Abstract
At nanoscale, quantum-dot cellular automata (QCA) defines a new device architecture that permits the innovative design of digital systems. Features of these systems are the allowed crossing of signal lines with different orientation in polarization on a Cartesian plane, the potential of high throughput due to efficient pipelining, fast signal switching, and propagation. However, QCA designs of even modest complexity suffer from the negative impact due to the placement of long lines of cells among clocking zones, thus resulting in increased delay, slow timing, and sensitivity to thermal fluctuations. In this paper, different schemes for clocking and timing of the QCA systems are proposed; these schemes utilize 2D techniques that permit a reduction in the longest line length in each clocking zone. The proposed clocking schemes utilize logic-propagation techniques that have been developed for systolic arrays. Placement of QCA cells is modified to ensure correct signal generation and timing. The significant reduction in the longest line length permits a fast timing and efficient pipelining to occur while guaranteeing a kink-free behavior in switching.
I. INTRODUCTION
IN THE PAST few decades, the exponential scaling in feature size and the increase in processing power have been successfully achieved by very large scale integration (VLSI) technology, mostly using CMOS; however, in the not-so-distant future [1], this technology will face serious challenges as the fundamental physical limits of its devices are reached. In recent years, there has been extensive research at nanoscale to supersede the conventional CMOS using the so-called emerging technologies. It is anticipated that these fundamentally different technologies can achieve extremely high densities and high operational speed. Among these new devices, quantum-dot cellular automata (QCA) not only gives a solution at nanoscale but also offers a new method of computation and information transformation (often referred to as processing-in-wire). In terms of feature size, it is projected that a QCA cell of a few-nanometer size can be fabricated through a molecular implementation by a self-assembly process [2], [3]. Sequential as well as combinational designs can be realized using the QCA. Designs based on QCA (such as carry-look-ahead adder, barrel shifter, microprocessors, and field-programmable gate arrays, have been presented in the technical literature [4]–[9].
VIII. CONCLUSION
The QCA has been advocated as a potential device architecture for nanotechnology. The QCA not only gives a solution at nanoscale but also offers a new method of computation and information transformation. However, the QCA designs of even modest complexity suffer from the disadvantage of long vertical lines in the placement of the cells, thus resulting in long delay, slow timing, inability to operate at higher (room) temperature, and sensitivity to thermal fluctuations.
