Design and Analysis of Low Power High Speed Hybrid logic 8-T Full Adder Circuit

Asian Journal of Applied Science and Technology (AJAST)
Volume 1, Issue 2, Pages 206-210, March 2017
© 2017 AJAST All rights reserved. www.ajast.net
Page | 206
Design and Analysis of Low Power High Speed Hybrid logic 8-T Full Adder Circuit
B.AnishFathima
Assistant Professor, Department of Electronics and Communication Engineering, Sri Krishna College of Engineering and Technology, Coimbatore, India.
Article Received: 10 March 2017 Article Accepted: 19 March 2017 Article Published: 20 March 2017
1. INTRODUCTION
1.1 Need for Low Power
The advantage of utilizing a combination of low-power
components in conjunction with low-power design techniques
is more valuable now than ever before. Now a day’s power is
the primary concern due to the remarkable growth and success
in the field of personal computing devices and wireless
communication system which demand high speed
computation and complex functionality with low power
consumption.
The motivations for reducing power consumption differ
application to application. In the class of micro-powered
battery operated portable applications such as cell phones, the
goal is to keep the battery lifetime and weight reasonable and
packaging cost low. For high performance portable computers
such as laptop the goal is to reduce the power dissipation of
the electronics portion of the system to a point which is about
half of the total power dissipation. Finally For the high
performance non battery operated system such as
workstations the overall goal of power minimization is to
reduce the system cost while ensuring long term device
reliability.
In the past, the major concerns of the VLSI designer were
area, performance, cost and reliability; power consideration
was mostly of only secondary importance. In recent years,
however, this has begun to change and, increasingly, power is
being given comparable weight to area and speed
considerations. Several factors have contributed to this trend.
Perhaps the primary driving factor has been the remarkable
success and growth of the class of personal computing devices
and wireless communications systems which demand
high-speed computation and complex functionality with low
power consumption. In these applications, average power
consumption is a critical design concern. From the
environmental viewpoint, the smaller the power dissipation of
electronic systems, the lower the heat pumped into the rooms,
the lower the electricity consumed and hence the lower the
impact on global environment.
1.2 Need for Energy efficient Adders
Adders are the most important components used in most of
the Digital signal processors for real time applications.
Adders are also used in the architecture of other arithmetic
operators like Subtractors, Multipliers, shifters, MACs, etc.
Many Adder architectures have been proposed by various
researchers over the years. Even today the researches on
Adder architectures are finding a major interest for achieving
energy efficient devices. The survey on conventional adder
architectures [3, 5-9] have proved there is a lot more scope for
optimization. One method of optimization is by using
different logic styles for different parts of the design instead
of using the same logic style throughout the design. This
method of designing with different logic styles in various
parts of the same circuit is called Hybrid circuit. Various
hybrid architectures were surveyed for reference [1, 2 and 4].
This paper presents a Hybrid full adder circuit consisting of
two different logic styles namely CMOS logic and Pass
transistor logic. This method has lesser power consumption,
and hence energy efficient. The remaining of this paper is
organized as follows. Section II presents the overview of
proposed hybrid full adder circuit, the two different logic
styles and the reason behind using them in this circuit. Section
III presents the working principle of the proposed design with
an example and the analysis of obtained results using Tanner
Tool of 180nm Technology. Section IV draws the most
relevant conclusions of this work.
2. PROPOSED HYBRID FULL ADDER CIRCUIT
2.1 Overview of existing systems
Full adders, being one of the most fundamental building
blocks of all the aforementioned circuit applications, remain a
key focus domain of the researchers over the years. Different
logic styles, each having its own merits and bottlenecks, was
investigated to implement1-bit full adder cells. The designs,
ABSTRACT
Addition is a fundamental arithmetic operation that is broadly used in many VLSI systems, such as application-specific digital signal processing
(DSP) architectures and microprocessors. This addition module is also the core of other arithmetic operations such as subtraction, multiplication,
division and address generation. The prime objective of this project is to design a full-adder having low-power consumption and low propagation
delay which may result in the efficient implementation of modern digital systems. This model is referred as “hybrid” because of the combination of
two different design logic styles namely CMOS logic and pass transistor logic. Performance parameters such as power, delay and hence energy were
compared with the existing designs such as complementary CMOS logic full adder. In the existing hybrid systems, over 28 transistors were used.
While the optimized hybrid full adder circuit reduces this count to 8 transistors, it still obtains better energy efficiency. Further the proper working of
proposed full adder is verified by applying it in a Ripple carry Adder circuit.

