World Applied Sciences Journal 11 (7): 808-812, 2010
ISSN 1818-4952
© IDOSI Publications, 2010
A New Full Swing Full Adder Based on a New Logic Approach
1
M.H. Ghadiry, 2M. Nadisenejani and 1M. Miryahyaei
Department of Computer Engineering, Islamic Azad University, Arak Branch, Arak, Iran
Department of Computer Engineering, Islamic Azad University, Ashtian Branch, Ashtian, Iran
1
2
Abstract: This paper presents a new low power full adder based on a new logic approach, which reduces power
consumption by implementing full adder using only one XOR module. In addition, output inverters have been
embedded in the full adder cell, which boost driving capability, reduce delay and provide complementary
outputs. Simulation has been carried out by HSPICE in 0.18µm bulk technology at 1.8V supply voltage. The
results show that the proposed circuit has less power and PDP than recently proposed full adders in the
literature.
Key words: Full adder
Low Power
Performance
Logic approach
INTRODUCTION
CMOS VLSI
Previous Works Review: There is a variety of full
adders in the literature for example there are 41 full
adders only in [2]. Many of them use XOR and XNOR as
intermediate signals [8-9]. There are full adders based
on only multiplexers or inverters. In terms of transistor
count, it varies from 6 in [4] up to 32 in TFA [10]. There
are full adders in dynamic style, such as [5, 11] and
current mode [4]. Including analysis of all full adders in
this study is not possible. Thus, six well-known full
adders with various numbers of transistors have been
selected in order to study and compare with the proposed
full adder cell.
Conventional CMOS [12] full adder in Fig.1 (a) with
28 transistors is a robust, high power and area full adder,
which has been designed, based on standard CMOS
topology. It has full-swing outputs that increase noise
margin and reliability. Due to high number of transistors,
its power consumption is high. Large PMOS transistor in
pull up network result in high input capacitances, which
cause high delay and dynamic power. However, using
inverters on the output nodes decreases the rise-time and
fall-time and increases the driving ability. It functions well
at low power supply voltages because it does not have
threshold loss problem.
SERF full adder, as shown in Fig. 1 (b), uses energy
recovery technique to reduce power consumption [13].
Ten transistors are used to implement this circuit. Thus,
it is low-cost and low-area cell. On the other hand,
outputs are not full-swing. Thus, it cannot work correctly
at low voltages and it exhibits high delay when the cells
are cascaded to make large full circuits.
One of the most important issues in VLSI design is
power consumption. With the continuously increasing
chips’ complexity and number of transistors in a chip,
circuits’ power consumption is growing as well. Higher
power consumption, raises chips’ temperature and
directly affect battery life in portable devices as it causes
more current to be withdrawn from the power supply.
High temperature afflicts circuit operation and reliability
so requires more complicated cooling and packaging
techniques [1].
Full adder is the fundamental unit in circuits used for
performing arithmetic operations such as multipliers,
compressors, large adders, comparators and parity
checkers [2]. Therefore, reducing power consumption in
full adders, will reduce the overall power consumption of
the whole system.
There are several issues related to the full adders.
Some of them are power consumption, performance, area,
noise immunity, regularity and good driving ability [3].
Several works have been done in order to decrease
transistor count and consequently decrease power
consumption and area [3-6]. In some designs, reducing
transistor count has been resulted in threshold loss
problem that causes non-full swing outputs [2-3, 7], low
speed and low noise immunity especially when they
are used in cascaded fashion.
The aim of this study is to design 1-bit low-power full
adder cell, based on a new logic approach without losing
driving capability.
Corresponding Author: M.H. Ghadiry, Department of Computer Engineering, Islamic Azad University, Arak Branch, Arak, Iran.
E-mail: m.hoseinghadiry@gmail.com.
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World Appl. Sci. J., 11 (7): 808-812, 2010
(b)
(a)
(c)
(d)
(e)
(f)
Fig. 1: Several full adder cells from the literature. (a) CCMOS. (b) Energy recovery full adder (SERF). (c) New version of
14T (N14T). (d) 16T. (e) Multiplexer based full adder (MB12T). (f)New version of hybrid pass logic with static
output drive full adder (26T).
