DESIGN AND IMPLEMENTATION OF AN IMPROVED CARRY INCREMENT ADDER

International Journal of VLSI design & Communication Systems (VLSICS) Vol.7, No.1, February 2016
10.5121/vlsic.2016.7103 21
DESIGN AND IMPLEMENTATION OF AN
IMPROVED CARRY INCREMENT ADDER
Aribam Balarampyari Devi1
, Manoj Kumar2
and Romesh Laishram3
1
M. Tech VLSI & Embedded System, National Institute of Technology, Manipur, India.
2
ECE Department, National Institute of Technology, Manipur, India.
3
ECE Department, Manipur Institute of Technology, Manipur, India.
ABSTRACT
A complex digital circuit comprises of adder as a basic unit. The performance of the circuit depends on the
design of this basic adder unit. The speed of operation of a circuit is one of the important performance
criteria of many digital circuits which ultimately depends on the delay of the basic adder unit. Many
research works have been devoted in improving the delay of the adder circuit. In this paper we have
proposed an improved carry increment adder (CIA) that improves the delay performance of the circuit. The
improvement is achieved by incorporating carry look adder (CLA) in the design of CIA contrary to the
previous design of CIA that employs ripple carry adder (RCA). A simulation study is carried out for
comparative analysis. The coding is done in Verilog hardware description language (HDL) and the
simulation is carried out in Xilinx ISE 13.1 environment.
KEYWORDS
CIA, CLA,RCA, delay, Verilog HDL.
1. INTRODUCTION
The demand of high-performance VLSI systems are increasingly rapidly for used in small and
portable devices, standard wireless and mobile devices, and rapidly advancing biomedical
instruments [1], [2]. The performance of a VLSI systems are basically decided by the speed of
operation, low power consumption and less design area. In the recent years due to the fast
growing technologies in mobile communication and computation, the demand for building low-
power VLSI systems has also increases. The technology of storage batteries does not advance at
the same rate as the microelectronics technology. Therefore the power available for mobile
systems is a very limited. So system designers have to comply with more constraints such as high
speed, high throughput, small silicon area, and at the same time, low-power consumption [1].A
complex digital system consists of many operational blocks and arithmetic logic unit (ALU) is
one of the important building block of such system. The main component of an ALU involves
several binary adders. Eventually an efficient adder design essentially improves the performance
of a complex digital system. The design of area and power-efficient high-speed data path logic
systems is one of the most substantial areas of research in VLSI system design. In binary adders,
the speed of operation is limited by the time taken in propagating the carry through the adder.
The sum for each bit position in a basic adder is obtained sequentially only after the previous bit
position has been calculated and a carry propagated into the next position. The development of
adder has seen tremendous growth since the design of basic half adder and full adder. Many
works have been dedicated to improve the ripple carry adder in terms of delay ,area, power

22
consumption etc. Since adders are used as modules in many complex digital circuit, improving
the performance of the digital adder would extensively speed up the execution of binary
operations inside such complex circuit that comprised of many adder blocks. In cell-based
VLSI design techniques the parameters may be well be characterized in terms of circuit area and
speed as well as suitability for logic optimization and synthesis[3]. Over the last decade many
different adder architectures for improving the binary addition have been design and proposed. In
this paper our main focus is to improve the delay performance of the binary adder circuit thereby
increasing the speed of operation.
The rest of the paper is organized as follows. Section 2 provides a brief review of the related work
done earlier on the design binary adders. In Section 3, the design of carry increment and modified
or improved carry increment adder is presented. In Section 4, analysis on the simulation results is
done and the conclusion is drawn in Section 5.
2. RELATED WORKS
Some of the binary adders are summarized in this section. The most basic simple adder is the
Ripple Carry Adder (RCA)[3][4] but it is the slowest with O(n) area and O(n) delay, where n is
the operand size in bits. Carry Look-Ahead (CLA)[5][6] have O(n· log(n)) area and O(log(n))
delay, but typically suffer from irregular layout. An area efficient CLA is proposed by the authors
in [7]. On the other hand, Carry Skip Adder(CSA)[8][9],carry increment adder (CIA) [6],[11]and
carry select adder (CSLA)[10], [12]-[16] provides a good compromise in terms of area and delay,
along with a simple and regular layout. Carry save adder have O(n) area and O(log n) delay. CLA
adders can be realized in two gate levels provided there is no limit on fan in/out. CSLA reduces
the computation time by pre-computing the sum for all possible carry bit values (i.e. ‘0’ and ‘1’).
After the carry becomes available the correct sum is selected using multiplexer. A power efficient
adder scheme is proposed in [17]. It is shown by the authors in [6],[18],[19] that among the
parallel adders carry increment adder has the best delay performance which is one of the most
important parameter in the high speed devices. This result prompted us to study the details of CIA
and an a modified architecture of CIA is proposed in this paper that further improves the delay of
the circuit. The architecture incorporates carry look ahead adder. The proposed design is a new
concept and to the best of our knowledge it has not been proposed earlier by any researchers. The
details of the design are explained in the next subsequent section.
3. CARRY INCREMENT ADDER (CIA)
In this section the design and working of CIA is presented. First the conventional design of CIA
which is based on RCA is elaborated. We named this design of CIA as CIA_RCA. Secondly the
modified CIA is presented and we named it as CIA_CLA.
3.1. CIA_RCA
The standard Carry Increment Adder (CIA) consists of RCA’s and incremental circuitry [18]. The
incremental circuit is designed using HA’s in ripple carry chain with a sequential order. The
addition operation is done by separating the total number of bits in to group of 4bits and addition
operation is performed by several 4-bit RCA’s. Instead of computing two partial sums for each
group and selecting the correct one, only one partial sum is calculated and incremented if
necessary, according to the input carry. Thus the second adder and the multiplexers in the carry-
select scheme can be replaced by a much smaller incremental circuit and the modified
architecture is the Carry Increment Adder (CIA)[19]. For example, an 8-bit CIA comprises of
two 4-bit RCA. The first block of RCA adds first 4-bits to produce 4-bit partial sum and a carry
output. Thus, first 4-bit of sum of CIA is directly obtained from first block of RCA. And the carry

