Title of Invention

A METHOD OF DRIVING A PASSIVE MATRIX-ADDRESSABLE DISPLAY OR MEMORY ARRAY OF CELLS

Abstract

In a method of driving a passive matrix display or memory array of cells comprising an electrically polarizable material exhibiting hysteresis, in particular a ferroelectric material, wherein the polarization state of individual cells can be switched by application of electric potentials or voltages to word and bit lines in the matrix or array, a potential on selected word and bit lines is controlled to approach or coincide with one of n predefined potential levels and the potentials on all word and bit lines are controlled in time according to a protocol such that word lines are sequentially latched to potentials selected among nword potentials, while the bit lines are either latched sequentially to potentials selected among nbit potentials, or during a certain period of a timing sequence given by the protocol connected to circuitry for detecting charges flowing between a bit line or bit lines and cells connecting thereto. This timing sequence is provided with a read cycle during which charges flowing between the selected bit line or bit lines connecting thereto are detected and a "refresh/write cycle" during which the polarization of the cells connecting with selected word and bit lines are brought to correspond with a set of predetermined values.

Full Text

Addressing of memory matrix The present invention concerns a method of driving a passive matrix-addressable display or memory array of cells comprising an electrically polarizable material exhibiting hysteresis, in particular a ferroelectric material, wherein the polarization state of individual, separately selectable cells can be switched to a desired condition by application of electric potentials or voltages to word and bit lines forming an addressing matrix, and wherein the method comprises establishing a voltage pulsing protocol with n voltage or potential levels, n > 3, such that the voltage pulsing protocol defines a timing sequence for individually controlling the voltage levels applied to word and bit lines of the matrix in a time-coordinated fashion, arranging said timing sequence to encompass at least two distinct parts, including a "read cycle" during which charges flowing between said selected bit line(s) and the cells connecting to said bit line(s) are sensed, and a "refresh/write cycle" during which polarization state(s) in cells connecting with selected word and bit lines are brought to correspond with a set of predetermined logical states or data values. Particularly the present invention concerns pulsing protocols for the addressing of individual crossing points in passive matrices used for data storage and display purposes. A major concern is the avoidance of disturbing non-addressed crossing points in the same matrices. Another important concern is to minimize the cumulative signal from non-addressed cells in such matrices during reading of stored data. Applications shall typically involve, but are not limited to, matrices containing a ferroelectric thin film that acts as non-volatile memory material. Passive matrix addressing'implies the use of two sets of parallel electrodes that cross each other, typically in orthogonal fashion, creating a matrix of crossing points that can be individually accessed electrically by selective excitation of the appropriate electrodes from the edge of the matrix. Advantages of this arrangement include simplicity of manufacture and high density of crossing points, provided the functionality of the matrix device can be achieved via the two-terminal connections available at each crossing point. Of particular interest in the present context are display and memory applications involving matrices where the electrodes at each crossing point sandwich a material in a capacitor-like structure, henceforth termed a "cell", and where the material in the cells exhibits polarizability and hysteresis. The latter property confers non-volatility on the devices, i.e. they exhibit a memory effect in the absence of an applied external field. By application of a potential difference between the two electrodes in a given ceil, the material in the cell is subjected to an electric field which evokes a polarization response, the direction and magnitude of which may be thus set and left in a desired state, representing e.g. a logic "0" or "1" in a memory application or a brightness level in a display application. Likewise, the polarization status in a given cell may be altered or deduced by renewed application of voltages to the two electrodes addressing that cell. Examples of passive matrix devices employing ferroelectric memory substances can be found in the literature dating back 40-50 years. Thus, W.J. Merz and J.R. Anderson described a barium titanate based memory device in 1955 (WJ. Merz and J.R. Anderson, "Ferroelectric storage devices", Bell.Lab.Record. 1, pp. 335-342 (1955)), and similar work was also reported by others promptly thereafter (see, e.g.'C.F. Pulvari "Ferroelectrics and their memory applications", IRE Transactions CP-3, pp. 3-11 (1956), and D.S. Campbell "Barium titanate and its use as a memory store", J. Brit. IRE 17 (7) pp. 385-395 (1957)). An example of a passive matrix addressed display rendered non-volatile by a ferroelectric material can be found in US patent No. 3 725 899 (W. Greubel) filed in 1970. In view of its long history and apparent advantages, it is remarkable that the passive matrix addressing principle in conjunction with ferroelectrics has not had a greater impact technologically and commercially. While important reasons for this may be traced back to the lack of ferroelectric materials that satisfy the full range (technical and commercial) of minimum requirements for the devices in question, a major factor has been certain inherent negative attributes of passive matrix addressing. Prominent among these is the problem of disturbing non-addressed crossing points. The phenomenon is well recognized and extensively discussed in the literature, both for displays and in memory arrays. Thus, the basics shall not be discussed here, but the reader is referred to, e.g.: A. Sobel: "Some constraints on the operation of matrix displays", IEEE Trans.Electron Devices (Corresp.) ED-18, p. 797 (1971), and L.E.Tannas Jr., "Flat panel displays and CRTs", pp.106 & seq., (Van Nostrand 1985). Depending on the type of device in question, different criteria for avoiding or reducing disturbance of non-addressed crossing points can be defined. Generally, it is sought to lower the sensitivity of each cell in the matrix to small-signal disturbances, which can be achieved by cells that exhibit a non-linear voltage-current response, involving e.g. thresholding, rectification and/or various forms of hysteresis. Although general applicability is claimed for the present invention, particular focus shall be directed towards ferroelectric memories, where a thin film of ferroelectric material is stimulated at the matrix crossing points, exhibiting a hysteresis curve as illustrated generically in fig.l. Typically, writing of a bit is accomplished by applying a voltage differential across the film at a crossing point, causing the ferroelectric to polarize or switch polarization. Reading is analogously achieved by applying a voltage of a given polarization, which either causes the polarization to remain unchanged after removal of the voltage or to flip to the opposite direction. In the former case, a small current will flow in response to the applied voltage, while in the latter case the polarization change causes a current pulse of magnitude larger than a predefined threshold level. A crossing point may arbitrarily be defined as representing a "0" bit in the former case, a "1" bit in the latter. A material with hysteresis curve as shown in fig.l will change its net polarization direction upon application of a field that exceeds Vc- However, partial switching shall take place upon application of voltages below this value, to an extent depending on the material in question. Thus, in a matrix with a large number of crossing points, repeated stimuli of non-addressed crossing points may ultimately degrade the polarization states in the matrix to the point where erroneous reading results. The amount and type of stimulus received by non-addressed crossing points in a cross-bar passive matrix during write and read operations depends on how the voltages are managed on all addressing