Abstract:
A device such as a smart card ( 100 ) capable of displaying data on a display panel ( 122 ). The panel ( 122 ) comprises an array of segments ( 130 ) coupled to a battery ( 124 ) for activating any or several of the segments. Microprocessor ( 114 ) is employed for storing and processing data. Switch ( 110 ) is operable by the user. Driver ( 120 ) includes electronic switches ( 220; 222 ) for activating a selection of the segments in accordance with the processed data to display segmented symbols on the panel ( 122 ). In order to extend battery life while keeping long-lasting quality performance, the polarity of the voltage applied to the segments is reversed in succession with respect to the previous activations.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to display panels, and particularly to a method and apparatus of activation thereof. More specifically, the invention concerns visual display panels typically though not exclusively, liquid crystal based display (LCD) panels. The terms “LC” or “LCD” shall therefore be used throughout the present specification to include as well other types of display devices such as polymer dispersed liquid crystal (PDLC) displays, electro-phoretic display, electro-chromic displays, etc., as will become apparent in view of the description below. Still more specifically the invention is focused on the implementation of display panels in smart cards. 
       BACKGROUND OF THE INVENTION 
       [0002]    For reasons associated with the basic physics of LCD, it has been accepted as common practice to activate the LC individual segments, combinations of which compose an eligible alpha-numeric (characters or digits), or other symbols, by continued alternating (+) and (−) voltage pulses, typically square waves at frequency of 30-90 Hz (cycles per second). It has been experienced that driving the segments otherwise, namely by non-alternate pulses, or pulses at frequencies other than specified, will cause meaningful deterioration of the display performance over time, such as decreased contrast. 
         [0003]    It is further known that voltage alternating frequency should preferably be higher than a human eye refresh rate, in order to prevent flickering of the display image during display activation. Therefore, LCD voltage is commonly alternating during display operation at rates higher than 25 Hz (cycles per second) 
         [0004]    Alternate voltage (or current) feeding is relatively high energy consumption. Hence, for devices powered by small size batteries the use of LCD panels has been heretofore ruled-out. 
       OBJECTS OF THE INVENTION 
       [0005]    It is therefore the major object of the invention to provide a method of electrical driving of panels suitable for use in devices operated by low capacity batteries. 
         [0006]    It is a further object of the invention to activate LCD segments by non-alternate pulses during their operational cycles. 
         [0007]    It is a still further object of the invention to provide a driver circuit adapted to reverse the polarity of the voltage pulses applied to any given segment after every operational cycle thereof, thereby discharging any residual capacitance accumulated during the previous cycle. 
         [0008]    It is a still further object to apply the present invention to what is known as “Smart Cards” namely, credit-card-like, data processing devices operable as OTP, e-purses, active identification cards and, others for applications as known in the art. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the invention there is provided a method of operating liquid crystal (as hereinbefore defined) display panels, comprising an array of segments drivingly coupled to DC power source, characterized in that the panel is intermittently activated for displaying information by applying a DC voltage pulse to a selection of segments comprised in said panel for given length of time and of a given polarity [(+) or (−)], and wherein during the next activation of the same selection of segments a similar DC voltage is applied but of a reverse polarity. 
         [0010]    According to another aspect of the invention there is provided a device, such as “Smart Card”, comprising means for storing data, means for processing the stored data, electric power supply means selectively operable by a user, means for deactivating the power source after a pre-set period of time, a display panel, comprised of segments, and means for activating selected segments in accordance with the processed data to display segmented symbols by said panel, characterized by means for reversing the polarity [(+) or (−)] of the activating means in succession with respect to every cycle of activation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and additional constructional features and advantages of the invention will be more readily understood in the light of the ensuing description of a preferred embodiment thereof, given by way of example only, with reference to the accompanying drawings, wherein:— 
           [0012]      FIG. 1  is a schematic representation of a smart card constructed and operative in accordance with an embodiment of the present invention; 
           [0013]      FIG. 2  is a block diagram of the smart card of  FIG. 1  after a first given activation thereof; 
           [0014]      FIG. 3  is a block diagram of the smart card of  FIG. 2  after an activation subsequent to the activation of  FIG. 2 ; 
           [0015]      FIG. 4  is a voltage vs. time diagram of the operational cycles. 
