最大延迟时间减小.     

Optimized carry lookahead adders with direct feeding

This paper describes improvements to the parallel prefix adder designs and optimization algorithms of Chan, Oklobdzija, Schlag, Thomborson and Wei. Our “direct feeding” (DF) adder design avoids large signal fanouts along critical adder paths. Our “random pruning” heuristic limits the time and space required to find near-optimal DF adders, so that the search process runs in a few minutes on a Sun-4 workstation. Our improved carry lookahead adders are well suited for static CMOS implementation; our improvements may be applied to other parallel prefix CMOS circuits. Simulations with Mentor Graphics' Lsim indicate that our best DIP adders are 12% to 20% faster than the carry lookahead adders presented by Chan et al.     

Performance analysis of low power high speed pipelined adders for digital /spl Sigma//spl Delta/ modulators

The carry skip adder (CSA) is widely assumed to outperform the carry lookahead adder (CLA) in terms of power and area. However, for pipelined adders used in digital SigmaDelta modulators (DDSM), it is shown that the CLA has similar performance to the CSA architecture when low bit blocks are used. Furthermore, the CSA outperforms the CLA in terms of glitch content and hence the CSA is more suitable for the operational frequencies of DDSMs     

3.3-V BiCMOS current-mode logic circuits for high-speed adders

New high-speed BiCMOS current mode logic (BCML) circuits for fast carry propagation and generation are described. These circuits are suitable for reduced supply voltage of 3.3-V. A 32-b BiCMOS carry select adder (CSA) is designed using 0.5-μm BiCMOS technology. The BCML circuits are used for the correct carry path for high-speed operation while the rest of the adder is implemented in CMOS to achieve high density and low power dissipation. Simulation results show that the BiCMOS CSA outperforms emitter coupled logic (ECL) and CMOS adders