Abstract: Reversibility plays an important role when energy efficient computations are considered. Reversible logic circuits have received significant attention in quantum computing, low power CMOS design, optical information processing and nanotechnology in the recent years. This study proposes a new fault tolerant reversible half-subtractor and a new fault tolerant reversible full-subtractor circuit with nanometric scales. Also in this paper we demonstrate how the well-known and important, PERES gate and TR gate can be synthesized from parity preserving reversible gates. All the designs have nanometric scales

Majid Haghparast

Mozhgan Shiri

Vahid Shahbazi

CiteSeerX

Research Journal of Applied Sciences, Engineering and Technology 4(22): 4566-4571, 2012
ISSN: 2040-7467
© Maxwell Scientific Organization, 2012
Submitted: December 30, 2011 Accepted: March 10, 2012 Published: November 15, 2012
Corresponding Author: Mozhgan Shiri, Department of Computer Engineering, Arak Branch, Islamic Azad University, Arak, Iran
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A Novel Nanometric Fault Tolerant Reversible Subtractor Circuit
1Mozhgan Shiri, 2Majid Haghparast and 1Vahid Shahbazi
1Department of Computer Engineering, Arak Branch, Islamic Azad University, Arak, Iran
2Department of Computer Engineering, Shahre-Rey Branch, Islamic Azad University,
Tehran, Iran
Abstract: Reversibility plays an important role when energy efficient computations are considered. Reversible
logic circuits have received significant attention in quantum computing, low power CMOS design, optical
information processing and nanotechnology in the recent years. This study proposes a new fault tolerant
reversible half-subtractor and a new fault tolerant reversible full-subtractor circuit with nanometric scales. Also
in this paper we demonstrate how the well-known and important, PERES gate and TR gate can be synthesized
from parity preserving reversible gates. All the designs have nanometric scales.
Keywords: Fault tolerant, nanometric circuits, nanotechnology, quantum computing, reversible logic,
subtractor
INTRODUCTION
Reversible circuits are composed of reversible gates.
In this circuits information don’t be lost. Reversible
circuits can produce unique output from each input and
vice versa, hence, there is a one-to-one correspondence
between input and output vectors (Thapliyal and Srinivas,
2006). So a reversible logic gate has an equal number of
inputs and outputs (k ×k) (Babu and Chowdhury, 2005).
In ideal conditions, a reversible circuit has zero
internal power dissipation because, it does not lose
information. Under R. Landauer’s research in the early
1960s, the amount of energy dissipated for every
irreversible bit operation is given by KTLn2  joules,
where K = 1.3806505 × 10!23 J/K is the Boltzmann’s
constant and T is the absolute temperature at which
operation is performed. But in 1973, Bennett showed that
KTLn2   joules of energy can be saved from a system as
long as the system permits the regeneration of the inputs
from produced outputs (Haghparast et al., 2009;
Haghparast and Navi, 2008a; Thapliyal and Gupta, 2006).
A reversible gate with n-inputs and n-outputs is
called a n × n reversible gate (Sastry et al., 2006). In a n
× n reversible function, there are 2n input rows and 2n
output rows in its truth table. In fact the output rows are
a permutation of the input rows in the truth table
(Kerntopf, 2002; Hung et al., 2006). Direct fan-outs from
the reversible gate and feedbacks from a gate output
directly to its inputs are not allowed (Sastry et al., 2006).
Classical logic gates are called irreversible since they
cannot uniquely reconstruct the input vector states from
the output vector states. 
Synthesis and designing of a reversible gate is
different from traditional logic gates (Haghparast and
Sheikh, 2011). Therefore, constructing a fault tolerant
reversible circuit is much more difficult than a
conventional logic circuit (Parhami, 2006). In this paper,
we propose a fault detection method based on parity
preserving reversible logic gates.
PRIMARY DEFINITIONS
Some of the main measures in synthesis a reversible
logic circuit are: Number of reversible logic gates,
Number of garbage outputs, Number of constant inputs,
Total quantum cost, and total logical calculations
(Haghparast et al., 2009).
Garbage outputs: One of the most important parameters
in designing a reversible circuit is its garbage outputs
(Hasan et al., 2004). In a reversible logic gate number of
inputs and outputs is equal but all the outputs are not
expected. The number of outputs added to make an n-
input k-output function reversible is named garbage
outputs. In fact they are needed to maintain reversibility.
A heavy price is paid for every garbage output (Parhami,
2006).
So, one of the major challenges in reversible logic