23
output of first RCA block is given as input to the cin of incremental circuit. Incremental circuit
consists of Half Adders (HA). Hence, the partial sum obtained from the second RCA block is
given to incremental circuit. The block diagram representation of an 8-bit CIA_RCA is as shown
in Figure 1.
Figure 1. Block diagram of CIA_RCA
3.2. CIA_CLA
In this subsection we present the modified carry increment adder i.e. CIA_CLA. We know that
RCA is the basic binary adder circuit and is quite popular because of its simple design. However
it suffers from the worst propagation delay affecting the overall performance of the system. It is
proved that CLA performs better than RCA in terms of delay at the expense of increased design
complexity. We have modified CIA_RCA by replacing the RCA with CLA block. It is quite
obvious because of the property of CLA, the overall delay performance will be improved. As
similar to CIA_RCA incremental circuit can be designed using HA’s in ripple carry chain with a
sequential order. The block diagram representation of CIA_CLA is as shown in Figure 2.
Figure 2. Block diagram of CIA_CLA

24
4. SIMULATION RESULTS
The simulation study of the proposed CIA_CLA is presented in this section. The RTL code is
written using Verilog HDL and the simulation and design synthesis is carried under Xilinx ISE
13.1 environment. The behavioural simulation result for verifying the design is done under
behavioural simulation by writing a test bench. The RTL schematic and technology view are also
generated using the synthesis tool. We have performed the experiment for addition of 8 bit binary
numbers. The CIA_RCA is built using two modules of 4-bit RCA and the incremental circuitry
by the half adders. Whereas CIA_CLA is built using two modules 4-bit CLA. The delay of 4-bit
RCA is about 12.008 ns but the delay of 4-bit CLA is about 10.006 ns. This indicates that CLA is
faster than RCA. Therefore it is quite obvious that CIA_RCA will be better than CIA_RCA in
terms of delay performance. The behavioural simulation result, RTL schematic, and technology
view schematic of CIA_RCA and CIA_CLA are shown in Figure 3 to Figure 8 respectively.
Figure 3. Simulation result of two 8-bit addition using CIA_RCA
Figure 4. RTL schematic view of CIA_RCA
Figure 5. Technology view of CIA_RCA

25
Figure 6. Simulation result of two 8-bit addition using CIA_CLA
Figure 7. RTL schematic view of CIA_CLA
Figure 8. Technology view of CIA_CLA
The comparison of CIA_RCA and CIA_CLA in terms delay, area and power consumption are
given in Table 1. The performance parameters like delay and area are obtained from the synthesis
report and the power dissipation or consumption is calculated using x-power analyzer which is
available with the Xilinx ISE design suite. It can be observed form Table 1 that the proposed
design has better delay performance which is the desired goal of this research without
compromising the power dissipation.
Table 1. Comparison of 8 bit CIA_RCA and CIA_CLA.
Design Area(LUT's) Area(Slices) Delay(ns) Power consumption (mW)
CIA_RCA 20 13 14.59 41
CIA_CLA 19 12 13.54 41

26
5. CONCLUSIONS
A modified carry increment adder is proposed in this paper using the faster carry look ahead
modules instead of the much slower ripple carry adder. By replacing the 4-bit RCA with a 4-bit
CLA the delay performance of the circuit is improved in the design without affecting the poer
dissipation of the circuit. The design is tested and verified by Verilog HDL coding and simulation
is carried out in Xilinx ISE 13.1 environment. In this paper we have carried out the design only
for two 8-bit binary addition but the design may be extended to higher order adder circuit. It may
pointed out that although CLA has better delay performance than RCA the circuit complexity of
CLA increases as the number of bit increases. The choice of a particular adder depends on the
intended application. Future work may be dedicated to study the complexity of CIA_CLA when
the number of bit is increased.
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AUTHORS
Aribam Balarampyari Devi is presently doing M.Tech. in VLSI & Embedded systems in NIT,Manipur.
Her research area includes digital sytems, Circuit design,VLSI design.
Manoj Kumar is presently working as Assistant professor in the department of Electronics &
Communication Engineering, National Institute of Technology, Manipur-India. His research area includes
digital electronics,VLSI design.
Romesh Laishram is presently working as Assistant professor in the department of Electronics &
Communication Engineering, Manipur Institute of Technology, Imphal-India. He has published more than
20 research papers in refereed journals and conferences. He has also publish one book on "WiMAX: Call
Admission Control". He is also a member of the editorial board in International Journal of Recent
Advances in Engineering & Technology (IJRAET). His research area includes signal processing, image
processing, Communication systems, VLSI design.

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