Reversible Logic Based Design and Test of Field Coupled Nanocomputing Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 134

the outputs. Thus for unidirectional stuck-at faults in conservative logic circuits,
counting the number of 1s in the inputs and the outputs would be the fault
detection scheme. The feedback in sequential circuits makes them untestable by
all 0s and all 1s test vectors. Moreover, in reversible logic fanout is not allowed.
Hence, we propose a technique that will take care of the fanout at the output of
the reversible latches and can also disrupt the feedback to make them suitable
for testing by only two test vectors, all 0s and all 1s. In other words, circuits will
have feedback while executing in the normal mode. However, in order to detect
faults in the test mode, our proposed technique will disrupt feedback to make
conservative reversible latches testable as combinational circuits. The proposed
technique is extended towards the design of two vectors testable master-slave
flip-flops, double edge triggered flip-flops, asynchronous set/reset D latch, D
flip-flop and counters. Thus, our work is significant because we are providing the
design of reversible sequential circuits completely testable for any unidirectional
stuck-at faults by only two test vectors. The reversible design of the double edge
triggered flip-flop, ring counter and Johnson counter are proposed for the first
time in literature.
Field coupled quantum dot cellular automata (QCA) computing is based on
majority voting, hence the designs based on conservative logic will be completely
different from those based on conventional CMOS. Single missing/additional cell
defects are prominent permanent defects in QCA circuits. We implemented the
Fredkin gate in the QCA technology and observed that all 0s and all 1s test
vectors cannot provide 100 % fault coverage for single missing/additional cell
defect in the QCA layout of the Fredkin gate. Thus, to have the 100 % fault
coverage for single missing/additional cell defect by all 0s and all 1s test vectors,
we identified the QCA devices in the QCA layout of the Fredkin gate that can
be replaced with fault tolerant components to provide the 100 % fault coverage.
Further, while designing a QCA sequential circuit, the designer may sometimes
prefer to sacrifice the reversibility to save the number of QCA cells while keeping
the test strategy to be the same, that is the design can still be tested by two
test vectors. Thus, we also propose a new conservative logic gate called Multi-
plexer Conservative QCA gate (MX-cqca) which is not reversible in nature but
has similar properties as the Fredkin gate of working as 2:1 multiplexer. The
proposed MX-cqca gate surpasses the Fredkin gate in terms of complexity(the
number of majority voter), speed and area. MX-cqca can implement the multi-
plexer function with 1 less majority gate than the Fredkin gate; it also requires
a smaller area and fewer QCA cells in QCA layout. The design and verification
of the QCA layouts were performed using the QCADesigner and HDLQ tools.
The chapter is organized as follows: Sect. 2 presents the background on con-
servative logic, the basics of QCA computing, such as QCA logic devices and
QCA clocking, related work etc.; Sect. 3 presents design of testable reversible
latches; Sect. 4 describes design of testable reversible master-slave flip-flops;
Sect. 5 presents design of testable reversible double edge triggered flip-flop; Sect. 6
shows the design of testable reversible complex sequential circuits; Sect. 7 dis-
cusses the application of the proposed two vectors, all 0s and all 1s, testing