New-14T, Fig. 1 (c), is an improvemnet from 14T
[14]. which has simultaneous XOR and XNOR
signals. Feedback transistors provide rail-to-rail outputs
in XOR-XNOR module. However, they prompt
high delay.
Fig. 1 (d) shows another improvemed version from
14T, which is called 16T [15]. It is the same as New-14T in
terms of the output modules. However, the XOR-XNOR
module has been modified to reduce delay and power
consumption. lower power and delay has been obtained
at the expense of 2 additional transistors. The XOR-XNOR
modules does not have full-swing outputs thus, the
transistors which have been connected to this module are
turned on or off slowly.
MB12T [16] has been implemented using six
multiplexers and 12 transistors. Each multiplexer is
implemented by pass-transistor logic with two transistors.
As shown in Fig. 1 (e), there is no VDD or GND
connection in this circuit. Therefore, short circuit power
can be decreased significantly. However, pass gates lack
of driving in cascaded mode and high fan-out that leads
to high delay.
26T full adder [9] in Fig. 1 (f), has used the output
module of CCMOS for generating COUT signal and also
modified version of SUM generator circuit of 16T with
additional inverters in order to increase driving ability and
performance. The XOR-XNOR module is the same
modoule as 16T, with two additional feedback transistors
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World Appl. Sci. J., 11 (7): 808-812, 2010
An Alternative Logic Approach: By replacing the logic 0
and 1 with the equivalent inputs at each case of input,
several tables like Table 1 can be produced. Proper
categorizing will result in reduction in number of gates
and power. Table 1 shows that implementing both COUT
and SUM based on signal B C make it possible to
generate them using only A and C inputs. The new logic
approach reduces the power consumption by eliminating
power-consuming XNOR gate.
In order to increase the driving ability for high fanout applications, output inverters are embedded in the
circuit by implementing COUT and SUM . Fig. 2 shows the
Table 1: Truth table for the proposed structures
C
B
A
0
0
0
0
0
0
1
0
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
0
1
1
1
0
proposed logic approach.
The Proposed Full Adder Cell (SS16T): Fig. 3 shows the
transistor realization of the cell. A full-swing XOR gate
from [8] has been used in this full adder. It uses 6
transistors and benefits from full-swing output, low delay
and power consumption. Two inverters in the path
between CIN and COUT act as a buffer decreasing the
propagation delay. Multiplexers are implemented using
pass gates with only two transistors. Full-swing XOR
module avoids double VT threshold loss problem at the
input node of the output inverters and high leakage power
consumption in the output inverters as well.
Fig. 2: The proposed logic to implement 1-bit full adder
cell with one XOR gate
Simulation Setup: All the netlists have been simulated
using HSPICE in 0.18µm bulk technology. Each input is
derived by two cascaded inverters. Two capacitances are
used as loads for COUT and SUM connections. Power
and delay of the inverters are included in the power and
delay calculation of the whole circuit. To study the
properties of the compared circuits in cascaded fashion,
4-bit ripple carry adder (RCA) has been implemented.
The performance of the under test circuits have been
evaluated in terms of worst-case propagation delay.
Propagation delay has been calculated from 50% of
voltage level of input to 50% of voltage level of output. In
this study, power means the total average power
consumption. Power delay product (PDP) has been
calculated from production of worst-case delay and
average power consumption. Transistors’ size has been
optimized in order to minimize the PDP.
To compare the cells, several input patterns have to
been applied to cover all the input cases. An input
pattern, which maximizes the power consumption for a
given cell, could exhibit less power consumption for
another cell. Different patterns also show different delays.
With three inputs and three frequencies, there are 27
different input patterns. All the 27 input patterns have
been applied to the under test circuits to fairly compare
them [15].
