Patent Publication Number: US-11024243-B2

Title: Electronic device display with charge accumulation tracker

Description:
This application is a continuation of U.S. patent application Ser. No. 16/113,132, filed Aug. 27, 2018, which is a continuation of U.S. patent application Ser. No. 15/890,517, filed Feb. 7, 2018, now U.S. Pat. No. 10,102,815, which is a continuation of U.S. patent application Ser. No. 14/722,620, filed May 27, 2015, now U.S. Pat. No. 9,922,608, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. 
     Liquid crystal displays contain a layer of liquid crystal material. Pixels in a liquid crystal display contain thin-film transistors and pixel electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the pixel. 
     There is a potential for ions in a liquid crystal display to move in response to applied electric fields. This can lead to charge accumulation on the pixels. Another cause of charge accumulation is dielectric polarization. Charge accumulation effects can produce visible artifacts on a display such as undesired flickering. 
     To minimize charge accumulation in a liquid crystal display, the polarity of the electric field applied to the pixels may be periodically reversed. For example, alternating positive polarity and negative polarity frames of image data may be displayed on the pixels of a liquid crystal display to prevent excess positive or negative charge accumulation. Although periodic polarity reversal can help reduce charge accumulation, charge accumulation issues may still arise in liquid crystal displays. Charge accumulation may arise, for example, in situations in which a software application or other content generator creates negative and positive frames of image data with unbalanced gray levels. The risk of undesired charge accumulation may be exacerbated in displays with a variable refresh rate. 
     It would therefore be desirable to be able to provide displays with enhanced charge accumulation mitigation capabilities. 
     SUMMARY 
     An electronic device may generate content that is to be displayed on a display. The display may be a liquid crystal display have an array of liquid crystal display pixels. Display driver circuitry in the display may display image frames on the array of pixels. The image frames may be displayed with positive and negative polarities to help reduce charge accumulation effects. 
     A charge accumulation tracker may analyze the image frames to determine when there is a risk of excess charge accumulation. The charge accumulation tracker may use information on gray levels in the displayed image frames, frame duration information, and frame polarity information as inputs. The charge accumulation tracker may compute a charge accumulation metric based on the gray levels, frame duration, and frame polarity. Weights that are retrieved from a look-up table or that are represented using a mathematical expression may be applied to the inputs of the charge accumulation tracker. For example, the charge accumulation tracker may apply weights to the inputs that vary as a function of gray level, image frame duration, and polarity. 
     The charge accumulation tracker may compute the charge accumulation metric for entire image frames or may process subregions of each frame separately. When subregions are processed separately, each subregion may be individually monitored for a risk of excess charge accumulation by comparing a charge accumulation metric for that subregion to a threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a computer or other device with display structures in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 6  is a top view of a portion of an array of pixels in a display in accordance with an embodiment. 
         FIG. 7  is a graph showing how the refresh rate of a display may be varied as a function of time in accordance with an embodiment. 
         FIG. 8  is a diagram showing how a charge accumulation metric may be computed and compared against a threshold in accordance with an embodiment. 
         FIG. 9  is a diagram of illustrative circuitry that may be used in operating a display with charge accumulation monitoring capabilities in accordance with an embodiment. 
         FIG. 10  is a graph showing how a weighting factor for use in computing a charge accumulation metric may vary as a function of gray level in accordance with an embodiment. 
         FIG. 11  is a graph showing how a weighting factor for use in computing a charge accumulation may vary as a function of the amount of time for which an image is displayed (image frame duration) in accordance with an embodiment. 
         FIG. 12  is a diagram showing how a block-based charge accumulation tracker may monitor charge accumulation for multiple subregions of a display in accordance with an embodiment. 
         FIG. 13  is a flow chart of illustrative steps involved in operating a display with charge accumulation monitoring capabilities in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1, 2, 3, and 4 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, watch, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have openings for components such as button  26 . Openings may also be formed in display  14  to accommodate a speaker port (see, e.g., speaker port  28  of  FIG. 2 ). In compact devices such as wrist-watch devices, port  28  and/or button  26  may be omitted and device  10  may be provided with a strap or lanyard. 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have an opening to accommodate button  26  (as an example). 
