Area Efficient QCA Barrel Shifter - onlinepresent.org

Advanced Science and Technology Letters Vol.144 (UBWCN 2017), pp.51-57 http://dx.doi.org/10.14257/astl.2017.144.07

Area Efficient QCA Barrel Shifter Nuriddin Safoev1 and Jun-Cheol Jeon2 Dept. of Computer Engineering, Kumoh National Institute of Technology 61, Daehak-ro, Gumi, Gyeongbuk 730-701, Korea [email protected] 2Corresponding Author: [email protected]

Abstract. Improving power consumption, physical dimension and also other aspects in CMOS is reaching its limited levels. Quantum-dot cellular automata (QCA) is such technology that offers new techniques of computation and data transmission. This paper presents binary barrel shifter that is used to realize shifting operations in arithmetic unit. The barrel shifter is proposed based on QCA technology. In contemporary arithmetic and logic unit in CPU, barrel shifter is used to shift a desired number of bits in a desired direction. Hence, it can be used as important component for designing QCA microprocessors. To overcome, we have compared proposed design to prior design. The suggested design is simulated in QCADesigner software and acceptable results are achieved as a better performance in the terms of cell numbers, delay and occupied area. The results show that the proposed structure performs equally well and better in case of previous design. Keywords: Quantum-dot cellular automata (QCA), Barrel shifter, Rotator, Nanoelectronics

1 Introduction Developing in the microelectronic industry depend upon the ever-declining size of transistors. According to scientific predictions, CMOS (complementary metal-oxide semiconductor) technology is expected to the end of its roadmap. Some serious challenges, impurity variations, high cost of lithography, short channel effect and other problems occurred with it. Quantum-dot cellular automata (QCA) is one of the alternative technologies that has several advantages over CMOS. It promises extremely low power consumption, high speed switching and small dimension [1-3]. The assumption is that all these advantages will result in the development of highly powerful and efficient computer architectures. QCA technology is first introduced by Lent et al in 1994[3,4]. In the last few years, several basic QCA elements: a QCA cell, small binary wire, and digital logic gate and also some combinational circuits, have been demonstrated and developed. A QCA is the computing with cellular automata composed of arrays of quantum-dot devices. Transmission of the data in QCA is realized using clocking technique. It is controlled by a tunneling barrier. According to the moving up or down of the tunneling barrier, the clocking technique consists of four stages: locking locked, relaxing and relaxed

ISSN: 2287-1233 ASTL Copyright © 2017 SERSC

Advanced Science and Technology Letters Vol.144 (UBWCN 2017)

However, this technology is still young technology and requires much technical work for building circuits. This paper presents QCA barrel shifter that can perform shifting operation in binary math. This component design is for natural size (4, 8, 16…) barrel shifters that perform logical right shift, arithmetic right shift, circular right shift and similar types logical left shift, arithmetic left shift, circular left shift operations depending on the instantiation parameters. A well designed barrel shifter is able to implement bidirectional information shifting [8,14]. The left and right operation is implemented through inversion of the input and output vectors. Several applications including arithmetic operations, variable length coding and bit indexing, all require shifting and rotating operations. Therefore, it can be one of the important building blocks for arithmetic logic application in QCA technology.

2 Related Researches 2.1 QCA In QCA, each QCA cell is represented by using a square structure that is prepared with four quantum dots and two excess electrons. Conceptually, two free electrons are pushed into the QCA cell, and then placed to the diagonal position due to their repulsion within the cell. According to the existing coulombic interaction between the electronic charges, they can occupy diagonal antipodal sites through tunnelling junction as a quantum mechanically [3]. QCA wire is such wire that is constructed by placing QCA cells side-by-side, as shown in Figure 1(a). In QCA paradigm the electrostatic influence on the neighbour cells is the key for information transfer. In the wire, the input cell value is taken as propagated value [5]. If the input is a “0”, then the information propagated from the input to the output will be zero or if the input is a “1”, then one will be transferred to the output. Hence, the all cells in the straight wire are kept the same polarization from the input to the output as shown in Figure 1(a) [3,13].

