Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Low Power and Area Efficient Multiplier Layout using Transmission Gate Sahil Gargi ME Student (ECE) National Institute of Technical Teachers’ Training & Research Chandigarh, India-160019 sahil1038@gmail.com
Abstract: This paper proposes the design and implementation of a 2-bit multiplier using fully automatic design and semicustom design. Any digital signal processor has adder and multiplier in its core unit. Low power and high speed mac units are in high demand and therefore make a significant place in today’s vlsi environment. Power consumption of cmos circuits is a major concern in vlsi design. The proposed design is made using transmission gate logicwhich helped in using less number of cmos. The multiplier circuit is first simulated using avlsi cad tool and thus the layout was generated. The proposed circuit is simulated by using 90nm cmos technology with supply voltage of 1.2v. It is found that semi-custom based design produced better results in terms of power dissipation and area. Keywords: Multiplier, VLSI, Transmission Gate, CMOS, Power, Area.
1.
INTRODUCTION
In most of the VLSI applications arithmetic operations like addition, subtraction, multiplication are commonly used. The time required for multiplication is one of the most dominant factor to evaluate the time needed for instruction cycle of a DSP chip. Performance of a DSP chip is largely limited by its multiplication performance. Demand and popularity of various devices depends on the small silicon area, higher speed, longer battery life and reliability [1]. For VLSI design, emphasis is generally laid on three constraints that is, power, area and delay. Interest has now widely grown in optimizing and minimising the chip area with least power consumption and still maintaining high performance.Statics shows that more than 70% instructions in microprocessor and most of DSP algorithms perform addition and multiplication [2]. Thus a multiplier plays a pivotal role in modern VLSI design and a high speed multiplier with low power consumption is the need of the hour. Minimising the power consumption is still a major concern for designers. Most techniques for saving power involve scaling of the power supply, which resulted in substantial increase in the leakage current. It has been found that there is fundamental connection between computation and power dissipation [3]. A method is proposed to somehow reduce the computation without any loss of information. Thus, with reduced computation, there is a reduction in power consumption in the design process. In this paper a fully automatic design is compared with semi-custom design in terms of all the design constraints. Binary multiplication can be performed using number of techniques. Parallel multiplication uses an array or tree of adders (half or full) to sum the partial products. A n*n multiplier requires, n*(n-2) full adders, n half-adders and n2 AND gates [4]. Shift and add algorithm was the conventional technique used to implement the design. It involved the basic multiplication principle. The partial
products are generated by the multiplication of the multiplicand with one multiplier digit followed by addition of shifted partial products. Traditionally shift and add algorithm has been implemented to design however this is not suitable for VLSI implementation and also from delay point of view[4]. Main advantage of binary multiplication is that the generation of intermediate products are easy [2]. Generally this technique of multiplicationisslow and accumulates a lot of area in hardware. Thus requires more power. With the help of Transmission Gate Logic (TGL) there is reduction in leakage power and leakage current in active mode [5]. TGL has proved to be an attractive alternative to static CMOS designs with respect to area, performance and power consumption [6]. It will improve the overall performance.It is designed by connecting both source to source and drain to drain terminals of NMOS and PMOS transistors [7].This logic style has a great functionality that can reduce the number of transistor counts [8]. A design with a high speed is resulted using a TGL with a reduced transistor count. Thus optimised low power circuit design is obtained. As the NMOS transistor is passing strong ‘0’ signal and PMOS transistors passes strong ‘1’ signal towards the output, the both transistors are turn on/off by an enable signal [7].Inverter followed by Transmission gate structure appears in many CMOS circuit design [9].The multiplier is constructed using AND gates and half adder. The half adder is made using transmission gate which is made using EX-OR and AND gates. The multiplier is schematised by DSCH3.1 CAD tool and its layout is generated using CMOS 90nm technology in Microwind 3.1 CAD tool [10]. 2. MULTIPLIER DESIGN This paper presents design approach for a 2-bit multiplier. Two types of multiplier designs are proposed here using Ex-OR and AND gates. A 2-bit multiplier will have a1, a0, b1, b0 as inputs and z3, z2, z1, z0 as outputs. The truth table is shown in Table 1. Table 1: Truth table of 2-bit multiplier
a1 0 0 0 0 0 0 0 0
Inputs a0 b1 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1
b0 0 1 0 1 0 1 0 1
z3
Outputs z2 z1
z0
1 1 1
1