       FIG. 4  shows how electronic device  10  may be a display such as a computer monitor, a computer that has been integrated into a computer display, or other device with a built-in display. With this type of arrangement, housing  12  for device  10  may be mounted on a support structure such as stand  30  or stand  30  may be omitted (e.g., to mount device  10  on a wall). Display  14  may be mounted on a front face of housing  12 . 
     The illustrative configurations for device  10  that are shown in  FIGS. 1, 2, 3, and 4  are merely illustrative. In general, electronic device  10  may be a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, or other wearable or miniature device, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Display  14  for device  10  may include pixels formed from liquid crystal display (LCD) components. A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42  for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     Display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  58  and  56  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer in the upper or lower portion of display  14  may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). 
     Backlight structures  42  may include a light guide plate such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes. 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . Light source  72  may be located at the left of light guide plate  78  as shown in  FIG. 5  or may be located along the right edge of plate  78  and/or other edges of plate  78 . 
     Light  74  that scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of plastic covered with a dielectric mirror thin-film coating. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce hotspots, compensation films for enhancing off-axis viewing, and brightness enhancement films (also sometimes referred to as turning films) for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint in the X-Y plane of  FIG. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. If desired, films such as compensation films may be incorporated into other layers of display  14  (e.g., polarizer layers). 
     As shown in  FIG. 6 , display  14  may include an array of pixels  90  such as pixel array  92 . Pixel array  92  may be controlled using control signals produced by display driver circuitry. Display driver circuitry may be implemented using one or more integrated circuits (ICs) and/or thin-film transistors or other circuitry. 
     During operation of device  10 , control circuitry in device  10  such as memory circuits, microprocessors, and other storage and processing circuitry may provide data to the display driver circuitry. The display driver circuitry may convert the data into signals for controlling pixels  90  of pixel array  92 . 
     Pixel array  92  may contain rows and columns of pixels  90 . The circuitry of pixel array  92  (i.e., the rows and columns of pixel circuits for pixels  90 ) may be controlled using signals such as data line signals on data lines D and gate line signals on gate lines G. Data lines D and gate lines G are orthogonal. For example, data lines D may extend vertically and gate lines G may extend horizontally (i.e., perpendicular to data lines D). 
     Pixels  90  in pixel array  92  may contain thin-film transistor circuitry (e.g., polysilicon transistor circuitry, amorphous silicon transistor circuitry, semiconducting-oxide transistor circuitry such as InGaZnO transistor circuitry, other silicon or semiconducting-oxide transistor circuitry, etc.) and associated structures for producing electric fields across liquid crystal layer  52  in display  14 . Each liquid crystal display pixel may have one or more thin-film transistors. For example, each pixel may have a respective thin-film transistor such as thin-film transistor  94  to control the application of electric fields to a respective pixel-sized portion  52 ′ of liquid crystal layer  52 . 
     The thin-film transistor structures that are used in forming pixels  90  may be located on a thin-film transistor substrate such as a layer of glass. The thin-film transistor substrate and the structures of display pixels  90  that are formed on the surface of the thin-film transistor substrate collectively form thin-film transistor layer  58  ( FIG. 5 ). 
     Gate driver circuitry may be used to generate gate signals on gate lines G. The gate driver circuitry may be formed from thin-film transistors on the thin-film transistor layer or may be implemented in separate integrated circuits. The data line signals on data lines D in pixel array  92  carry analog image data (e.g., voltages with magnitudes representing pixel brightness levels). During the process of displaying images on display  14 , a display driver integrated circuit or other circuitry may receive digital data from control circuitry and may produce corresponding analog data signals. The analog data signals may be demultiplexed and provided to data lines D. 
     The data line signals on data lines D are distributed to the columns of display pixels  90  in pixel array  92 . Gate line signals on gate lines G are provided to the rows of pixels  90  in pixel array  92  by associated gate driver circuitry. 
     The circuitry of display  14  may be formed from conductive structures (e.g., metal lines and/or structures formed from transparent conductive materials such as indium tin oxide) and may include transistors such as transistor  94  of  FIG. 6  that are fabricated on the thin-film transistor substrate layer of display  14 . The thin-film transistors may be, for example, silicon thin-film transistors or semiconducting-oxide thin-film transistors. 