lines in the matrix during these operations, henceforth termed the "pulsing protocol". The choice of voltage pulsing protocol depends on a number of factors, and different schemes have been proposed in the literature, for applications involving memory materials exhibiting hysteresis. Examples of prior art shall now be given. US patent No. 2 942 239 (J.P. Eckert, Jr. & al.) descloses pulsing protocols for memory arrays with magnetic cores, each with a magnetic hysteresis curve analogous to the ferroelectric one shown in fig.l. Although claiming general applicability for memory elements exhibiting bistable states of specific teachings on magnetic data storage where separate contributions to the total magnetic flux in each cell are added or subtracted from several independent lines intersecting in each cell. This is reflected in how cells are linked up in the proffered embodiments, with a readout protocol providing superposition of a slow, or "background" biasing stimulus being applied to all or a subset (e.g. a column or a row) of the cells in the matrix, and with a fast selection pulse being applied between the crossing lines containing the addressed cell. No teachings are given on efficient voltage protocols for two-terminal, capacitor-like memory cells combining high speed, random access to data with restoration of the destructively read information. US patent No. 3 002 182 (J.R. Anderson) concerns the problem of polarization loss by partial switching of ferroelectric memory cells in passive matrix addressed arrays of ferroelectric-filled capacitors. To reduce the partial switching polarization loss during writing, this patent teaches the use of simultaneous application of addressing pulses to an addressed row and column such that the former executes an electrical potential swing of typically +2Vs/3 to +3Vs/4 (where Vs is the nominal switching voltage) while the latter swings to a negative value sufficient for the potential difference between the electrodes at the selected crossing point to reach the value Vs. With the remaining columns being switched to a potential in the range +Vs/3 to + Vs/4, only the selected cell in the matrix is subjected to a significant switching field, and partial switching at the other crossing points is strongly reduced (the reduction depends on the material properties of the ferroelectric, in particular the shape of the hysteresis curve and the magnitude of the dielectric constant). In an alternative pulsing scheme, the same patent teaches the application of additional "disturbance compensating pulses" subsequent to each writing operation, where the selected row is clamped at zero potential while the selected and non-selected columns are pulsed to +Vs/4 to +Vs/3 and -Vs/4 to -Vs/3, respectively. The latter operation is claimed to reduce the partial switching induced loss of polarization even further. No physical explanation was provided for this choice of pulsing scheme, however, which appears to rely to a large degree on the inventor's empirical experience with the ferroelectric materials of his day, in particular barium titanate. While the basic choice of polarities appear plausible and indeed intuitive to the person skilled in the art of ferroelectrics, the description given is insufficient to provide an adequate guide to selection of pulse magnitudes and timing in concrete terms for generalized cases. For reading out the stored information or clearing the cells before a writing operation, the inventor proposes the application of the full switching voltage -Vs to the selected row or rows, referring to "a manner well known in the art". Selection of the column electrode voltages is treated in a nebulous fashion. It may appear that the selected column electrode is clamped at ground, with all non-selected column electrodes biased to -Vg/S or -Vs/4 (cf. fig. 4B in US patent No. 3 002 182). However, this leads to a voltage load of 2Vs/3 to 3Vs/4 on the non-selected cells in the same row as the selected cell, with obvious danger of partial switching. Thus, it would at best seem that the invention shall be poorly suited for situations where a large number of read operations are involved between each write, and the general applicability to realistic ferroelectric devices appears doubtful. US patent No. 3 859 642 (J. Mar) discloses a memory concept based on a passive matrix addressing scheme, where an array of capacitors with programmable bistable capacitance values is subjected to a two-level excitation during the reading cycle. The memory function resides in the bistability of the capacitors, which are assumed to be of the metal-insulator-semiconductor (MIS) type or equivalent, exhibiting a hysteresis loop which is centered around an offset voltage and well removed from the zero offset point. Writing of data is achieved by biasing the row and column lines crossing at the selected capacitor to polarities +V and -V, respectively, ^ alternatively to -V and +V, respectively, depending on which of the two bistable states is to be written. The resulting net bias is thus 4-2V on the selected capacitor, and does not exceed an absolute magnitude V on non-selected capacitors, where V is defined as being below threshold for writing. Partial writing is apparently not considered to be a problem, and no particular provisions are described in that connection beyond the simple scheme referred here. Thus, the teachings of US patent No. 3 859 642 cannot be seen as having any prior art significance relative to the subject matter of the present invention. A one-third voltage selection scheme for addressing a ferroelectric matrix arrangement is disclosed in US patent No. 4 169 258 (L.E. Tannas, Jr.). In this case, the x- and y lines in a passive matrix addressing arrangement are subjected to a pulsing protocol where (unipolar) voltages with relative magnitudes 0, 1/3, 2/3 and 1 are applied in a coordinated fashion to all x and y lines. Here, voltage value 1 is the nominal voltage amplitude employed for driving a given cell from a logic state "OFF" to "ON", or vice versa, with the typical coercive voltage being exemplified as a value between 1/2 and 2/3. An important limitation of the scheme taught in the patent is that the pulse protocols are predicated upon all cells starting out with the same initial polarization magnitude and direction ("OFF"), i.e. the whole matrix must be blanked to an "OFF" state before a new pattern of states can be written into the matrix cells. Furthermore, any "ON" state on the same y-line as the addressed cell shall receive a disturb pulse of magnitude 2/3 in the direction of the "OFF" state, leading to partial switching in most known ferroelectrics. While these limitations may be acceptable in certain types of displays and memories, this is not the case in the vast majority of applications. Total blanking is not subsumed under what Tannas Jr. terms the conventional method "one-half selection scheme", which is described in detail in the cited US Patent No. 4 169 258. However, the latter scheme exposes the non-selected cells to disturbing pulses of relative value ½. This is generally deemed unacceptable for all practical memory applications employing traditional ferroelectric materials such as inorganic ceramics. Furthermore, the one-half voltage selection scheme is only described in terms of single switching events in the addressed cells, which destroy the pre-switching polarization states. A three level voltage pulsing protocol is disclosed in US patent No. 5 550 770 (Kuroda). This pulsing protocol is intimately linked with an active ferroelectric memory device having a higher level of integration than the usual active ferroelectric matrices with memory cells of the IT-IC type. Kuroda segments the memory device into memory blocks such that all bit lines (or data lines as termed by Kuroda), are connected with a switch element in the form of a field-effect transistor, particularly of the so-called IGFET (insulated gate field-effect transistor) type. The outcome is that Kuroda ends up with a memory matrix with fewer switch elements or transistors linked with the memory cells than is the case of the prior art active memory matrices. All word and bit lines in Kuroda's memory device are before a write or read cycle kept on zero voltage potential. In order to initialize a write or read cycle the transistors must be turned on by applying a voltage level which must be as large as the sum of the polarization switching voltage Vo and the effective