           [0016]      FIG. 5  is a general block diagram of a driver according to a preferred embodiment of the invention; 
           [0017]      FIG. 6  is schematic representation of the switching array of  FIG. 5  operatively connected to a group of segments; and 
           [0018]      FIGS. 7   a  and  7   b  are schematic representations of positive and negative polarity activation modes of the segments of  FIG. 6 , respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    As shown in  FIG. 1  the smart card generally denoted  100  is of a regular plastic credit card size and shape, normally 1.2  mm  or less thick. It comprises a display panel  122 , an embedded operating switch  110  and, optionally, smart-card chip  116  for downloading data from external sources. 
         [0020]    Referring to  FIG. 2 , a user wishing to obtain, e.g. alpha-numeric data through the display panel  122 , may operate switch  110 , thereby a signal is transmitted to a microprocessor  114 , which preferably stores data that was previously retrieved during connection of the smart card chip  116  to an external reader or an ATM machine, or any other suitable sources. Alternatively, microprocessor  114  may retrieve data stored in the smart card chip  116  upon pressing on switch  110 . 
         [0021]    The microprocessor  114  communicates with a display driver  120  (details of which are given below with reference to  FIGS. 5-7 ) which, in turn, communicates with the display panel  122  for displaying the requested or available information. Such information may commonly be presented by a digital alpha-numerical display but may also be presented by a cartographical, matrix or any other pictorial display depending of the application. 
         [0022]    In the example of  FIG. 2 , the data displayed on display panel  122  is an authentication number of a user. Any other data may be displayed, such as a telephone number, an identification number, banking information and bank account data, or a one-time password (OTP) for card holding authentication in telephone or internet credit card transactions. Different types of information and data may be provoked by successive pressing the switch  110 . This data may be used, for example, for an ill patient suffering from a disease involving memory loss, such as Alzheimer. The data may be used to aid an ill patient, in times of memory loss, in retrieving important personal information. 
         [0023]    In the case of a banking smart card, the display may present financial information, such as an account balance, the last transaction, or any other suitable information. It is appreciated that various types of information may be visualized alternately on display panel  122 , through successive pressing actions on switch  110  and/or by employing more than one switch. 
         [0024]    It is appreciated that the smart card  100  may be used for any suitable purpose, besides storing financial data and medical data, or generating OTP. 
         [0025]    Power is supplied to the smart card  100  via a battery  124  mostly of the dry-cell type, which is preferably embedded in the smart card  100 . A power booster  126  may be utilized in conjugation with battery  124 , to increase or decrease the battery voltage, thereby supplying power with suitable electrical voltage to the display panel  122  via the display driver  120 , which supplies an operating voltage to appropriate segments of display panel  122 , so as to display information on the display panel  122 . 
         [0026]    The display panel  122  may be any suitable display, as well known in the art. Typically, the display panel  122  may be a capacitive, non bi-stable display, i.e. a display in which suitable voltage is applied to the activate segments and continuously charge them in order for the information to be displayed. Examples for capacitive and non bi-stable displays are liquid crystal displays (LCD) or polymer dispersed liquid crystal displays (PDLC). 
         [0027]    It is known that capacitive, non bi-stable displays, typically, cannot withstand long-term DC voltage, and therefore the polarity of the charging voltage is alternated during a display operation period, i.e. the period of operation of the display panel  122  in which information is continually displayed on the display panel  122 . 
         [0028]    In conventional driving schemes, generally, the polarity of the electric voltage is alternated sequentially during the display operation period (AC voltage). In the present invention the segments of the display panel  122  are charged by direct voltage (DC voltage) without alternating the voltage polarity during the entire display operation period. It is appreciated that in the case of the present invention, the display panel  122  displays information for a relatively short period of time, either pre-determined by the microprocessor  114  or governed by the pressing period on the switch  110 . A typical such display operation period may be in the range of 10 to 60 seconds. The display panel  122  may shut off between such short display operation periods, in order to save power of the battery  124 . 
