Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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A Survey on Low Power VLSI Designs Raj Kumari1, Madhu Priya2, Mr. Subhash Chand3

1,2PG Scholar, Department of ECE, NITTTR, Chandigarh, India 3Associate Engineer HCL Infosystems, Noida, India

1kumari09raj@gmail.com, 2madhu19feb.89@gmail.com, 3thakur.subhash578@gmail.com

Abstract- In today’s modern electronics industries energy or power efficiency is most important feature to increase the speed, portability, reliability, popularity and efficiency of electronic products. Reduction in power consumption or low power requirement for a system adds features of low cost, high speed, more efficiency and reliability. CMOS technology is a popular name in the field of low power systems. In the field of CMOS technology various methods are used to make the systems more power efficient like, use of Sleepy transistors, Stack method in which transistor length or width is increased to get reduction in leakage power, use of pre-computation technique with the use of BDD (Binary Decision Diagram), use of SRAM (Static Random Access Memory) for high speed operations. In this paper we survey low power systems in which various techniques are used to reduce the power consumption in different circuit areas of the system to get more power efficient and cost effective electronic systems. Keywords: Power consumption, Leakage power, Flip-Flop, CMOS, BDD, Sleepy Transistor, Adders.

I. INTRODUCTION In VLSI a few years ago cost, reliability, area, performance, delays were considered as of major concern. But as the time passes power becomes an area of major concern. Now-a-days lower power consumption is one of the important challenges for VLSI system designers. It is important to save power in the electronic circuits without compromising state integrity or performance [3]. Also in this era of integration because of the popularity of portable electronics products or battery operated electronics systems,

low power systems becomes more popular. On reducing the power consumption battery life increases as well as overheating of the circuits can be removed. A major application of low power consumption is in the field of mobile technology, as these are battery operated devices. There are so many power factors in a circuit i.e. switching power, short circuit power, power in capacitance, gate current, source leakage current, input rise time, leakage power which affects the power consumption of a device. And by controlling any of these, we can control the power consumption in an electronics circuit. We analyze several existing designs to verify different power optimization techniques at different circuit levels. Reduction in power also increases the reliability and efficiency of a device. With this important need of low power consumption systems development of CMOS technology comes into existence. These devices are best known for their low

power requirements. But it is not enough to use only CMOS devices to minimize the power requirement of system. In CMOS devices power dissipation depends on charging and discharging circuit nodes, where capacitors are connected and switching of these capacitor transitions per clock cycles. Another important dependence factor is transition due to the short circuit current flowing from supply to ground and one more is leakage current in the circuit. The results of these factors are switching activity power, short circuit power and leakage current power. To minimize the total dissipated power in the circuit we have to minimize all these three powers by applying optimization at different levels of abstraction i.e. circuit, logic, architecture and system level. This also focuses on industrial and academic research to minimize power dissipation at various abstraction levels. One another reason for low power requirement is that, due to the expensive packaging techniques, cooling management system, which increases the density by a huge factor on chip.

II. TECHNIQUES FOR LOW POWER CONSUMPTION

In this section, we bring the main schemes which we have surveyed from large number of papers. In these all energy efficient techniques power abstraction is being done at different levels of circuit [1].

Power Reduction at Logic Level Power dissipation at logic level is most difficult to be optimized or minimized, because of latches and flip-flops. At logic level it is most important to reduce power in flip-flops and clock distribution networks (CDN) [5]. One technique utilized is clock gating. At logic level flip-flops and latches are basic elements. In flip-flops we observed if we use Pulse triggered flip-flop designs then because of their shorter discharging paths they will consume low power in the circuit.

A second case at logic level is a large amount of power is consumed in clocking. And this power can be minimized or optimized if we replace simple flip-flops with multi bit flip-flops [9]. Power can be minimized because in multi bit, two bit flip-flops can share the same clock. Hence power consumed by clocking can be reduced further by replacing several flip-flops with multi bit flip-flops. Therefore less

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number of clock sinks exists, and that clock network would have smaller power consumption.

One more pre-computation technique is used in CMOS circuits to minimize power consumption i.e. use of BDD (Binary Decision Diagrams) [14]. BDD is directed acyclic graph (DAG), which represents a Boolean function as a sum of disjoint product form [18]. BDD are binary decision trees for any expression and these are MUX (Multiplexer) based realization. Number of MUX required for a circuit depends on the BDD tree [11]. This MUX realization for a circuit is done using different power reduction techniques.

Power Reduction at Arithmetic and Logical Unit (ALU) In most of the electronic circuits which are designed using VLSI technique an arithmetic logical unit is present which is used to perform arithmetic operations and calculations like addition, subtraction, multiplication and division are performed. And it is noticeable that most of the power is consumed at this part of circuit. To reduce this power consumption we can use different adder circuits in ALU which have less power requirements. We have surveyed some of these circuits which reduces the power consumption by an important factor by applying different techniques.