threshold voltage of the IGFET, Then Kuroda selects a word line by means of a word line decoder. A single bit line is selected by turning a first switch transistor ON while keeping another switch transistor OFF, these switch transistors being connected between each single bit line and an output line from a bit line decoder. Unselecting a bit line is then done by turning the first transistor OFF and the second transistor ON. For the write and read cycle of the voltage pulsing protocol Kuroda applies a three-level scheme incorporating the so-called one-half voltage selection scheme and claims that what is termed "stress" on unselected word and bit lines in his memory device becomes comparable to the "stress" that occurs in fully active memory matrices, i.e. with memory cells of the IT-IC type. As clearly set forth in Kuroda in col. 17 his voltage pulsing protocol does not appear suitable for passive matrix-addressable ferroelectric memories listed as Prior Art 1 in table 1 in the same column. The higher integration level achieved by the memory device of Kuroda is thus in some degree offset by having to resort to a memory cell selection scheme that first involves the selection of a memory block and then the selection of word lines as known in the prior art, while the selection of bit lines has to resort to a selector device equipped with two switching MOSFETs for every bit line in a block column. This enables Kuroda to employ a three-level protocol with the one-half voltage selection scheme involving a voltage of Vs/2 (Vo/2 in Kuroda) that results in a disturb (stress) level on unaddressed memory cells comparable to that achievable in fully active matrix-addressable memories. It should furthermore be noted that Kuroda does not allow parallel write and read, only bit by bit read and write, as only a single write and a single sense amplifier can be connected in each block column of his memory, although Kuroda of course, offers the possibility of simultaneous write and read of individual memory cells in other memory block segments of his memory matrix. Thus, in passive matrix-addressable memory and display applications where it is desired to be able to change the logic content of individual cells without disturbing other cells or having to blank and reset the whole device, there is a clear need for improvement over the existing prior art. Hence it is a major object of the invention to provide voltage vs. time protocols for driving the x and y passive matrix addressing lines in nonvolatile memories exhibiting ferroelectric-like hysteresis curves so as to minimize disturbance of non-selected memory cells during writing as well as reading of data to/from said memories. It is a further object of the invention to describe voltage protocols that reduce charging/discharging transients and thus to achieve high speed. It is a yet further object of the invention to describe voltage protocols that permit simple, reliable and cheap electronic circuitry to perform drive and sense operations on the memory matrices. The above objects as well as other advantages and features are achieved with a method according to the invention which is characterized by selecting one voltage level having zero value, another voltage level equal to a polarization switching voltage Vs and at least one additional voltage level having a value between 0 and Vs and, in case the voltage pulsing protocol comprises more than three voltage levels, at least another additional voltage level having a value between 0 and Vs, or at least another additional voltage level having a value between 0 and Vs and an additional voltage value having a value larger than Vs, the intervals between succeeding and following voltage levels in the voltage pulsing protocol in any case having the same values; selecting one or more pairs of voltage levels as a pair of active voltage levels such that the potential difference between the voltage levels in said one or more pairs of active voltage levels is Vs or higher; selecting one or more voltage levels as quiescent voltage levels such that at least one quiescent voltage level has a value between 0 and Vs; selecting individual memory cells for an addressing operation in the form of writing data thereto or reading data therefrom inherently in the voltage pulsing protocol by applying each of the voltage levels of a pair of said active voltage levels to respectively a word line and a bit line crossing at the memory cell to be selected; keeping before initializing a write or read cycle all word and bit lines latched to one of said one or more quiescent voltage levels; performing a write operation in the write cycle of said defined timing sequence by latching a word line to a voltage level of a pair of said active voltage levels, and either one or more bit lines to the other voltage level of said pair of active voltage levels or to a quiescent voltage level being as close as possible to the voltage level applied to said word line, thereby activating the word and bit lines to perform the writing operation on a selected memory cell by either setting a definite polarization state in the cell, changing an existing polarization state of the cell, or leaving an existing polarization state of the cell unaltered, said polarization state being predefined as representing data values stored in the memory cells, while inactive word lines and inactive bit lines during the write operation are latched to said at least one quiescent voltage level or, in case more than one quiescent voltage level are used, switched from a quiescent voltage level to another or switched to another voltage level, whereby in any case the difference between said voltage levels shall not exceed Vs; performing a read operation in the read cycle of said defined timing sequence by latching a word line and one or more bit lines respectively to either of the voltage levels of a pair of said active voltage levels and sensing the charge flowing between one or more active bit lines and respectively one or more memory cells connecting with said bit line or bit lines, said charge flow being indicative of a polarization state of respective said one or more memory cells, said polarization state being predefined as representing data values stored in a memory cell, while inactive word lines and inactive bit lines during the read operation are latched to a quiescent voltage level or, in case more than one quiescent voltage level and/or more than one pair of active voltage levels are used, are switched from a quiescent voltage level to another quiescent voltage level or switched to another voltage level, whereby in any case the difference in said voltage levels shall not exceed Vg; and returning after terminating a write or read cycle all word lines and bit lines to a quiescent voltage levels; the selection of voltage levels for active lines according to the voltage pulsing protocol in any case taking place in regard of whether a polarization state of a memory cell shall be set, remain unchanged, or reset in the write operation, while the selection of voltage levels latched to the inactive word and bit lines among quiescent voltages or other voltage levels takes place in the write and read operation in regard of the voltage levels applied to the active word and bit lines in these operations so as to minimize capacitive couplings between active and inactive lines and a possible disturb of unaddressed memory cells. According to the invention it is advantageous allowing one or more bit lines to float in response to charges flowing between a bit line and the cells connecting to the bit line during the read cycle, and latching all voltages on the word and bit lines during the refresh/write cycle. In a first advantageous embodiment of the invention the values n = 3, nWORD "3, and nBIT - 3 are selected in case voltages across non-addressed cells do not significantly exceed Vs/2, where Vs is the voltage across the addressed cell during read, refresh and write cycles. In a second advantageous embodiment of the invention the values n = 4, nWORD = 4, and nBIT = 4 are selected in case voltages across non-addressed cells do not significantly exceed Vs/3, where Vs is the voltage across the addressed cell during read, refresh and write cycles. In a third advantageous embodiment of the invention the values n = 5, nWORD = 3, and nBIT = 3 are selected in case voltages across non-addressed cells do not significantly exceed Vs/3, where Vs is the voltage across the addressed cell during read, refresh and write cycles. It is according to the invention preferred to subject non-addressed cells along an active word line and along active bit line(s) to a maximum voltage during the read/write cycle that deviates by a controlled value from the exact values Vs/2 or Vs/3, and it is then preferable subjecting non-addressed cells along an active word line to a voltage of a magnitude that exceeds the exact values Vs/2 or Vs/3 by a controlled voltage increment, and at the same time subjecting non-addressed cells along selected active bit lines to a voltage of a magnitude that is less than the exact values Vs/2 or Vs/3 by a controlled voltage decrement, the controlled voltage increment and voltage decrement preferably being equal to each other. It is according to the invention advantageous adding a controlled voltage increment δ1 to potentials inactiveWL of inactive word lines and adding a controlled voltage increment 82 to potentials OinactivcBL of inactive bit lines, where 5i = 82 = 0 corresponds to the voltage pulsing protocols with maximum Vs/2 or Vs/3 voltage exposure on non-selected cells. In this connection is preferably 5i = 82 ?