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Unformatted text preview: Asynchrony in Quantum- Dot Cellular Automata Nanocomputation: Elixir or Poison? Mariagrazia Graziano, Marco Vacca, Davide Blua, and Maurizio Zamboni Politecnico di Torino T HE CELLULAR AUTOMATA principle has been suc- cessfully applied to electronic digital circuits, lead- ing to the quantum-dot cellular automata (QCA) concept. 1 The general principle of this concept is that a QCA cell has two different charge configura- tions, representing the two logic values 0 and 1 (see Figure 1a). These building blocks are placed a short distance from each other on the same plane. The electrostatic interaction between neigh- bor cells drives the information through the circuit. Two main implementations of this theoretical princi- ple exist: molecular QCA, in which the cell is a com- plex molecule, and magnetic QCA (MQCA), 3 in which the cell is a single-domain nanomagnet (see Figure 1b). Although MQCA allow speeds (hun- dreds of MHz) lower than the molecular ones (a few THz), they offer several specific advantages. These include small area, low power consumption, and the possibility to combine computation and storage on the same circuit. 5 Most importantly, though, they offer experimental feasibility with technology thats currently available. 3 The International Technology Roadmap of Semiconductors thus mentions that MQCA are worth studying to prove whether cellular automata can generally replace CMOS (see http://public.itrs.net/ Links/2010ITRS/Home2010.htm). QCA might offer many advantages in replacing CMOS, but many issues and constraints come with this technology (as we detail in the Considerations and Constraints sidebar). Asynchrony has been intro- duced when designing QCA circuits at the architectural level, and its tempting to think of this approach as a magical elixir. Is it, though? Or, because of the over- head it introduces, is this approach more of a poison? In this article, thats exactly what we intend to discern. Specifically, were interested in applying and testing Wave Semiconductors asynchronous Null Convention Logic (NCL) to QCA technology. Tabrizi- zadeh et al. proposed a general NCL implementation applied to QCA circuits, 6 whereas we presented a spe- cific solution for magnetic circuits in previous work. 2,7 That research informed the system that we currently propose, which is globally asynchronous (the handshake protocol to allow information propa- gation) and locally synchronous (the clock system to enable information propagation). Some traditional designs have used globally asynchronous, locally syn- chronous (GALS) circuits recently to successfully le- verage the burdens caused by interconnect delays 8 or by different synchronization subsystems....
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This note was uploaded on 11/28/2011 for the course COMP 790 taught by Professor Staff during the Fall '08 term at UNC.
- Fall '08