Generally transistors are used to build these important adder circuitry. As with advancement in integration technology the size of chips shrinking day by day and density of transistors or other components is increasing by a huge amount [12]. And because of this leakage currents increases considerably, thus increases the power consumption. Various techniques are used to reduce this power consumption and one of these techniques is Stack method using sleepy transistors [21]. A sleepy transistor method gives a leakage power reduction with 12.486 ụw as compare to other methods. In this method power reduction can be achieved by increasing length or width , by using clock gating, by implementing multiple threshold CMOS etc [13].

Also a major use of ALU is in Mobile technology or in microprocessors [10]. Thus an important application of low power consumption is in the field of mobile technology. As these are using adders in ALU and also these are battery operated devices. To save power we generally put device in stand-by mode in which a portion of circuit is disconnected from power or shut down. As mentioned above a leakage current also consume power when it is off. Now to reduce this leakage power here again we can utilize the properties of Sleepy transistor. In this technique of power gating Sleep transistor is added between actual ground rail and circuit ground or virtual ground. Thus device is turned off to cut-off the leakage current. In this way sleepy transistor is used in

applications of low power consumption to reduce to total consumed power.

In high performance and portable applications to increase energy efficiency one more adder used in ALU is SERF full adder [24]. SERF full adder is an optimized full adder which is based on XOR gate [19] [20] [22]. Full adders are utilized in multiplier modules of DSP filters.

By knowing the fact that in case of adders speed is limited by time taken by carry to propagate through adder, Which also increases the total power consumption of adders. An adder CSLA (Carry Select Adder) [45] is developed which is a compromise between small area longer delay and longer area shorter delays [29]. It uses multiple pairs of ripple carry adder (RCA) to generate partial sum and carry by considering carry input either 0 or 1[30]. Then final sum and carry are selected by multiplexer.

Power Reduction at Circuit Level Power reduction at circuit level can be done by reducing power consumption across transistors or components used on the chip.

One technique to reduce power at circuit level is to use MOS transistor in di-erent modes i.e. weak and strong inversion layers. These di-erent modes of transistors can be implemented using CNN (Cellular Neural Network) [34]. CNN model is main characteristic of low power consumption, programmability [25]. Chua—Yang model is generally used for simple circuits which is a special type of analog, non-linear processor array [43]. Due to regularity, parallelism and local connectivity found in CNN circuit array it is used for VLSI implementation. To reduce power consumption we process the small currents using MOS transistors in weak inversion for all di-erential pairs.

Second low power consumption technique is to use Scaling and Voltage Islands [44]. This technique partitions the circuit into multiple voltage and frequency islands which reduces the dynamic and leakage power. This technique is similar to the Sleepy transistor technique in which transistor switches off the portion of circuits which are in rest state or not active, to reduce the power consumption. Same is done here using voltage Islands [26]. Voltage Islands reduces the power by selectively shutting down the different portion of chip and run only selected parts at different voltage and frequency levels. This is done at software level with the compiler support using voltage islands. This technique has better performance, energy savings and voltage scaling of memory.

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Power Reduction at Bus Interfaces An idea to reduce power dissipation in circuits comes from the fact that an important part of power is dissipated by the buses [39], which are used for the communication of circuit components. Thus to reduce power dissipation efforts are made for low power bus design.

As a result a technique Bus – invert (BI) [33] [46] Coding is designed to reduce the power dissipation by reducing the number of bus transitions. It is very simple and usable technique because of which a large number of BI (Bus-Invert) algorithms are developed [35]. A major drawback of Bus-Invert technique is, increase in bus areas and because of which degradation occurs into its transition [36].

Therefore a new technique is introduced to overcome this limitation of BI technique. This new technique is TBIC (Two-bit Bus Invert Coding) [37]. It divides an n-bit bus into n/2 sub-buses of width 2 and BI coding is individually applied to each sub-bus [38] . It reduces the bus transition by about 45.7%. Hence total power dissipation reduces as the bus transitions get reduced.

III. CONCLUSION In this paper, we have surveyed various energy efficient techniques from device level to thee architecture level to overcome the problem of power dissipation. Through this whole survey we observed how devices, circuits and architecture within the design space can be optimized for minimum energy consumption. There are so many power factors in a circuit like switching power, short circuit power, power in capacitance, gate current, source leakage current, input rise time, leakage power, which affect the power consumption of a device. And by controlling any of these factors we can control the power consumption in a circuit. It is observed that a number of parameters like leakage current, input supply voltage level, frequency of operation, load capacitance, rise time, switching power, power dissipation capacitance and loading effect directly affects the power consumption of a circuit. Thus if, power consumption is reduced it results in so many benefits like, no requirement of heat sinks because of very less heat generation in the circuit due to low power. Also circuit size get reduced due to the removal of heat sinks or other components associated with high temperature. Due to small sizes reliability of system also increases and cost of product get reduced. Low power consumption also increases the portability and life of battery in all the battery operated systems. In future we will work on an energy efficient design which includes these all power consumption reduction techniques at

different circuit levels and thus get minimum total power consumption.

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