Image Processing Reference
In-Depth Information
clocking zone to a minimum. The reason to do this is that as wire length grows,
the probability that a QCA cell will switch successfully decreases, in proportion to
the distance of this particular cell from a clamped (frozen) input at the beginning of
the wire. Consequently, for shorter wires, it is most likely that all cells making up
the wire will switch successfully. Additionally, short wire lengths result in circuit
operation in higher clock rates [23]. That is because, before the cells in a given zone
can change phase, every cell within this zone must make appropriate polarization
changes that need time. Therefore, the longer the wire is, the longer time is needed
for a signal to propagate along its length. In general, the maximum wire length that
can successfully propagate a signal from one end to the other is twenty eight (28)
90-degree cells, or twenty seven (27) 45-degree cells, acquired by simulation on an
experimental setup [22].
At nonzero Kelvin temperatures, higher operating temperatures lead to higher
thermal fluctuations which increase the probability of a kink occuring (in which
a QCA cell aligns differently from its expected polarization) [1]. It has been
proven [44, 66] that the maximum QCA line length for kink-free operation is given
by:
E
k
k
b
T
N
≤
e
(4.3)
where
E
k
represents the energy required for a QCA cell to encounter a kink,
k
b
is the
Boltzmann constant and
T
is the operating temperature. For the proposed designs
the maximum wire length would be kept under a desired value so that it can operate
to higher possible temperatures.
It is also important to keep the area of the clocking zones to a minimum. This has
to be done to increase uniformity and consequently manufacturability. Furthermore,
by keeping the area to minimum, wire lengths are also kept to a minimum and
consequently the circuit can operate at higher temperatures with no kink occurrence.
On the other hand, if we shrink a phase block to a single cell this block will cause
functional failures. So to make up with this vulnerability we must set a minimum of
2 cells for the length of a phase block.
Another problem arising with complex QCA designs is that usually large amounts
of white space-wasted area is left between cells [40, 42]. In the proposed designs the
use of clocking zones with many cells should be avoided and consequently the QCA
cells should be uniformly distributed into the clocking zones. The clocking zones
should also be designed in such a way so that the uncovered areas would be as small
as possible. Finally, to confront this problem, the total area covered by QCA cells
should be minimized by keeping the distance between binary wires as close as pos-
sible according to QCA design rules [22, 23]. As a result, the proposed architecture
would not leave large amount of area unused.
In order to obtain a successful and stable design the following issues should be
considered:
•
keep the length of the wires to a minimum
•
keep the area of the clocking zones to a minimum
•
set the minimum of 2 cells for the length of a phase block
•
the uncovered areas would be as small as possible
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