^ 0. It is according to the invention considered advantageous controlling a quiescent potential (the potential imposed on the word and bit lines during the time between each time the voltage pulsing protocol is employed) to have the same value on all word and bit lines, i.e. a zero voltage is imposed on all cells. Further it is according to the invention considered advantageous selecting the quiescent potentials on one or more of the word and bit lines among one of the following: a) System ground, b) Addressed word line at initiation of pulsing protocol, c) Addressed bit line at initiation of pulsing protocol, d) Power supply voltage (Vcc) • It is also according to the invention considered advantageous selecting the potential on a selected bit line or bit lines in a quiescent state such that it differs from that at the onset of a floating period (read cycle), and bringing said potential from a quiescent value to that at the onset of the floating period, where it is latched for a period of time comparable to or exceeding a time constant for charging the bit line ("pre-charge pulse"). According to the invention it is considered advantageous preceding the read cycle with a voltage shift on inactive word lines, whereby the non-addressed cells on an active bit line are subjected to a voltage bias equal to that occurring due to the active bit line voltage shift during the read cycle, said voltage shift on the inactive word lines starting at a selected time preceding said voltage shift on the active bit line, and terminating at the time when the latter voltage shift is initiated, in such a way that a perceived voltage bias on said non-addressed cells on the active bit line is continuously applied from the time of initiation of said voltage shift on the inactive word lines and up to the time of termination of said voltage shift on the active bit line ("pre-charge pulse"). Finally it is according to the invention considered advantageous applying a pre-read reference cycle which precedes the read cycle and is separated from it by a selected time, and which mimics precisely the voltage pulsing protocol and current detection of said read cycle, with the exception that no voltage shift is imposed on an active word line during the pre-read reference cycle, and employing a signal recorded during the pre-read reference cycle as input data to the circuitry that determines the logic state or a data value of the addressed cell, in which case the signal recorded during the pre-read reference cycle may be subtracted from a signal recording during the read cycle. The basic principles of the invention and exemplary embodiments shall now be described below and with reference to the appended drawing figures, wherein fig. 1 shows a principle drawing of a hysteresis curve for a ferroelectric memory material, fig. 2 a principle drawing of a passive matrix addressing arrangement with crossing electrode lines, and cells containing a ferroelectric material localized between these electrodes where they overlap. fig. 3 the sum of voltage steps around a closed loop in the matrix. fig. 4 a read and write voltage protocol requiring three separate voltage levels to be controlled on the word- and bit lines, fig. 5 an alternative variant of the three level voltage protocol in fig. 4, fig. 6 a read and write voltage protocol requiring four separate voltage levels to be controlled on the word- and bit lines, fig. 7 an alternative variant of the four level voltage protocol in fig. 6, fig. 8 a read and write voltage protocol requiring five separate voltage levels to be controlled on the word- and bit lines, fig. 9 an alternative variant of the five level voltage protocol in fig. 8, figs, 10-13 alternative voltage protocols to those shown in figs. 6-9, the difference being that pre-charging pulses on inactive word lines are now included, fig. 14 an example of a read and write protocol involving a pre-read reference cycle, and fig. 15 a readout scheme based on full row parallel detection. The general background and the basic principles of the present invention shall now be discussed in some detail. An essential aspect of the present invention is to control the time-dependent voltages on all the x and y lines in the matrix in a coordinated fashion according to one of the protocols described hereinafter. These protocols ensure that no non-addressed cell (crossing point) in the matrix experiences an interline voltage exceeding a predetermined value which is well below a level at which disturbance or partial switching occurs. It is understood that the materials constituting the memory function in displays and memory devices as per the instant invention exhibit hysteresis as exemplified in a generic fashion in fig. 1. Relevant materials are electrets, ferroelectrics or a combination of the two. For simplicity, it shall be assumed in the following that the material in question is a ferroelectric, but this shall not restrict the generality of the present invention. As a consequence of prior exposure to electric fields, the material is assumed to reside in one of two polarization states when in zero external field, represented by the points +PR and -PR in fig.l. Application of a voltage across the cell containing the ferroelectric causes the latter to change its polarization state, tracing the hysteresis curve in a manner well known to the person skilled in the art of ferroelectrics. For convenience, the hysteresis curve in fig.l is shown with the voltage rather than the field along the abscissa axis. Below shall be described how, in a passive matrix configuration, voltages can be applied to the crossing word- and bit lines in such a fashion that a single, freely chosen cell in the matrix experiences a potential difference Vs between the two electrodes crossing at that point which has sufficient magnitude to cause the ferroelectric to switch its polarization direction in either positive or negative direction (depending on the polarity of the applied field between the electrodes) and ending up at one of the points +PR or -PR on the hysteresis curve after removal of the externally imposed field. At the same time, no other cell in the matrix shall be subjected to a potential difference that causes an unacceptable (according to prior defined criteria) change in the polarization state. This is ensured by the potential difference across non-addressed cells (the "disturbing voltage") never exceeding + Vs/n, where n is an integer or non-integer number of typical value of 2 or more. Depending on the required switching speed, etc, the nominal switching voltage Vs employed for driving the polarization state of the ferroelectric is typically selected considerably larger than the coercive voltage Vc (cf fig.l). However, it cannot be chosen arbitrarily large, since the pulsing protocols described here shall only reduce the disturbing voltage to a certain fraction (typically 1/3) of Vs, which level should be less than VQ. Before proceeding to a discussion of specific pulsing protocols, it may be useful to review the problem in a generalized fashion, with reference to the matrix shown in fig. 2. For easy reference and to conform with standard usage, it is henceforth referred to the horizontal (row) and vertical (column) lines as "word lines" (abbreviated: WL) and " bit lines" (abbreviated: BL), respectively, as indicated in the figure. It is desired to apply a voltage that is sufficiently high to switch a given cell, either for defining a given polarization direction in that cell (writing), or for monitoring the discharge response (reading). Accordingly, the cell is selected by setting the potentials of the associated word and bit lines (the "active" lines) such that: At the same time, the numerous word- and bit lines that cross at non-addressed cells must be controlled in potential such that the disturbing voltages at these cells are kept below the threshold for partial switching. Each of these "inactive" word- and bit lines cross the active bit- and word line at a non-addressed cell. Referring to fig. 2, one notes that four distinct classes of cells can be defined in the matrix, according to the perceived voltages across the cells: (charge/discharge currents) and reducing the complexity of the driving circuitry, resulting in pulsing protocols such as those described below. One example is an overall shift in potentials by adding or subtracting the same voltage to all four levels. potential). As was mentioned above, partial switching may represent a serious problem at voltage levels of Vs/2, rendering three-level protocols unacceptable. However, the degree of partial switching at a given applied voltage depends explicitly on the ferroelectric material in question. Referring to fig.l, materials with a square shaped hysteresis curves shall in many applications yield acceptable performance. Recently, certain classes of ferroelectrics such as organic polymers have received much attention as memory substances in advanced data storage concepts. In addition to other attractive features, theses materials exhibit hysteresis curves far more square shaped than those of the ceramic ferroelectrics that have traditionally dominated developments in the field of ferroelectric-based non-volatile memory devices. Thus, it has become relevant to define pulsing protocols which can satisfy the requirements of realistic and optimized electronic device designs. Following upon the partial switching problems that discouraged development and exploitation of early efforts based on three-level switching protocols, these aspects have received very little attention, which the present invention sets out to remedy. Now examples of preferred embodiments shall be given. Figs. 4 and 5 illustrate some three-level pulsing protocols according to the present invention, comprising a complete read cycle and a refresh/write cycle. Only the pulse diagrams for the active word- and bit lines are shown. The inactive word lines nnay be kept stable at Vs/2 throughout the read/write cycle, as may the inactive bit lines. Alternatively, the latter may during the read cycle each be connected with a separate sense amplifier, which would be biased near the bit line voltage when the bit line clamp is released (full row readout). In the diagrams shown in figs. 4 and 5, the time markers are as follows: to: Word line latched, active pulldown to 0 (fig.4) or pullup to Vs (fig. 5) ti : Bit line clamp released - sense amplifier ON t2 : Bit line decision - data latched t3 : Word line returned to quiescent Vs/2 t4 : Write data latched on bit lines t5 : Word line pulled to Vs (fig. 4) or zero (fig. 5) - set/reset capacitors t6 : Word line returned to quiescent Vs/2 ty : Bit lines actively returned to Vs (fig. 4) or zero (fig. 5) clamp tg : Read/write cycle complete The read cycle investigates the state of the polarization of the addressed cell. Depending on the polarization direction, the read operation may leave the * polarization unchanged, or it may reverse the polarization direction (destructive read). In the latter case, the information must be refreshed if it is desired avoid loss of stored data. This implies that the polarization must be driven in the opposite direction of the read operation in an appropriate cell (not necessarily the one that was read) somewhere in the matrix. This is achieved by the part of the protocol dedicated to refresh/write, as shown. The two branches in the bit line voltage protocol correspond to the cases where the polarization is left unchanged and reversed, respectively. An isolated write operation is trivially achieved by omitting the preceding read operation. As shown in figs, 4 and 5, it is clear that non-addressed cells shall not receive voltages exceeding ½ of the nominal switching voltage, neither during reading or refresh/writing periods. In addition, one notes that there are included event delays in the pulsing sequence to facilitate transient ring-down and latching of data. Depending on how the memory device is to be operated, the bit line potential in the quiescent state (i.e. between read/refresh/write cycles) may be chosen to match that of the bit line at the start of the read cycle (cf. figs. 4 and 5) or it may match the quiescent potential of the word line (not shown here). In the former case, appropriate when cycling is intense and at high speed, charging currents at the start of the read cycle are minimized. In the latter case, long-term effects of an imposed field in the cells (e.g. imprint) are avoided. It should be clear that the examples shown in figs. 4 and 5 may be modified (e.g. by concurrent shifting of all potentials, or by minor departures from exact voltage levels in the three-level scheme shown) without departing from the essential principles illustrated therein. Example 2: Four-level (Vs/3) switching protocol As described above, by employing at least 4 different potential levels on the word and bit lines, one can ensure that no non-addressed cell experiences a voltage exceeding 1/3 of the nominal switching voltage. Figs. 6 and 7 illustrate two variants of a preferred scheme for reading as well as refreshing/writing data, according to the present invention. Here, the time markers are as follows: Apart from the increased voltage level complexity, the basic features are similar to those referred above in connection with the three level schemes. Now, however, no non-addressed cell is exposed to a voltage exceeding Vs/3 in the course of a complete read/write cycle, which shall cause only minor partial switching in most ferroelectric materials of relevance here. Again, several variants on a common theme are possible. Thus, figs. 6 and 7 show a return to zero applied voltage across all cells in the quiescent state (cf. the above discussion under the three-level switching protocol), which corresponds to word and bit line potentials of 2Vs /3 or Vs /3, whereas other potential levels on the word- and bit lines are possible in the quiescent state that either yield zero voltages across the cells or voltages of absolute value <_lvs such variants shall be assumed obvious to the skilled person and not pursued in further detail here.> The timing diagrams in figs. 6 and 7 are equivalent in principle, one being an "inverted" version of the other. In practice, however, one may be preferred over the other. Thus, the scheme shown in fig.6, implies a voltage at the sense amplifier input during the read cycle near Vs . In the scheme of fig.7, however, the voltage is near zero. This may permit the use of low voltage components with a single high voltage pass transistor per bit line. Example 3: Five-level (Vs/3) switching protocol A class of seemingly more complex, but in certain respects more simply implemented pulsing protocols involve the application of five different potential levels to the word- and bit lines during a complete read/write cycle. Explicit examples of two preferred embodiments are shown in figs. 8 and 9. The time markers are as follows: Here, a fifth voltage level, Vcc, is involved. It is typically of magnitude 4Vs/3, and is applied to the active word line during the reading (fig. 9) or refresh/write (fig. 8) cycle. One notes that while the four-level schemes in figs. 6 and 7 require all word and bit lines to be driven at four levels in the course of the complete read/write cycle, the five-level schemes in figs, 8 and 9 require only three separate voltage levels to be applied to the word lines and three separate but not identical voltage levels to be applied to the bit lines. This provides opportunities for optimization and simplification of the driving and sensing electronics supporting the device. Further simplification can be realized by choosing 4Vs/3 = Vcc close to the power supply voltage. Example 4: Switching protocols involving pre-charging of non-addressed