synthesis is to minimize the garbage outputs (Hasan et al.,
2004; Thapliyal et al., 2009).
Quantum cost: The quantum cost of a reversible gate is
the number of 2×2 reversible gates or quantum logic gates
required    in    designing    it.   The  quantum cost of all
Res. J. Appl. Sci. Eng. Technol., 4(22):4566-4571, 2012
4567
reversible 2×2 gates is taken as 1 and the cost of all
the1×1 reversible gates such as NOT gate is assumed to
be zero. Thus, any reversible gate is performed by using
1×1 NOT gates and 2×2 reversible gates (Thapliyal and
Ranganathan, 2009).
Constant inputs: The inputs that are added to a n × k
function to make it reversible are called constant inputs
(Haghparast et al., 2009).
Reversible logic gates: 
NOT gate: NOT gate is a 1×1  gate represented as shown
in Fig. 1. Since it is a 1×1gate, its quantum cost is zero
(Thapliyal and Ranganathan, 2009; Rangaraju et al.,
2010).
Feynman Gate (FG): Among the exist 4! = 24 2×2
reversible gates, Feynman gate which is shown in Fig. 2
is the most important gate. It can be represented as Iv =
(A, B) Ov = (P = A,Q = ArB), where, Iv and Ov  are input
and output vectors. Since it is a 2×2 gate, it has a quantum
cost of 1 Thapliyal and Ranganathan (2009) and Saiful
(2010). 
Feynman double Gate (F2G): Feynman double gate as
a 3×3 reversible gate is shown in Fig. 3. It can be showed
as:
Iv = (A, B, C) , Ov = (P = A, Q = ArB, R = ArC)
where, Iv nd Ov are input and output vectors respectively.
F2G gate is a Feynman gate with one more input and one
more output which the control input ‘A’ defines a second
controlled NOT operation (Haghparast and Navi, 2008b).
It has the quantum cost of 2. 
Toffoli Gate (TG): Toffoli gate is a 3×3 two-through
reversible gate as shown in Fig. 4. It means that two of its
outputs are same as the inputs. This gate can be
represented as:
Iv = (A, B, C) , Ov = (P = A, Q = B, R = ABrC)
where, Iv and Ov are input and output vectors respectively.
Toffoli gate is one of the most popular reversible gates
and it has quantum cost of 5 (Thapliyal and Ranganathan,
2009; Chung and Wang, 2007). 
Peres Gate (PG): Peres gate also known as New Toffoli
Gate (NTG) is a 3×3 reversible logic gate. It can be
described as:
Iv = (A, B, C) , Ov = (P = A, Q = ArB, R = ABrC)
where, Iv and Ov are input and output vectors respectively.
This gate is constructed by a Toffoli gate that followed by
Fig. 1: NOT gate
Fig. 2: Feynman gate
Fig. 3: Feynman double gate
Fig. 4: Toffoli gate
Fig. 5: Peres gate
Fig. 6: Fredkin gate
Fig. 7: New gate
a Feynman gate. It is shown in Fig. 5. Its quantum cost is
4 which is less than quantum cost of a Toffoli gate
(Haghparast et al., 2009). 
Fredkin Gate (FRG): Fredkin gate is shown in Fig. 6. It
can be described as:
Iv = (A, B, C) , Ov = (P = A, Q = ¾BrAC, R = ¾CrAB)
where, Iv and Ov are input and output vectors,
respectively. It   is   known as swap gate. If control input
Res. J. Appl. Sci. Eng. Technol., 4(22):4566-4571, 2012
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Fig. 8: TR gate
Fig. 9: New fault tolerant gate
A = ’0’ then inputs B and C are showed clearly in outputs,
else if A = ’1’ then inputs B and C are swapped and
showed in outputs. It has quantum cost of 5 (Haghparast
and Sheikh, 2011; Saiful, 2010). 
New gate (NG): Another one of the interesting gate is
New gate which is represented in Fig. 7. The advantage of
selecting the New gate is that, this gate can produce all
the basic gates. It has the quantum cost of 7 (Hasan et al.,
2003; Haghparast and Navi, 2007). 
TR gate: TR gate is a 3× 3 reversible gate and it has
mapping between its inputs and outputs as:
I A B C O P A Q A B R AB Cv v= = = = ⊕ = ⊕( , , ), ( , , )
'
where, Iv and Ov are input and output vectors,
respectively. It is shown in Fig. 8. The quantum cost of
this gate is 6. TR gate can work singly as a half-subtractor
circuit furthermore, with two TR gates we can design a
reversible full-subtractor (Thapliyal and Ranganathan,
2009). 
New Fault Tolerant (NFT):
NFT gate can be defined as:
Iv = (A, B, C), Ov = (P = ArB, Q = B’CrAC’,
R = BCrAC’)
where, Iv and Ov are input and output vectors,
respectively. This gate is shown in Fig. 9 and its quantum
cost is 5. It can implement all Boolean functions
(Haghparast and Navi, 2008b). 
Parity preserving reversible gates: Parity preserving is
one of the error detection methods in digital logic
systems. Using error detection methods in reversible logic
circuits is together with some problems because in these
circuits, fan-out and loops are not permitted. 
Table 1: Truth table of the parity preserving F2G gate
A B C P Q R
0 0 0 0 0 0
0 0 1 0 0 1
0 1 0 0 1 0
0 1 1 0 1 1
1 0 0 1 1 1
1 0 1 1 1 0
1 1 0 1 0 1
1 1 1 1 0 0
Table 2: Truth table of the parity preserving FRG gate
A B C P Q R
0 0 0 0 0 0
0 0 1 0 0 1
0 1 0 0 1 0
0 1 1 0 1 1
1 0 0 1 0 0
1 0 1 1 1 0
1 1 0 1 0 1
1 1 1 1 1 1
