Reversible Logic Based Design and Test of Field Coupled Nanocomputing Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 133

will be a significant factor of the overall heat dissipation. For example, in new
Superconductor Flux Logic (SFL) family based on nSQUID gates, the energy
dissipation in conventional logically irreversible architectures is close to few kT
ln2 per logic operation. By employing reversible logic, the energy dissipation per
nSQUID gate per bit measured, at 4 K temperature is below the thermodynamic
threshold limit of kT ln2 [ 55 ]. Therefore, reversible logic is being investigated for
its promising applications in power-ecient nanocomputing [ 17 , 32 , 35 ].
Further, quantum dot cellular automata (QCA) is one of the emerging field
coupled nanotechnologies in which it is possible to implement reversible logic
gates [ 19 , 37 , 38 ]. QCA makes it possible to achieve circuit densities and clock
frequencies beyond the limits of existing CMOS technology [ 70 , 71 ]. The logic
states of 1 and 0 are represented by the position of the electrons inside the QCA
cell as illustrated in Fig. 2 (b). Thus, when the bit is flipped from 1 to 0 there is no
actual discharging of the capacitor as in conventional CMOS. Hence, QCA does
not have to dissipate all its signal energy during transition. Further, propagation
of the polarization from one cell to another is because of interaction of electrons
in adjacent QCA cells. As there is no movement of electrons from one QCA
cell to the other, hence there is no current flow. Thus, QCA has no dissipation
in signal propagation. Therefore, QCA has significant advantage compared to
CMOS technology in terms of power dissipation. Further in contrast to CMOS,
the cells in QCA are connected to 4 clocking zones, each lagging behind by 90 ⓦ
in phase. QCA clocking helps in the successive transfer of information from one
clock zone to the next [ 25 , 34 ]. Therefore, we have information flow from the
input to the output in a pipelined fashion. Thus, QCA cells are inherently suit-
able for pipeline and systolic designs [ 15 ]. QCA computing can be implemented
in semiconductor, molecular and magnetic platforms. Researchers are currently
targeting magnetic and molecular QCA, several smaller circuits in magnetic and
molecular QCA have been fabricated and tested [ 3 , 7 , 26 , 33 , 52 , 53 ]. Theoretical
studies have also shown that molecular QCA can operate at room temperature
at THz of speed [ 36 ].
Several works can be found in the literature for QCA design such as adders,
multipliers, shifters, memories, FPGA, synthesis etc. [ 9 , 10 , 24 , 28 , 45 , 64 , 73 , 76 ].
Due to high error rates in nano-scale manufacturing, the major goal in QCA and
other nanotechnologies is to have devices with reduced error rates [ 37 ]. In the
manufacturing process for QCA, defects can occur in the synthesis and depo-
sition phases. However, defects are more likely to occur during the deposition
phase [ 62 ].
In this work, we propose the design of testable sequential circuits based on
conservative logic gates. It has shown in [ 61 ] that the combinational circuits
based on conservative logic gates outperform all the circuits implemented using
classical gates in the area of testing. Further, any combinational circuit based on
conservative logic gates can be tested for classical unidirectional stuck-at faults
using only two test vectors. The two test vectors are all 1s, and all 0s [ 61 ]. This is
because whenever there are unidirectional faults in combinational conservative
logic circuits, the number of 1s in the inputs will differ from the number of 1s in