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Fig. 4. (A) A magnetic field can be generated by a current flowing through a wire
placed under the magnets plane. (B) Clock zones layout is made by parallel strips that
follows the wires used to generate the magnetic fields.
To simulate QCA circuits a dedicated simulator, called QCADesigner, was
developed [ 28 ]. Unfortunately QCADesigner does not support magnetic circuits.
To simulate NML circuits two paths can be followed. First of all low level mag-
netic simulators, like OOMMF [ 29 ], NMAG [ 30 ]or[ 31 ], can be used. Low level
simulators allow to obtain the most accurate simulation, but they are very slow
and only small circuits can be simulated due to the high memory usage.
As a second option circuits can be studied using a RTL model [ 32 ]. The idea
is to describe using the VHDL language a CMOS circuit that behaves exactly like
its NML counterpart. For example, starting from the NML circuit of Fig. 5 (A),
its RTL model can be built as shown in Fig. 5 (B) and then described with VHDL
language. Registers are used to model the propagation delay. This is possible due
to the intrinsic pipelined behavior of the technology. Ideal logic gates without
delay are used to model the logic function. The logic gate set available in this
technology is based on majority voters [ 16 ], AND/OR gates [ 33 ] and crosswires
[ 13 ], that are particular blocks that allows to cross two wires on the same plane.
At every register one of the three clock signals shown in Fig. 5 (C) is then applied.
In this way complex circuits can be easily described using VHDL and simulated
using the powerful CAD tools available in CMOS technology, like Modelsim [ 34 ].
We applied this model successfully designing complex NML circuits in [ 26 , 27 , 35 ].
2 ToPoliNano
Both simulation mechanisms available have their flaws. The situation is summa-
rized in Fig. 6 .
To have the most accurate results physical simulators are required, but they
can be used only on very small circuits due to their computational require-
ments. At the same time, while the RTL model is a powerful tool that allows
fast description and simulation of complex circuits, it gives only estimations of
real circuits performance, because a lot of informations on the circuit layout are
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