cells on active bit lines So far, primary focus has been on avoiding partial switching of non-addressed cells. However, it is also desirable to design switching protocols that simultaneously minimize the effect of parasitic current flows within the memory matrix during the read cycle: In memory matrices based on passive matrix addressing, the area data storage density is maximized by using matrices that are as large as possible. This implies that each matrix should contain the largest possible number of crossing points between word and bit lines, and any given bit line must consequently cross a large number of word lines. When a given word and bit line crossing is selected, the large number of non-selected crossing points between the bit line and all of the non-selected crossing word lines constitute a correspondingly large number of parasitic current leakage paths (capacitive, inductive, ohmic) which may add up to slow down the device and reduce the contrast ratio of as-read logic "T's and "0"s. One method of reducing the effect of parasitic currents on the determination of logic states is to pre-charge the non-addressed cells on the active bit line to a level corresponding to that which would be approached during the reading of the active cell. This procedure is implicit in the voltage protocols shown in figs. 6-9. At time point 2, i.e. prior to applying the read voltage step to the active word line (at time point 3 in the figures) the active bit line voltage is shifted to its read cycle value, creating a voltage bias between the active bit line and all word lines. This initiates the spurious current flows in all the non-active cells on the active bit line. These currents are typically transient, reflecting polarization phenomena in the cells, and die out or are greatly diminished after a short time. Thus, by making the time gap between time points 2 and 3 sufficiently long, the spurious current contributions to the switching currents sensed during the reading cycle are greatly diminished. Certain limitations adhere to this scheme: If the time gap between time points 2 and 3 becomes very long, it has obvious implications on the data access speed and overall read cycle time. Additionally, the cumulative effect of repeated cycling with long pre-charging times may be to cause partial switching and imprint, which was sought avoided by haying zero voltage across all cells in the quiescent state. The voltage protocol diagrams in figs. 6-13 do not show the sense amplifier timing, which may vary from case to case, depending upon the dynamics of the polarization switching and spurious current response in the addressed and in the non-addressed cells. The sense amplifiers must be activated after time point 2 to avoid the spurious current transient from the non-addressed cells, and not much later than time point 3 in order to capture any polarization reversal current in active cells that are switched by the read cycle. One notes that by advancing the time point 2 well ahead of time point 3, not only the inactive cells on the active bit line are subjected to an early voltage bias of magnitude I Vs/3 1, but also the active cell. Thus, some of the switching charge in the active cell is drained away before the sense amplifier has been connected. The magnitude of this effect, which is undesirable since it reduces the read signal, depends on the polarization characteristics of the memory material in the cells and may range from negligible to significant. In the latter case, one may implement a slight modification of the voltage protocol by introducing a voltage shift on the inactive word lines as illustrated in figs. 10-13. The leading edge of the shift occurs at time point 0, and the trailing edge coincides with the leading edge of the active bit line voltage shift at time point 2. By precisely controlling the trailing and leading edge shifts at time point 2, the voltage across the non-addressed cells on the active bit line shall rise from zero to a magnitude I Vs/3 | at time point 0 and remain unchanged at this value until time point 5, i.e. after completion of the read cycle. The time point 2 may now be optimized for the readout process in the active cell, without limitations relating to driving the pre-charge transient in the non-addressed cells. As can be seen from figs. 10-13, the yoltage across non-addressed cells is always maintained at less than a magnitude I Vs/3 I in these modified schemes, but 4 voltage levels are now involved on the word lines in the five-level protocols, compared to three levels previously. Example 5: Switching protocols involving a reference pre-read cycle Another scheme for circumventing or alleviating the problems relating to parasitic currents in non-addressed cells on active bit lines shall now be described. For concreteness, refer to, e.g. the four-level timing diagram shown in fig. 6. The pre-charge scheme described in the above paragraphs implies that the active bit line has been shifted to its read cycle value at time point 2, and ensuing parasitic currents have been significantly reduced by the time the active word line is switched at time point 3. The logic state in the addressed cell is determined by the sense amplifier which records the charge flowing to the bit line during a defined time interval that starts near the time point 3 and stops before the time point 4. Ideally, such pre-charge schemes shall enable detection of the charge flowing in response to the shifting of the active word line at time point 3, without interference from parasitic currents through cells at inactive word lines. In practice, the parasitic currents may die down slowly and/or have an ohmic (i.e. non-transitory) component such that some parasitic charge is captured by the sense amplifier. Although the magnitude of the parasitic current component flowing through each non-addressed cell on the active bit line may be small, the currents from hundreds or thousands of non-addressed cells on the active bit line may add up to become very significant, corrupting the readout results. Assuming stable and predictable conditions, such a parasitic contribution may in principle be removed by subtracting a fixed amount of charge from that recorded by the sense amplifier during the reading cycle. In many instances, however, the magnitude and variability of the parasitic contribution makes this inappropriate. Thus, in addition to the manufacturing tolerances for the device, the fatigue and imprint history may vary within wide limits between different cells in the same memory device and even on the same bit line, and the parasitic current may depend strongly upon the device temperature at the time of read-out. In addition, the parasitic current associated with a given non-addressed cell on the active bit line may depend on which logic state it is in. In that case the cumulative parasitic current from all non-addressed cells on the active bit line shall depend on the set of data stored in those cells, which defies prediction. In order to obtain a true measure of the cumulative parasitic currents in connection with a given read-out event, one may implement a pre-read reference cycle as exemplified in fig. 14. The pre-read cycle immediately precedes the read-out cycle and differs from the latter in only one respect, namely that the active word line is not shifted at all. The sense amplifier is activated in precisely the same time slot relative to the bit line voltage shifts as is the case in the subsequent read cycle. Thus, the cumulative charge detected during the pre-read cycle shall correspond very closely to the parasitic current contributions captured during the read cycle, including contributions from the active cell. The detected charge from the pre-read cycle is stored and subtracted from that recorded during the read cycle, yielding the desired net charge from the switching or non-switching transient in the active cell. Clearly, the effects of fatigue, imprint, temperature and logic states are automatically taken care of by this referencing scheme. An important prerequisite is that the pre-read cycle must not materially alter the parasitic current levels in the read cycle. Thus, the delay between time points P6 and 0 (cf. fig. 14) must be sufficient for pre-read cycle transients to die down. In certain cases, two or more successive pre-read cycles may be employed to obtain a reproducible parasitic current response prior to the read cycle. However, this increases complexity