NanoMagnet Logic: An Architectural Level Overview - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 223

Fig. 2. 3-phase overlapped clock system. (A) Clock signal waveforms. 3 clock signals
with a phase difference of 120 ◦ , partially overlapped, are used to assure a correct signals
propagation. (B) Detailed signal propagation through a simple NML wire. 6 different
time step can be identified thanks to this clock system. When magnets of a clock zone
are switching (SWITCH) magnets on their left are in the HOLD state and act as an
input, while magnets on their right are in the RESET state so they have no influence.
elements. A different clock signal is applied to every clock zone. Different clock
schemes can be used. One of the most simple clock scheme uses three overlapped
clock signals [ 18 ], with a phase difference of 120 ⓦ (Fig. 2 (A)). Figure 2 (B) shows
an example of NML wire and how its state changes when the clock signals are
applied. Three different states can be identified: HOLD when no clock signal is
applied, RESET when the clock signal is applied and SWITCH when the clock
signal is slowly removed. When magnets of a clock zone are switching (SWITCH
state), they see on their left magnets that are in the HOLD state and therefore
act as an input. Magnets on the right are in the RESET state and they have
no influence on the signals propagation. To obtain an errorless signal propaga-
tion magnets of a clock zone must be forced in the RESET state before neighbor
magnets start to switch (see Fig. 2 (B)). This is obtained by overlapping the clock
waveforms [ 18 ].
1.1 Designing Circuits with Clock Zones Constraints
The clock mechanism is central to the entire NML (and QCA) technology and it
is the source of the main problems encountered at architectural level. Particularly
in case of NML implementation four possible clock mechanisms can be identified,
as shown in Fig. 3 . The first mechanism uses a magnetic field generated by a
current flowing through a wire placed under the magnets' plane [ 13 , 19 ], as shown
in Fig. 3 (A). The second mechanism uses Spin-Torque coupling with a current
flowing through the magnets themselves [ 20 - 22 ], as can be seen from Fig. 3 (B).
In this case the basic element is no more a simple magnet but it is a Magneto-
Tunnel Junction (MTJ). The third clock mechanism, shown in Fig. 3 (C), uses the
mechanical stress induced by the strain of a piezoelectric layer to force magnets
in the RESET state [ 23 , 24 ]. In this case the clock mechanism is no more based
on a current but on a voltage. Finally, a 4th clock topology is available as shown
in Fig. 3 (D). In this case, magnets are made by Cobalt/Platinum multilayered