Page | 207
reported so far, may be broadly classified into two categories:
1) static style 2) dynamic style. In the existing system,
full-adder has to obtain an intermediate signal and its
complement, which are then used to drive other blocks to
generate the final outputs. The overall propagation delay and
the power consumption of the full-adder depend on the delay
and its complement generated. So, to increase the operational
speed of the full-adder, it is necessary to develop a new logic
structure that does not require the generation of intermediate
signals to control the selection or transmission of other signals
located on the critical path.
In most hybrid techniques, XOR, and XNOR functions were
simultaneously generated by pass transistor logic module by
using only six transistors. CMOS module produces full swing
outputs of the full adder but at the cost of increased transistor
count and decreased speed. Although the hybrid logic styles
offers promising performance, most of these hybrid logic
adders suffered from poor driving capability issue and their
performance degrades drastically in the cascaded mode of
operation if the suitably designed buffers are not included.
2.2 Overview of the proposed system
In this proposed system Pass Transistor Logic and CMOS
logic are used hence hybrid adder. In the proposed work,
consideration is given to the sizing of the MOS transistors
during the initial design step. A low transistor count full adder
cell using the new XOR cell is also presented. A PTL based
3-transitors XOR and XNOR circuits presented in had full
output voltage swing and better driving capability. To
compare the performance of new circuit and test their driving
capability, an adder circuit is built with the proposed XOR
and XNOR circuits.
An XOR/XNOR function with low circuit complexity can be
achieved with only 3 transistors in PTL. It uses only six
transistors to produce both an XOR and the complementary
XNOR function. The circuit has full voltage-swing and
negligible static power dissipation. The entire model is
divided into 3 modules, each module performing a specified
task. Here the propagation delay and power consumption is
reduced.
2.3 Hybrid technique of the proposed system
2.3.1 Pass transistor logic
The Pass-Transistor Logic (PTL) is a better way to implement
circuits designed for low power applications. The low power
pass transistor logic and its design and analysis procedures
were reported. The advantage of PTL is that only one PTL
network (either NMOS or PMOS) is sufficient to perform the
logic operation, which results in smaller number of transistors
and smaller input loads, especially when NMOS network is
used. Moreover, VDD-to GND paths, which may lead to
short-circuit energy dissipation, are eliminated. As the
designs with fewer transistor count and lower power
consumption are pursued it becomes more and more difficult
and even obsolete to keep full voltage swing operation. In
pass transistor logic, the output voltage swing may be
degraded due to the threshold loss problem. That is, the output
high (or low) voltage is deviated from the VDD (or ground)
by a multiple of threshold voltage. The reduction in voltage
swing, on one hand, is beneficial to power consumption. On
the other hand, this may lead to slow switching in the case of
cascaded operation such as ripple carry adder. At low
operation, the degraded output may even cause circuit
malfunction. A formal design procedure for realizing a
minimal transistor CMOS pass network XOR cell is
presented.
The advantages of pass transistor based realization are ratio
less, lower area due to smaller number of transistors and
lesser power dissipation because of no static power and short
circuit power dissipation. Some of the disadvantages of pass
transistor logic are higher delay and multi threshold voltage
drop. Pass transistor logic family is used to overcome the
limitations of pass transistor circuits. Some of the techniques
are:
Insertion of buffers to avoid long delay of a chain of
pass transistor logic.
Use of swing restoration circuit to overcome
multi-threshold voltage drop (vout = vdd-vtn).
Use of dual rail logic to generate complementary
control signals.
Most of full adder designs are based on three module
implementation i.e. XOR, sum, carry modules. The
4-Transistor XOR (XNOR) module is used to design pass
transistor logic based full adder. But this type of design raises
severe threshold voltage loss problems. The signal
degradation in the output causes signal degradation in the
further modules. Finally the signal degradation in the circuit
causes a multi-threshold loss problem at the output stage.
These multi-threshold loss problems can be reduced by using
complementary outputs. The complementary outputs are
obtained by using 5-Transistor XOR-XNOR design.
