Patent Publication Number: US-6710624-B2

Title: Programmable array logic circuit macrocell using ferromagnetic memory cells

Description:
This application is a 371 of application Ser. No. PCT/US01/01790 filed Jan. 20, 2001, which claims benefit of 60/177,478 filed Jan. 21, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates generally to a programmable array logic circuit macrocell using ferromagnetic memory cells. More particularly, the present invention uses a non-volatile ferromagnetic memory cell to temporarily store binary data. 
     2. The Background Art 
     Programmable logic devices have any number product sets, usually in groups of four (4), eight (8), sixteen (16) or more bits, although often in groups of ten (10). The arrays are programmed for application-specific tasks to be performed within digital electronic circuits. The fusible link types cannot be re-programmed, but those employing EEPROM and Flash can. For those PALs which use fusible links, the data in the “D” registers is lost at power off. For those that use EEPROM and Flash as replacements for the “D” registers, data is not lost at power off time. 
     Up to the present, traditional PALs have used “D” type flip-flops for product registers. Lately, however, some fabricators have begun using EEPROM and Flash technology to replace these. These last two technologies have drawbacks, however. EEPROMs are cumbersome to re-program, both are slow to re-program, exhibit “write fatigue,” thereby limiting their useful life, and must be mass-written to re-program. 
     SUMMARY OF THE INVENTION 
     It has been recognized that it would be advantageous to develop a programmable array logic circuit macrocell using ferromagnetic memory cells. More particularly, the present invention uses a non-volatile ferromagnetic memory cell to temporarily store binary data. 
     It is an advantage of the invention to have the ferromagnetic memory cells or bits to store data even when there is no power provided to the circuitry, thus saving power during operation of the programmable logic circuitry, and ensuring that there is no loss of the data should there be a temporary power shut down. 
     Additionally, the ferromagnetic cells provide for indefinite number of switching actions on the data without degradation to the capacity to store data therein, thus eliminating “write fatigue”. 
     The invention provides an integrated circuit, comprising a programmable OR array, a programmable AND array, coupled to the programmable OR array, and a macrocell output circuit. The macrocell uniquely has a ferromagnetic bit and sensor coupled to store a remnant output signal, and an output buffer, coupled to output the remnant output signal upon receiving a output enable signal. The macrocell may further include a DQ register that contains the ferromagnetic bit. The DQ register may also include a drive coil, which at least partially surrounds the ferromagnetic bit. Drive coils may have a bi-directional current that sets the polarity of the ferromagnetic bit. The bi-directional current may be switched by two sets of transistor pairs. The two sets of transistor pairs may, in turn, be gated by first and second transistor respectively. The first and second transistors may be responsive to a DATA signal that is received when a CLOCK signal is received. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawing, which together illustrate by way of example, the features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a preferred embodiment of the present invention. 
     FIG. 2 is a schematic of a preferred embodiment of the present invention. 
     FIG. 3 is a cross sectional view of a single ferromagnetic memory bit. 
     FIG. 4 is a schematic of a field programmable logic device capable of utilizing the present invention. 
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications of the inventive features illustrated herein, and any additional applications of the principles of the invention as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
     For the purpose of providing background material which may in some respects illustrate the state of the are, the following books are herein incorporated by reference for non-essential material: “Programmable Logic Handbook,” fourth edition, by Monolithic Memories Inc., 2175 Mission College Blvd., Santa Clara, Calif.; and “Practical Design Using Programmable Logic,” by D. Pellerin and M. Holley, Prentice Hall, library of Congress no. TK7872.L64 1991. 
     The following United States patents are herein incorporated by reference for their supporting teachings: 
     U.S. Pat. No. 5,300,830, is a programmable logic device macrocell with an exclusive feedback and exclusive external input lines for registered and combinatorial modes using a dedicated product term for control. 
     U.S. Pat. No. 5,287,017, is a programmable logic device macrocell with two OR array inputs. 
     U.S. Pat. No. 5,324,998, is a zero power re-programmable flash cell for a programmable logic device. 
     U.S. Pat. No. 5,295,097, is a nonvolatile random access memory. 
     U.S. Pat. No. 5,867,049, is a zero setup time flip flop. 
     U.S. Pat. No. 5,298,803, is a programmable logic device having low power microcells with selectable registered and combinatorial output signals. 
     The invention is used in the normal arena in which PALs are employed, but where it may be advantageous to retain the last data stored after removal of power to the circuit as well as the simplicity of random re-programming. This PAL circuit could be in all manner of digital circuitry, including computers, robotics, telephones, automotive circuits, security devices, and so on. The advantages over prior art lie principally in that PAL devices using ferromagnetic digital memory cells for product registers would enjoy non-volatility, high-speed in situ random re-programmability and indefinite long life span. 