Input = 1

(a)

A

Output = 1 B

M(A,B,C) C

Input = 1

(b)

Output = 0

(c)

Fig. 1. QCA basic structures: (a) QCA wire, (b) inverter chain, (c) QCA majority gate

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Copyright © 2017 SERSC


Figure 1(b) illustrates an inverter chain. In the inverter chain, the information changed between 0 and 1 as it is being propagated in the chain and it depends on the initial input cell polarization [6,12]. Figure 1(c) shows the form of majority voting gate. There are three inputs A, B, C, one middle cell and the output of the majority gate. Therefore, the majority gate is composed of five QCA cells and calculated with the following equation:

𝑀(𝐴, 𝐵, 𝐶) = 𝐴𝐵 + 𝐵𝐶 + 𝐴𝐶; By using the majority gate, logical an AND gate and logical an OR gate can be implemented. The AND gate is implemented by fixing one of the input values to 0 and the OR gate is implemented by fixing one of the input values to 1. Thus, these are important in the design of logical gates in QCA circuits [5,7]. 2.2 Barrel Shifter A barrel shifter is a combinational logic circuit that has N data inputs, N data outputs and also a set of control inputs that specify how to shift the data between input and output. Barrel shifters are used for shifting and rotating data which is demanded in several applications like floating point adders, variable-length coding, and bitindexing. However, It is commonly used as a part of a microprocessor CPU that can typically specify the direction of shift (left or right), shifting types (logical, circular or arithmetic), and also the amount of shift (typically 0 to N-1, sometimes 1 to N bits) [8]. We begin our study of barrel shifters by presenting a block diagram of 4-bit right shifter circuit. This block diagram is presented by author [9] as a four bit shifter taking inputs X3, X2, X1, X0 and outputting Y3, Y2, Y1, Y0. X3

C

X2

X1

X0

A

S

Y3

Y2

Y1

Y0

Fig. 2. Block diagram of 4 bit right shifter

The shifter is controlled by three control signals, such as “A” for arithmetic shift, “C” for circular shift and “S” for realizing shift operation. Neither of these will have effect if S=0, calling for no shift.


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As a comparison, prior paper is taken to be implemented by author Vetteth et al[10]. In their 4-bit barrel shifter, 2:4 decoder is used to select the appropriate shifting unit. Their design contains four shifting units that generate the output of the barrel shifter by using serially connected OR gates. Blocks are controlled by decoder. Each OR gate made by fixing one of the inputs to the majority gate to 1. The design assumes that all shifting units output 0 unless selected. The decoder is used to select the appropriate shifting unit that means once activated, the incoming bits are shifted.

Fig. 3. Prior QCA 4-bit barrel shifter

4 Proposed Barrel Shifter The proposed 4-bit right QCA barrel shifter is designed, as shown in Figure 4. Fundamentally, the shifter is made up four performance units that are shifting mode (S), input unit (X0, X1, X2, X3), controlling unit (A, C) and resulting unit (Y0, Y1, Y2, Y3) [11]. In this circuit, X3 as a most significant bit and X0 as a least significant bit have been generated by controlling unit and expected results have been received. Inputs X1 and X2 are directly propagated to the final majority gates. The logic block diagram of the proposed shifter is shown in Figure 2.

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Fig. 4. Design of proposed barrel shifter in QCA

The layout of the shifter composed of 16 conventional majority gates and 5 QCA inverters. For getting strong signal throughout the wires, we implement cross-over bridges in the structure. The design uses 498 regular cells and the output is generated after 9 clock phases. Approximately 0.5 µm^2 area is occupied by the structure.


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Fig. 5. Simulation results: circular(left side) and logical(right side) barrel shifter

5 Conclusions In this paper, we have proposed 4-bit right barrel shifter in QCA. An accurate simulation has been taken using QCADesigner tool version 2.0.3 and the design has been also compared according to the cell count, area, and delay with the existing design, as shown in Table.1. Hence, it can be easily accessed by other purposes, due to all the output lines come from the same direction. The proposed circuit is multilayer structure. Furthermore, multilayer QCA circuits can potentially consume much less area and strong signal as compared to planar circuits. So the new design is efficient and stable for implementing other QCA arithmetic applications. Our future goal is to design an efficient arithmetic logic unit(ALU) architecture in QCA technology. Therefore, we are going to use the proposed architecture in our goal. There exist further opportunities for optimization which could lead to densities greater than reported in this work and could be taken up for further studies. Table 1. Comparison result of barrel shifter

Delay

Complexity

Total area (µ𝑚2 )

In Ref. [10]

2021cells

2.4

(No. clock phases) 11

Proposed design

498 cells

0.5

9

Circuits

Acknowledgments. This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (NO. NRF2015R1A2A1A15055749). 56



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