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ISU CPRE 583 - A magnetoelectronic

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A Magnetoelectronic Macrocell Employing Reconfigurable Threshold Logic Steve P. Ferrera University of Illinois Urbana-Champaign, IL [email protected] Nicholas P. Carter University of Illinois Urbana-Champaign, IL [email protected] ABSTRACT In this paper, we introduce a reconfigurable fabric based around a new class of circuit element: the hybrid Hall effect (HHE) magnetoelectronic device. Because they incorporate a ferromagnetic element, HHE devices are inherently non-volatile, retaining their state without a power supply. In addition, HHE devices are extremely well-suited to implementing threshold logic circuits, which allows many complex logic functions to be implemented in fewer gates than are required in systems based on AND-OR logic. We present the design of an HHE-based reconfigurable macrocell based on two-level threshold logic that can be configured on a cycle-by-cycle basis while internally storing non-volatile configuration data and computation state. The performance of this macrocell is characterized, and compared to that of competing technologies, showing that it has a significantly better power-delay product when implementing complex functions of many inputs. Categories and Subject Descriptors B.6.1 [Logic Design]: Design Styles – combinational logic; B.7.1 [Integrated Circuits]: Types and Design Styles – advanced technologies General Terms Performance, Design, Theory Keywords Magnetoelectronic circuits, lookup table, non-volatility, PLA/CPLD, threshold logic, wired-AND logic. 1. INTRODUCTION Since the discovery of the giant magnetoresistance (GMR) effect, a number of magnetoelectronic devices have been developed that take advantage of the properties of ferromagnetic materials to provide non-volatile data storage [1,2]. Recently, researchers at the Naval Research Lab have developed a new class of magnetoelectronic device, the hybrid Hall effect (HHE) device, which can be reprogrammed from cycle to cycle to implement a variety of logic functions with non-volatile storage of the result. In [3], we presented a set of circuit designs for reconfigurable logic gates based on HHE devices, including interface logic that allows these circuits to be integrated into CMOS systems. These circuits, which are based on a single HHE device and a small number of CMOS transistors, can be reconfigured to implement AND, OR, NAND, and NOR gates with multiple inputs. In addition, we presented circuits that use HHE devices to provide non-volatile storage for conventional SRAM cells. However, HHE devices are more versatile than these circuits would indicate. State switching in an HHE device is dependent on whether or not the magnitude of the input current to the device is large enough to generate a magnetic field capable of changing the magnetization state of the device’s ferromagnetic element. This permits HHE-based circuits to efficiently implement reconfigurable threshold logic, which often allows the implementation of complex logic functions in fewer gates than AND-OR designs. In this paper, we present a set of designs for reconfigurable threshold logic based on HHE devices. These circuit designs occupy an intermediate point between current SRAM-based reconfigurable logic and EEPROM wired-AND systems. At one extreme, designs based on SRAM lookup tables (LUTs) provide extremely high functional coverage, being able to implement any Boolean function of their inputs. However, LUTs suffer from geometric increases in area as their number of inputs increases, and do not provide any non-volatile storage. At the opposite end of the spectrum, EEPROM based programmable logic arrays Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. FPGA’04, February 22–24, 2004, Monterey, California, USA Copyright 2004 ACM 1-58113-829-6/04/0002…$5.00. FunctionalCoverageSRAMLUTLogicHHEThresholdLogicEEPROMWired-ANDLogic# of Function InputsFigure 1. Functional coverage vs # of function inputs. 143(PLAs) retain their configuration without power, and efficiently implement functions with many inputs, but are limited in the set of functions they can implement. As diagrammed in Figure 1, HHE-based threshold logic is capable of efficiently implementing functions with more inputs than LUT-based designs can efficiently support, but can implement a wider range of functions than PLA-based systems. In the next section, we describe the HHE device and its operation. In section 3, we introduce the concepts behind threshold logic. Section 4 discusses the implementation behind an HHE-based reconfigurable macrocell. Section 5 presents SPICE simulation results for our designs, including comparisons with other technologies. In sections 6 and 7, we discuss related work and propose future work respectively. Lastly, we conclude in section 8. Although we mainly focus on implementations employing HHE devices, the concepts presented in this paper can be transferred to designs utilizing other magnetoelectronic devices. 2. HHE DEVICE 2.1 Device Description and Operation Figure 2 shows an HHE device [4,5] consisting of one or more input wires (the top bar in the figure) that pass over a region of ferromagnetic material (the middle bar in the diagram). If the magnitude of the current along the input wire is sufficiently large, the magnetic field it generates will magnetize the ferromagnetic element in either the left or right direction, depending on the direction of current flow. Figure 2 shows both possible current directions and their associated magnetization states. Since ferromagnetic materials retain their magnetization state in the absence of an external magnetic field, the magnetization state of the ferromagnetic element can be used to store a binary value, interpreting one direction of magnetization as a logic 1 and the other as a logic 0. To observe the magnetization state of the ferromagnetic element, a bias current Ibias is passed through the conductor at the base of the device in a direction perpendicular to the magnetic field. As specified by the Hall Effect [6], the interaction of this current and the magnetic field generated by the


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