     A PAL having product registers (flip flops) that use non-volatile ferromagnetic memory/storage cells for product storage registers is described wherein the non-volatile ferromagnetic memory cells, comprised of single stick-like ferromagnets whose aspect ratio is greater than 1:1 (height to width, e.g. 2:1 etc.), are fabricated normal to the plane of the substrate with a sensor centered intimate to, and at one end of the ferromagnets are used for these functions. The magnetic polarity of each ferromagnetic memory cell bit is coerced into reversal using write, or state change circuitry, comprised of a coil which surrounds, up to and including 270°, and is intimate to, the ferromagnetic stick. The write current in each coil is bi-directional, depending on the desired magnetic movement, or polarity, of the bit. The output of the sensor reflects the binary condition of the bit, and is used as binary data output for product registers. 
     FIG. 1 is a schematic of a dual output storage register  10 , or flip-flop, which is comprised of a ferromagnetic memory cell consisting of micron or sub-micron sized ferromagnetic bit whose aspect ratio is greater than 1:1 (i.e., 2:1, etc.), deposited normal to the plane of the substrate with its attendant support circuitry. Uniquely, there is a ferromagnetic stick (bit)  1 , which is at least partially, but not necessarily completely, surrounded by a write drive coil  2 . The bi-directional current in the write drive coil sets the polarity of the bit and is switched by two sets of transistor pairs, or switches. The first transistor pair being Q 3  and Q 6 , having reference number  4   a , and the second pair being Q 4  and Q 5 , having reference number  4   b . These switches are in turn gated by Q 1  and Q 2 , having reference number  7 . With a signal on the DATA  21  line and a positive-going pulse on the CLOCK  20  line, either Q 7  or Q 8  will be activated to allow the memory cell  1  to be set to one binary position or another. Other features of the drawing are: capacitor C 1 , diode D 1 , resistors R 3  tied to ground, and R 4  and R 5  which are both tied to power+. 
     On a second portion of the flip-flop  10 , or storage register circuit, there is a ferromagnetic bit field sensor  3 , which is capable of sensing remnant polarity stored in the bit  1 . It is noted that the dashed line is meant to illustrate the fact that the sensor  3  is to be physically located proximate the top or bottom of the ferromagnetic bit  1 , but is illustrated at a spaced relationship for schematic purposes only and not meant to illustrate physical layout as is shown in FIG.  3 . Additionally, amplifiers U 1  and U 2 , having reference number  5 , respond based on the Hall voltage that appears across the sensor  3 . The outputs are fed to the transistors Q 10  and Q 11 , reference number  6 , whose outputs, in turn, are “Q” and “Q*”. In summary, the DATA is set into the cell with the rising CLOCK line and stored in the ferromagnetic memory bit  1  despite any power shutdown after being stored. Thereafter, the output in this case employs two operational amplifiers whose differential outputs are fed; one to the Q output and the other, Q* to the product feedback circuit. 
     Since the ferromagnetic bit does not change state when power to the circuit is off, when power has thereafter been restored subsequent to a power off, the storage registers will have retained their critical data. Additionally, even while there is power to the programmable logic device, it is noted that additional power is not required to maintain the stored state in the flip-flop  10 , thus, saving overall power consumption. 
     FIG. 2 is a representation of the “DQ” type register or flip-flop  10  of FIG.  1  and is part of the output macrocell circuitry  22  that receives data from an OR array  24  from a typical PAL  30 , (programmable array logic). It is noted that one skilled in the art will easily realize that the bit  1  will store a remnant polarity in the ferromagnetic material that the sensor  3  will be able to sense. Thus, creating a remnant output signal that will cause the amplifiers  5  to simultaneously and constantly open or close the gates of output transistors  6  accordingly; and where the polarity of the bit will exist even after the power to the circuitry has been turned off. To output the data signal Q, output enable signal line  32  will pulse to enable the output amplifier  34 , or output buffer. 
     FIG. 3 depicts, for example, a potential cross-section of a micron or sub-micron scale ferromagnetic memory cell with the sensor shown on the bottom. There is a ferromagnetic storage element  11 , or “bit,” a sensor  12 , a sample drive line  14 , which is made of Al, Cu or any other suitable conductor material. Additionally there is a cutaway of the set (write) drive coil  13  that wraps around bit  11 . In this depiction, matrix interconnects are not shown for the sake of clarity. The entire memory cell rests on substrate  19 , which can be silicon, glass, GaAs, or other suitable material. Insulation layer  15  resides between the ferromagnetic bit  11  and the set coil  13 . Material layer  15  can be of a material such as SiO 2  or Si 3 N 4 , etc. There is an overall insulation sections  16 ,  17  and  18 , again made of SiO 2 , Si 3 N 4 , or other suitable material. 
     Several methods may be employed to make this cell, including, but not limited to, electroplating, sputtering, E-beam deposition, chemical vapor deposition and molecular beam epitaxy. 
     Referring to FIG. 4, there is an embodiment of the invention being utilized in a field programmable logic device  30 . One skilled in the art will recognize that there is a programmable AND  28  array with inputs and outputs. Similarly, there are two programmable OR arrays  24  and  26  that rout signals through various levels or numbers of logical OR gates before the sum of products are sent to designated macrocells. Additionally, there is illustrated macrocell  22 . It is noted that the macrocells have one of the flip-flops  10  located therein. 
     It is understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements. Thus, while the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, variations in size, materials, shape, form, function and manner of operation, assembly and use may be made, without departing from the principles and concepts of the invention as set forth in the claims.