         [0029]    It is a particular feature of the present invention that the charging polarity of the DC voltage alternates upon different display operation periods of the display panel  122  of smart card  100 , and preferably upon successive display operation periods of the display panel  122  of smart card  100 . For example, as seen in  FIG. 4 , during a display operation period T 1 , which corresponds to display operation of the smart card  100  in  FIG. 2 , the DC voltage polarity is constantly positive. Upon subsequent display operation of the smart card  100  (after a shut-off period), as shown in  FIG. 3 , the DC voltage polarity is constantly negative, as seen in a subsequent display operation period T 2 , which corresponds to the display operation of the smart card  100  in  FIG. 3 . As seen in  FIG. 4 , the DC voltage polarity is alternated to positive in operation period T 3 , remains positive in subsequent operation period T 4 , and then alternated to negative polarity in operation period T 5 . It is appreciated that any suitable sequence of positive and negative DC voltage polarities may be practiced, as preferred and as predetermined by the microcontroller  114 . Preferably, the polarity of the DC voltage may alter staggeredly, namely upon each subsequent display activation of the smart card  100 . 
         [0030]    It is appreciated that a substantial portion of the display operation power, supplied by the battery  124  through power booster  126 , may be consumed by the charging operation of the appropriate display segments. Thus, use of power provided by the battery  124  is significantly reduced through the DC voltage alternating polarity operation scheme described herein above with reference to  FIG. 4 , since the charging polarity is constant during a single display operation period, wherein if the polarity would alternate once or more during a single display operation period of the smart card  100 , the use of power would significantly be increased due to increased number of charging and discharging events of the display segments, typically capacitive segments, of display panel  122 . 
         [0031]    It is appreciated that the charging polarity at the initial operation of the smart card  100  may be negative or positive. 
         [0032]    Accumulation of a net DC voltage on the segments of the display panel  122 , caused by the constant polarity during a single display operation period or cycle of the smart card  100 , is typically and statistically diminished or fully annulled upon many subsequent operations of the smart card  100 . This can be seen by observing the group of segments operated in the display panel  122  and designated by reference numeral  130  in  FIGS. 2 and 3 . As seen in  FIG. 4 , the polarity during T 1  is positive, thus the charge accumulated on segments  130  is positive. In a subsequent operation of the smart card  100 , as shown in  FIG. 3 , the polarity during T 2  is negative thus the net DC voltage on that segments, here designated by the reference numeral  132  in  FIG. 31  is minimized and balanced, and may even be fully annulled. 
         [0033]    Thus, upon a multiplicity of operations of the smart card  100  the DC charge accumulation is substantially minimized or even eliminated by being statistically neutralized by discharge so as to achieve negligible net DC effect. 
         [0034]    Preferably, the display time durations (T 1 , T 2 , T 3 , . . . T n ) may be constant. Alternatively, time durations may be different from each other, in which case that difference may preferably be random. Yet alternatively, the time operation periods of display  122  may be different, but such that over a large number of operation periods the overall time of positive DC voltage operation will be similar to the overall time of negative DC voltage operation. 
         [0035]    Preferably, the polarity of the DC voltage is alternated in every operation period. Alternatively, the polarity of the DC voltage may be alternated randomly, or according to a pre-determined sequence that balances the number of positive and negative operation periods over a large number of operation periods. 
         [0036]    It is appreciated that charge neutralizing may occur over long term operation of the display, and over a large number of operating periods According to the operation conditions as described herein above, the probability of any segment of display  122  (such as, for example, segments  130  in  FIG. 2 ) to be operated by a positive or negative voltage polarity over a large enough number of operation periods is similar, and the accumulative positive DC voltage time and negative DC voltage time of that segment (such as segments  130  in the above mentioned example) are substantially balanced, or even cancelled. 