and total readout time. Inspection of fig. 14 in conjunction with the four level pulse protocol shown in fig. 6 shows how the pre-read reference cycle principle may be implemented for the other pulse protocols covered by the present invention, by trivial extension of the example given in the present instance. Example 6: Switching protocols involving offset voltages Yet another scheme for circumventing or alleviating the problems relating to parasitic currents in non-addressed cells on active bit lines shall now be described. According to Equation (2) above, the minimum disturbing voltage on non-addressed cells is Vs/3 (cf Equation (3)) and the preferred embodiments described in conjunction with the four- and five-level switching protocols were shown to achieve this. As will be discussed below, it may in certain instances be preferable to deviate somewhat from this criterion. Given that the memory cells exhibit certain characteristics regarding their electrical impedance and switching properties, it is possible to achieve a low parasitic current load on the bit line during read operations, while at the same time keeping disturbance of the non-addressed cells at a low level. The magnitude of 5 must be selected with due consideration to two conflicting requirements: On the one hand, it should be as large as possible in order to minimize parasitic current contributions to the active bit line. On the other hand, it should be as small as possible in order to minimize the disturbance of non-addressed cells. In practice, a decision must be made based on the specific conditions prevailing in each case. Furthermore it is well-known to persons skilled in the art that the electrically polarizable materials used as the storage or memory medium in displays and memories can have a non-linear voltage-current response characteristic which may be exploited with advantage when implementing switching protocols involving offset voltages. Such non-linear response characteristic may however, also be dependent on the specific material and its treatment and factors which in the present context may depend on the pulsing protocol parameters actually used as well as design and scale factors. This implies that it will be impossible to generalize about a beneficial exploitation of non-linear voltage-current response in non-addressed cells, but that any specific embodiment involving this kind of response must be subject to the heuristics as applicable in each case. However, any heuristics of this kind shall be considered to lie outside the scope of the present application. Example 7: Full row readout An alternative route to reducing or eliminating the spurious current contributions from non-addressed cells along active bit lines during readout is illustrated in fig. 15. All word lines except the active one are clamped at a potential close to that at the sense amplifier input (defined as zero in fig. 15). For readout of data, the active word line is brought to the potential VREAD, which causes currents to flow through the cells on the crossing bit lines. The magnitudes of the currents depend on the polarization state in each cell and are determined by the sense amplifiers, one for each bit line as shown. This scheme provides several advantages: - Voltages across all non-addressed cells are very close to zero, eliminating leakage currents that may otherwise corrupt the readout from the addressed cells. - The readout voltage VREAD May be chosen much higher than the coercive voltage without incurring partial switching in non-addressed cells. This allows for film switching speeds approaching the intrinsic switching speed of the polarizable material in the cells. - The scheme is compatible with large matrix arrays. - The high degree of parallelism makes possible a large data readout rate. Since the readout is destructive, it shall in many cases be necessary to write data back into the memory device. This can be achieved by one of the pulsing schemes described in the previous paragraphs. A different set of cells in the memory device from those that were read may be chosen for refresh, e.g. in conjunction with caching. Possible disadvantages of this scheme are largely related to the increased demands on the circuitry performing the driving and sensing functions. Thus, the simultaneous switching of all cells on a lone word line shall cause a large current surge on that line (implies a need for low source impedance in the driver stage and low impedance current paths. Also potential for cross-talk within the device). Furthermore, in order to avoid loss of data a separate sense amplifier is needed on each bit line. With the highest possible density of cells in the passive matrix, this poses a crowding problem at the edge of the matrix where the sense amplifiers are connected. * The switching protocols described above make possible the controlled switching of polarization direction of any given cell in a passive matrix arrangement, without subjecting non-addressed cells to disturbing voltages that exceed «Vs/3. As described in the examples above, the pulsing protocols are directly applicable to the reading of logic states in memory cells that either experience no polarization switching during the read cycle, defined as being in e.g. a logic "0", or switch the direction of the polarization, correspondingly defined as being in a logic "1". Initialization of the memory could involve the writing of O's in all cells, which in the case above would imply performing a read pulse cycle (destructive read). Writing would then be achieved by applying the pulse sequence for changing the polarization in those cells that shall store a logical "1" while leaving the rest of the cells unchanged. Subsequent reading of data from the memory would then require a refresh cycle to be implemented in those cases where it is desired to retain data in the memory following the destructive read. The refresh protocol would require a complete read/refresh pulse sequence in cases where other cells are used for renewed storage than those that were read destructively to provide the data. On the other hand, if the same cells are used, those cells that were read as logic "0" can be left unchanged and only those that contained a "1" need to be exposed to polarization switching. WE CLAIM : 1. A method of driving a passive matrix-addressable display or memory array of cells comprising an electrically polarizable material exhibiting hysteresis, in particular a ferroelectric material, wherein the polarization state of individual, separately selectable cells can be switched to a desired condition by application of electric potentials or voltages to word and bit lines forming an addressing matrix, and wherein the method comprises establishing a voltage pulsing protocol with n voltage or potential levels, n > 3, such that the voltage pulsing protocol defines a timing sequence for individually controlling the voltage levels applied to word and bit lines of the matrix in a time-coordinated fashion, arranging said timing sequence to encompass at least two distinct parts, including a "read cycle" during which charges flowing between said selected bit line(s) and the cells connecting to said bit line(s) are sensed, and a "refresh/write cycle" during which polarization state(s) in cells connecting with selected word and bit lines are brought to correspond with a set of predetermined logical states or data values, and wherein the method is characterized by selecting one voltage level having zero value, another voltage level equal to a polarization switching voltage Vs and at least one additional voltage level having a value between 0 and Vs and, in case the voltage pulsing protocol comprises more than three voltage levels, at least another additional voltage level having a value between 0 and Vs, or at least another additional voltage level having a value between 0 and Vs and an additional voltage value having a value larger than Vs, the intervals between succeeding and following voltage levels in the voltage pulsing protocol in any case having the same values; selecting one or more pairs of voltage levels as a pair of active voltage levels such that the potential difference between the voltage levels in said one or more pairs of active voltage levels is Vs or higher; selecting one or more voltage levels as quiescent voltage levels such that at least one quiescent voltage level has a value between 0 and Vs; selecting individual memory cells for an addressing operation