Fig. 3: The proposed full adder cell (SS16T)
to make outputs full swing. It is a high-performance and
low-power XOR-XNOR module with full-swing outputs.
This design trades power and performance with area.
All the intermediate signals are rail-to-rail and the
outputs have good driving capability. Therefore, it
shows high performance in cascaded fashion and
large loads. Using both VDD and GND connection
for generating COUT causes to speed up in carry
propagation. Pass gate design together with full-swing
signals reduce the leakage and short-circuit current and
make it a low power circuit.
Proposed Full Adder: In this section, first, a new logic
approach to design low power full adder is proposed.
Afterward the proposed full adder will be illustrated.
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World Appl. Sci. J., 11 (7): 808-812, 2010
(a)
(d)
(b)
(e)
(c)
(f)
Fig. 5: Power and delay comparison vs. supply voltage, output load and frequency. (a) Power consumption vs. supply
voltage. (b) Power consumption vs. output load (c) Delay vs. supply voltage (d) Delay vs. output load (e) PDP
vs. supply voltage (f) PDP vs. output load.
RESULTS AND DISCUSSION
inverts and those which not, widens as load increases.
26T, SS16T and CCMOS show noticeably less delay than
MB12T, SERF, 16T and N14T. SS16T’s delay is higher
than 26T due to non full-swing signals of multiplexers that
drive output inverters. N14T and SERF have highest PDP
followed by the next group containing CCMOS, 16T and
MB12T.
In order to have a closer comparison, the values
obtained from simulation at 1.8V and 10fF output load
have been brought in Table 2. They have been sorted
based on power consumption. The first circuit in power
ranking is SS16T with 63 µW. However, it is not a very
good choice for high-speed applications. 26T has higher
power consumption and performance than SS16T.
Considering PDP, the superior circuit is SS16T.
Fig. 5 (a), (b) and (c) shows power consumption,
delay and PDP vs. supply voltage. It starts from 1V to
1.8V. Both 26T and SS16T have less power consumption
than other four circuits and SS16T’s power consumption
is less than 26T. SERF does not function properly at 1V,
therefore, there is no mark for SERF at this voltage.
Although, SS16T shows higher delay than 26T, its PDP
still is less than 26T. CCMOS and SERF are behind 26T
and SS16T in terms of PDP.
Fig. 5 (d), (e) and (f), demonstrate the effect of output
load on power, delay and PDP respectively. As load
increases power, delay and PDP increase as well. The gap
between those circuits, which drive the loads thorough
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World Appl. Sci. J., 11 (7): 808-812, 2010
4.
Table 2: Simulation Results for 1.8V, 10fF and 100MHz
PWR
DLY
PDP
Max. Freq.
(GHz)
Circuit
(uW)
(ns)
(e-15)
SS16T
63
0.56
36.5
26T
69
0.55
37.9
1.82
MB12T
72
0.79
56.8
1.27
16T
80
0.73
58.4
1.37
NEW14T
98
0.74
72.5
1.35
SERF
121
0.63
76.2
1.59
CCMOS
161
0.61
98.2
1.64
1.72
5.
6.
7.
All the other circuits have higher PDP than 26T and
SS16T. In addition to power and delay, maximum
frequency has been calculated based on delay.
Apparently, 26T obtained highest frequency. For normal
applications, which the main aim is to get lowest PDP,
SS16T is a proper circuit.
8.
CONCLUSION
9.
A full-swing full adder cell based on a new logic
approach was proposed in this paper. The new full adder
employed one XOR gate as intermediate signal in order to
reduce number of gates and power consumption. It has
complementary outputs and output inverters to increase
driving ability. The proposed circuit was simulated using
HSPICE and compared to several full adders in the
literature. The results show that there are improvements
in terms of power consumption and PDP compared to
simulated circuits. In addition, simulation for high output
loads show that full-swing full adders with output
inverters have less delay than those circuits using no
output inverter or with non-full swing outputs.
Furthermore, the study shows that, lowering the number
of transistors does not always mean power reduction.
10.
11.
12.
13.
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