         [0037]    Reference is now made to  FIG. 5 , which is a simplified block diagram of a display driver constructed and operative in accordance with an embodiment of the present invention. As seen in  FIG. 5 , display driver  120  of  FIGS. 2-3  is generally comprised of a command decoder  210 , a clock generator  212 , a sequencer  214  and a switching array  216 . 
         [0038]    The command decoder  210  interprets the commands received from the microprocessor  114  and initializes the operation of the sequencer  214  accordingly. 
         [0039]    The clock generator  212  generates the timely pulses required for synchronizing the data transmission to the display, as commonly practiced in the art. Sequencer  214  transforms the input received from the command decoder  210  to a vector of binary signals corresponding to the segments and common plane of display panel  122  and transmit that vector of signals to the switching array  216 , which switches the individual segments and common plane of display  122  accordingly to either ground voltage or to positive voltage received from the power booster  126 . 
         [0040]    Reference is now made to  FIG. 6 , which is a simplified block diagram of the switching array  216  of  FIG. 5 , constructed and operative for driving a segmented display having a number n of segments  218  and a common plane  219  which is underlying all n segments, as well known in the art. As seen in  FIG. 6 , the switching array  216  is comprised of a number n of two-state electronic switches  220 , each of which is in electrical communication with a single segment of the n segments of display  122 , and a two-state electronic switch  222  in electrical communication with the common plane of display  122 . As seen in  FIG. 6 , each of switches  220  and  222  receives an appropriate electronic signal from sequencer  214 , and switches the output voltage for the respective segment or common accordingly, to either ground voltage or positive DC voltage received from the power booster  126 . Hence, each of the n display segments and display common plane accommodates, in each operation period, either ground (zero) voltage or DC positive voltage as dictated by the driver  120 . 
         [0041]    Reference is now made to  FIGS. 7   a  and  7   b , which are simplified schematic illustrations of few of segments  218  and common plane  219  of display  122 , in two DC voltage operative state examples.  FIG. 7   a  demonstrates an example of the DC voltage configuration of few segments  218  and of common plane  219  during an operation period number N, as generated by driver  120  as detailed herein above with reference to  FIG. 5  and  FIG. 6 .  FIG. 7   b  demonstrates an example of the DC voltage configuration of few segments  218  and of common plane  219  during an operation period number N+K, as generated by driver  120  as detailed herein above, with reference to  FIG. 5  and  FIG. 6 . 
         [0042]    In the example of  FIG. 7   a , the segments numbered  1  and n assume positive DC voltage, while the segments numbered  2  and i and the common plane  219  assume ground (zero) voltage. As a consequence, during that operation period (number N), segments  1  and n exhibit DC voltage difference there-across and are activated while segments  2  and i exhibit zero voltage difference there-across and are not activated. Yet as a consequence, the polarity of the DC voltage across the activated segments  1  and n is positive, namely the segment  218  assumes a higher voltage than the common plane  219 . 
         [0043]    In the example of  FIG. 7   b , the segments numbered  1  and  2  and the common plane  219  assume positive DC voltage, while the segments numbered i and n assume ground (zero) voltage. As a consequence, during that operation period (number N+K), segments i and n exhibit DC voltage difference there-across and are activated while segments  1  and  2  exhibit zero voltage difference there-across and are not activated. Yet as a consequence, the polarity of the DC voltage across the activated segments i and n is negatives namely the common plane  219  assumes a higher voltage than the segment  218 . 
         [0044]    As demonstrated by  FIGS. 7   a  and  7   b , through the utilization and switching of either ground (zero) or positive DC voltage, the driver  120  can activate each of the individual segments  218  of display  122  by DC voltage of either positive or negative polarity. 
         [0045]    It is appreciated that the method of polarity alternation upon display operation period may be employed in any suitable type of display and for any type of data storage means and driving methods. 
         [0046]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications which would occur to persons skilled in the art upon reading the specifications and which are not in the prior art.