Table 3: Truth table of the parity preserving NFT gate
A B C P Q R
0 0 0 0 0 0
0 0 1 0 1 0
0 1 0 1 0 0
0 1 1 1 0 1
1 0 0 1 1 1
1 0 1 1 1 0
1 1 0 0 1 1
1 1 1 0 0 1
Consequently using these ways may increase the number
of gates and number of garbage outputs (Haghparast and
Navi, 2008b).
For parity preserving a reversible circuit, each gate
must be parity preserving. So for constructing parity
preserving reversible circuits we need to parity preserving
reversible gates. Simply we can verify that which gate is
parity preserving by comparing the input parity to the
output parity. In the other words, these gates satisfy the
equation: 
ArBr C = PrQrR. (Haghparast and Navi, 2008 a,b;
Parhami, 2006).
Among the discussed reversible gates, only the gates
F2G, FRG and NFT are parity preserving gates. Which
means that triple of them satisfy the relation ArBr C =
PrQrR.
Truth tables of F2G, FRG and NFT gates are
presented in Table 1, 2 and 3, respectively (Haghparast
and Navi, 2008 a,b).
RESULTS ANALYSIS
New design of parity preserving reversible subtractor
circuits: Half-subtractor circuit executes A!B operation
if A and B be input variables thus, the equations of the
borrow and difference are as follows:   Diff   =   ArB,
 Borr  =  A’B.  Similarly,  Full-ubtractor circuit performs
Res. J. Appl. Sci. Eng. Technol., 4(22):4566-4571, 2012
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Fig. 10: Proposed parity preserving reversible half-subtractor circuit
Fig. 11: Proposed parity preserving reversible full-subtractor circuit
Fig. 12: Proposed parity preserving PERES gate
Fig. 13: Proposed parity preserving TR gate
A!B!C  operation if A, B and C be input vectors  so, the
equations of borrow and difference are as follows:
 Diff A B C Borr A B A C BC= ⊕ ⊕ = ⊕ ⊕, ' '
(Thapliyal et al., 2005)
Although there are some works on reversible
subtractor circuits but so far haven’t reported any acts on
reversible subtractor circuits with parity preserving. In
this section, we show our new proposed design of parity
preserving reversible half-subtractor circuit and parity
preserving reversible full-subtractor circuit. The circuit
shows how F2G gate and FRG gate can be used to design
a parity preserving reversible half-subtractor and parity
preserving reversible full-subtractor.
Parity preserving reversible half-subtractor circuit:
The proposed parity preserving half-subtractor circuit is
shown in Fig. 10. As you see, this circuit is composed of
two gates F2G and FRG. Since the quantum cost of F2G
Res. J. Appl. Sci. Eng. Technol., 4(22):4566-4571, 2012
4570
and FRG are 2 and 5, respectively so, the quantum cost of
this circuit is 7. The proposed circuit requires two
constant inputs and produces two garbage outputs.
Our proposed parity preserving half-subtractor circuit
can be used in constructing fault tolerant reversible
circuits. 
Parity preserving reversible full-subtractor circuit:
The suggested reversible parity preserving full-subtractor
can be built using these gates of F2G gate and one FRG
gate. This circuit can be implemented as shown in Fig. 11
thus, the proposed circuit uses 4 reversible parity
preserving gates. It produces 4 garbage outputs and
requires only one constant input. Constructing the
proposed circuit has been resulted in generation of the
quantum cost of 11.
Our proposed parity preserving full-subtractor circuit
can be used for designing fault tolerant reversible systems
which is the necessary requirement of nanotechnology
based systems.
New design of parity preserving Peres gate: In this part
we demonstrate how Peres gate can be realized by using
three parity preserving reversible gates, which is shown in
Fig. 12. Thus, this circuit requires three gates (two F2G
gates and one FRG gate) and produces 2 garbage outputs.
The proposed parity preserving PG gate uses two constant
inputs and it has the quantum cost of 9.
New design of parity preserving TR gate: In this
section we show how FRG gate and F2G gate can be used
to implement the TR gate. Our proposed design is
depicted in Fig. 13. The proposed reversible parity
preserving circuit needs three reversible gates (one FRG
gate and two F2G gates) and it produces two garbage
outputs. Also, it has the quantum cost of 9. 
CONCLUSION
In this study, we proposed a novel parity preserving
reversible half-subtractor circuit and a novel parity
preserving reversible full-subtractor circuit. We also
designed parity preserving PG implementation using two
kinds of parity preserving gates. Furthermore we showed
the parity preserving TR gate implementation. All the
scales are in the nanometric area.
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A Novel Nanometric Fault Tolerant Reversible Subtractor Circuit

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A Novel Nanometric Fault Tolerant Reversible Subtractor Circuit

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