2.3.2 The CMOS logic
In all static CMOS circuits, a rising output transition is caused
by one of the pMOS branches, where a falling output
transition is always caused by one of the nMOS branches. The
delay associated with a certain transition is dictated by the
strength (or weakness) of the corresponding branch. Thus,
with the help of some simulations, the designer can identify
which branches are responsible for particular outputs. This
will enable the designer to modify the dimensions of only the
relevant transistors. The pMOS transistors, due to the lower
mobility of holes, have a gain factor that is 2 to 3 times lower
than that of an nMOS transistor. For the same current driving
capacity the pMOS transistor should have channel width that
is roughly 2 to 3 times wider than an nMOS transistor. Larger
transistors have larger current driving capability, making
them faster. The problem is that larger transistors also have
higher input capacitances and higher drain parasitics which
slows down overall operation speed. Transistors within the
same branch are usually drawn side-by-side on the same
active area. Thus, they should have the same size to simplify
the layout. For some common blocks there are many different
alternatives with varying advantages and disadvantages. As

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an example, XOR and MUX type circuits are hard to realize
in standard CMOS logic whereas in Pass transistor logic their
realization is quite simple. A very interesting design style for
efficient layouts is the branch based logic style. The branch
based logic style is a special case of the standard static CMOS
designs where both the nMOS and pMOS block of the circuit
are designed using parallel branches of series-connected
transistors between the VDD (or GND) and the output node.
2.3.3 Hybrid Designs Using XOR and XNOR
All hybrid designs use the best available modules
implemented using different logic styles or enhance the
available modules in an attempt to build a low power
full-adder cell. Generally, the main aim is to reduce the
number of transistors in the adder cell and consequently to
reduce the number of power dissipating nodes. This is
achieved by utilizing intrinsically low power consuming logic
styles like TGA, TFA or simply pass-transistors. There are
three main components to design a hybrid full adder circuit.
These are XOR or XNOR, Carry generator and Sum
generator. Hybrid full adder as shown in Figure has been
designed with pass logic circuit cogenerates the intermediate
XOR - XNOR and hence improves outputs. The carry
generator module of hybrid full adder is a complementary
CMOS logic style based MUX. In this circuit, worst case
delay problems of transitions from 01 to 00 and from 10 to 11
are solved by adding two series p-MOS and two series n-MOS
transistors respectively. Although this modification improves
the speed of XOR/XNOR circuit, these additional transistors
increase the power consumption of the full adder cell.
To produce sum, hybrid uses another XOR circuit which is
implemented with pass transistors. The output inverter
restores the output voltage level and improves the driving
capability for cascading. It uses 26 transistors having a full
swing logic, balanced output and good output drivability at
low voltage. Another hybrid designed full adder is a
combination of low power transmission-gates and pseudo
n-MOS gates as depicted in Figure. XOR gate consists of a
pMOS transistor and an nMOS transistor that are connected in
parallel, which is a particular type of pass-transistor logic
circuit. There is no voltage drop at output node but it requires
twice the number of transistors to design similar function.
A novel 8-transistor XOR–XNOR based Full adder circuit
that generates XOR and XNOR outputs simultaneously. This
circuit provides a full voltage swing (i.e., 0V for logic 0 and
1.8V for logic 1) at low supply voltage. The reported
XOR–XNOR Full adder circuit is based on complementary
pass-transistor logic using only one static inverter instead of
two static inverters as in the regular CPL style XOR circuit.
The first half of the circuit utilizes only NMOS pass
transistors for the generation of the XOR and XNOR outputs.
The cross-coupled PMOS transistors are connected between
XOR and XNOR output to alleviate threshold problem for all
possible input combinations and reduce short-circuit power
dissipation. The circuit is inherently fast due to the high
mobility NMOS transistors and the fast differential stage of
cross-coupled PMOS transistors. It indicates the functioning
of the XOR/XNOR circuit.
3. WORKING PRINCIPLE OF FULL ADDER
CIRCUIT
The proposed full adder circuit is represented by three
blocks Module 1 and module 2 are the XNOR modules that
generate the sum signal (SUM) and module 3 generates the
output carry signal (Cout).
3.1 Carry generation module
In the proposed circuit, the output carry signal is
implemented by the transistors Mp7, Mp8, Mn7, and Mn8.
The input carry signal (Cin) propagates only through a
single transmission gate (Mn7 and Mp7), reducing the
overall carry propagation path significantly.
3.2 Operation of the proposed full adder
The sum output of the full adder is implemented by XNOR
modules. The inverter comprised of transistors Mp1 and Mn1
generate B’, which is effectively used to design the controlled
inverter using the transistor pair Mp2 and Mn2. Output of this
controlled inverter is basically the XNOR of A and B. But it
has some voltage degradation problem, which has been
removed using two pass C transistors Mp3 and Mn3.