in the form of writing data thereto or reading data therefrom inherently in the voltage pulsing protocol by applying each of the voltage levels of a pair of said active voltage levels to respectively a word line and a bit line crossing at the keeping before initializing a write or read cycle all word and bit lines latched to one of said one or more quiescent voltage levels; performing a write operation in the write cycle of said defined timing sequence by latching a word line to a voltage level of a pair of said active voltage levels, and either one or more bit lines to the other voltage level of said pair of active voltage levels or to a quiescent voltage level being as close as possible to the voltage level applied to said word line; performing a read operation in the read cycle of said defined timing sequence by latching a word line and one or more bit lines respectively to either of the voltage levels of a pair of said active voltage levels and sensing the charge flowing between one or more active bit lines and respectively one or more memory cells connecting with said bit line or bit lines, said charge flow being indicative of a polarization state of respective said one or more memory cells, and said polarization state being predefined as representing data values stored in a memory cell, and returning after terminating a write or read cycle all word lines and bit lines to a quiescent voltage level, 2. A method according to claim 1, characterized by activating the word and bit lines to perform the writing operation on a selected memory cell by either setting a definite polarization state in the cell, changing an existing polarization state of the cell, or leaving an existing polarization state of the cell unaltered, said polarization state being predefined as representing data values stored in the memory cells, while inactive word lines and inactive bit lines during the write operation are latched to said at least one quiescent voltage level or, in case more than one quiescent voltage level are used, switched from a quiescent voltage level to another or switched to another voltage level, whereby in any case the difference between said voltage levels shall not exceed Vs- 3. A method according to claim 1, characterized by latching during a read operation inactive word lines and inactive bit lines to a quiescent voltage level or, in case more than one quiescent voltage level and/or more than one pair of active voltage levels are used, switching said word and bit lines from a quiescent voltage level to another quiescent voltage level or to another voltage level, whereby in any case the difference in said voltage levels shall not exceed Vs. 4. A method according to claim 1, characterized by selectmg m any case voltage levels for active lines according to the voltage pulsing protocol in regard of whether a polarization state of a memory cell shall be set, remain unchanged, or reset in the write operation. 5. A method according to claim 1, characterized by selecting the voltage levels latched to the inactive word and bit lines among quiescent voltages or other voltage levels in the write and read operation in regard of the voltage levels applied to the active word and bit lines in these operations so as to minimize capacitive couplings between active and inactive lines and a possible disturb of unaddressed memory cells. 6. A method according to claim 1, characterized by allowing one or more bit lines to float in response to charges flowing between a bit line and the cells connecting to the bit line during the read cycle, and latching all voltages on the word and bit lines during the refresh/write cycle. 7. A method according to claim 1, characterized by selecting the values n = 3 and nWORD = 3 and nBIT = 3, in case voltages across non-addressed cells do not significantly exceed Vs/2, where Vs is the voltage across the addressed cell during read, refresh and write cycles. 8. A method according to claim 1, characterized by selecting the values n = 4 and UWORD ^ 4 and neu ^ 4, in case voltages across non-addressed cells do not significantly exceed Vs/3, where Vs is the voltage across the addressed cell during read, refresh and write cycles. 9. A method according to claim 1, characterized by selecting the values n = 5 and UWORD = 3 and UBIT = 3, in case voltages across non-addressed cells do not significantly exceed Vs/3, where Vs is the voltage across the addressed cell during read, refresh and write cycles. 10. A method according to claim 1, characterized by subjecting non-addressed cells along an active word line and along active bit line(s) to a maximum voltage during a read and write cycle that deviates by a controlled value from the exact values Vs/2 or Vs/3, 11. A method according to claim 6, characterized by subjecting non-addressed cells along an active word line to a voltage of a magnitude that exceeds the exact values Vs/2 or Vs/3 by a controlled voltage increment, and at the same time subjecting non-addressed cells along selected active bit lines to a voltage of a magnitude that is less than the exact values Vs/2 or Vs/3 by a controlled voltage decrement. 12. A method according to claim 7, characterized by the controlled voltage increment and voltage decrement being equal to each other. 13. A method according to claim 1, characterized by adding a controlled voltage increment 51 to potentials CD inactiveWL of inactive word lines and adding a controlled voltage increment 52 to potentials OjnactiveBL of inactive bit lines, where 51 - 52 = 0 corresponds to voltage pulsing protocols with maximum Vs/2 or Vs/3 voltage exposure on non-selected cells. 14. A method according to claim 13, characterized by 51 = δ2 # 0. 15. A method according to claim 1, characterized by controlling a quiescent potential (the potential imposed on the word and bit lines during the time between each time the voltage pulsing protocol is employed) to have the same value on all word- and bit lines, i.e. a zero voltage is imposed on all cells. 16. A method according to claim 1, characterized by selecting quiescent potentials on one or more of the word-and bit lines among one of the following: a) System ground, b) Addressed word line at initiation of pulsing protocol, c) Addressed bit line at initiation of pulsing protocol, d) Power supply voltage (Vcc)- 17. A method according to claim 1, characterized by selecting the potential on a selected bit line or bit lines in a quiescent state such that it differs from that at the onset of a floating period (read cycle), and by said potential being brought from a quiescent value to that at the onset of the floating period, where it is latched for a period of time comparable to or exceeding a time constant for charging the bit line or bit lines ("pre-charge pulse"). 18. A method according to claim 1, characterized by preceding the read cycle with a voltage shift on inactive word lines, whereby the non-addressed cells on an active bit line are subjected to a voltage bias equal to that occurring due to the active bit line voltage shift during the read cycle, said voltage shift on the inactive word lines starting at a selected time preceding said voltage shift on the active bit line, and terminating at the time when the latter voltage shift is initiated, in such a way that a perceived voltage bias on said non-addressed cells on the active bit line is continuously applied from the time of initiation of said voltage shift on the inactive word lines and up to the time of termination of said voltage shift on the active bit line ("pre-charge pulse"). 19. A method according to claim 1, characterized by applying a pre-read reference cycle which precedes the read cycle and is separated from it by a selected time, and which mimics precisely the voltage pulsing protocol and current detection of said read cycle, with the exception that no voltage shift is imposed on an active word line during the pre-read reference cycle, and by employing a signal recorded during the pre-read reference cycle as input data to circuitry that determines the logic state or a data value of an addressed cell. 20. A method according to claim 19, characterized by the signal recorded during the pre-read reference cycle being subtracted from a signal recorded during the read cycle. 21• A. method of driving a passive matrix-addressable display or memory array of cells, substantially as herein described with reference to the sccompanying drawings.

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