Analyzing the truth table of a full adder, the condition for
Cout generation has been deducted as follows:
If, A = B, then Cout = B; else, Cout = Cin.
The parity between inputs A and B is checked by A _ B
function. If they are same, then Cout is same as B, which is
implemented using the transmission gate realized by
transistors Mp8 and Mn8. Otherwise, the input carry signal
(Cin) is reflected as Cout which is implemented by another
transmission gate consisting of transistors Mp7 and Mn7.We
use thin and thick gates for CMOS and transistor gates
respectively to reduce the power consumption and
propagation delay.
Fig.1. Working of the proposed full adder

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Fig.2. Schematic of proposed full adder
3.3 Working of the optimized full adder circuit
The working principle of the optimized full adder is presented
below. The working is divided into 3 modules and each of
these modules performs its intended operation. This
optimized circuit is a hybrid full adder which combines PTL
logic and CMOS logic.
3.4 Carry and sum generation modules
In the module1, XOR logic is used. 2 inputs are given to the
circuit A and B. These two inputs are XORed and the output is
considered to be S1. To consider an example say A=0, B=1
then A XOR B= 1.
Fig.3. Schematic of the optimized Full adder circuit
In the module 2 XOR logic is used .The S1 from the first
module and Cin is given as the input and here the operation is
S1 XOR Cin the output is the SUM for the given set of the
problem. If Cin =1, then S1 XOR Cin =0 (SUM).
Fig.4. Working of the optimized full adder
In the module 3 MUX logic is used thus we have a 2*1 mux
logic that is two inputs and one output, a selection line is also
used the inputs are SI and Cin the input A is the selection line
which is denoted by SL. The output of this MUX operation is
the carry. Thus the carry is 1 which is the Cout. The condition
for this carry generation is: “If A=0, Cout=Cin, If A=1,
Cout=S1”
3.5 Results and Analysis
Table 1. Comparative Analysis of Full Adders
The table 1 shows the comparison among the conventional
CMOS full adder, the hybrid full adder and the optimized full
adder we have calculated number of transistors used in all the
three from which we can infer that the proposed optimized full
adder uses only 8 transistors while the hybrid full adder
system uses 16 and the conventional uses 28. We have

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calculated the power consumed and propagation delay from
which we have calculated the power delay product which is
nothing but the energy and hence from the above details we
can conclude that the energy is reduced up to 23% in the
optimized full adder.
3.6 Application of proposed Hybrid full adder
The proper working of the proposed hybrid full adder design
has been verified by applying the circuit in a Ripple Carry
Adder (RCA). Ripple Carry Adders were the conventional 4
bit adders used for many arithmetic operations. They are
known for their simple architecture. The drawback associated
with RCA is the propagation delay associated with the
propagation of carry signal in between full adder blocks.
Fig.5. Schematic of the Ripple Carry Adder
This Figure shows the schematic view obtained from S-Edit
of RCA from Tanner Tool. The different full adders namely
conventional proposed hybrid full adders are called in
correspondingly for different methods. Similar results were
achieved for RCA circuit analysis also. The results are
displayed in the following table.
Table 2: Comparative Analysis of Full Adders
The table 2 shows the comparison among the conventional
CMOS RCA, the hybrid full adder_RCA and the optimized
full adder_RCA. There is a clear reduction in the number of
transistors. We have calculated the power consumed and
propagation delay from which energy has been analysed,
resulted in an energy reduction by 27.2% compared to
conventional circuit.
4. CONCLUSION
In this work, a hybrid full adder circuit and a ripple carry
adder circuit using the proposed hybrid full adder circuit are
designed and analyzed using TANNER SOFTWARE. This
full adder is compared with the conventional CMOS based
full adder. Hybrid Full adder is designed with two different
logic designs namely CMOS logic and Transmission gate
logic. Further optimization of the proposed full adder is also
done for better results by replacing the transmission gates
with the pass transistor logic. These systems are analyzed for
power consumption, propagation delay and hence energy
(power delay product), using T-SPICE. From these
computations, it is concluded that the optimized full adder is
energy efficient when compared with the hybrid full adder
circuit. The energy is reduced up to 23%.
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Design and Analysis of Low Power High Speed Hybrid logic 8-T Full Adder Circuit

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Design and Analysis of Low Power High Speed Hybrid logic 8-T Full Adder Circuit