     As shown in  FIG. 6 , pixels such as pixel  90  may be located at the intersection of each gate line G and data line D in array  92 . A data signal on each data line D may be supplied to terminal  96  from one of data lines D. Thin-film transistor  94  (e.g., a thin-film polysilicon transistor, an amorphous silicon transistor, or an oxide transistor such as a transistor formed from a semiconducting oxide such as indium gallium zinc oxide) may have a gate terminal such as gate  98  that receives gate line control signals on gate line G. When a gate line control signal is asserted, transistor  94  will be turned on and the data signal at terminal  96  will be passed to node  100  as pixel voltage Vp. Data for display  14  may be displayed in frames. Following assertion of the gate line signal in each row to pass data signals to the pixels of that row, the gate line signal may be deasserted. In a subsequent display frame, the gate line signal for each row may again be asserted to turn on transistor  94  and capture new values of Vp. 
     Pixel  90  may have a signal storage element such as capacitor  102  or other charge storage elements. Storage capacitor  102  may be used to help store signal Vp in pixel  90  between frames (i.e., in the period of time between the assertion of successive gate signals). 
     Display  14  may have a common electrode coupled to node  104 . The common electrode (which is sometimes referred to as the common voltage electrode, Vcom electrode, or Vcom terminal) may be used to distribute a common electrode voltage such as common electrode voltage Vcom to nodes such as node  104  in each pixel  90  of array  92 . As shown by illustrative electrode pattern  104 ′ of  FIG. 6 , Vcom electrode  104  may be implemented using a blanket film of a transparent conductive material such as indium tin oxide, indium zinc oxide, other transparent conductive oxide material, and/or a layer of metal that is sufficiently thin to be transparent (e.g., electrode  104  may be formed from a layer of indium tin oxide or other transparent conductive layer that covers all of pixels  90  in array  92 ). 
     In each pixel  90 , capacitor  102  may be coupled between nodes  100  and  104 . A parallel capacitance arises across nodes  100  and  104  due to electrode structures in pixel  90  that are used in controlling the electric field through the liquid crystal material of the pixel (liquid crystal material  52 ′). As shown in  FIG. 6 , electrode structures  106  (e.g., a display pixel electrode with multiple fingers or other display pixel electrode for applying electric fields to liquid crystal material  52 ′) may be coupled to node  100  (or a multi-finger display pixel electrode may be formed at node  104 ). During operation, electrode structures  106  may be used to apply a controlled electric field (i.e., a field having a magnitude proportional to Vp-Vcom) across pixel-sized liquid crystal material  52 ′ in pixel  90 . Due to the presence of storage capacitor  102  and the parallel capacitances formed by the pixel structures of pixel  90 , the value of Vp (and therefore the associated electric field across liquid crystal material  52 ′) may be maintained across nodes  106  and  104  for the duration of the frame. 
     The electric field that is produced across liquid crystal material  52 ′ causes a change in the orientations of the liquid crystals in liquid crystal material  52 ′. This changes the polarization of light passing through liquid crystal material  52 ′. The change in polarization may, in conjunction with polarizers  60  and  54  of  FIG. 5 , be used in controlling the amount of light  44  that is transmitted through each pixel  90  in array  92  of display  14  so that image frames may be displayed on display  14 . 
     Charge accumulation issues may arise from repeated application of electric fields across liquid crystal material  52 ′ using applied voltages Vp-Vcom of a single polarity. Accordingly, the polarity of the electric field may be periodically alternated. As an example, in odd frames a positive voltage Vp-Vcom may be applied across material  52 ′, whereas in even frames a negative voltage Vp-Vcom may be applied across material  52 ′. To ensure that charge accumulation effects are not present (even when periodically reversing the polarity of the image frames), device  10  can incorporate charge accumulation monitoring functionality. For example, a charge accumulation tracker can be implemented in device  10  that monitors display  14  for excessive charge accumulation conditions. If suitable criteria are satisfied (i.e., if a calculated charge accumulation level exceeds a predetermined charge accumulation threshold for all or part of display  14 ), appropriate remedial actions may be taken. 
     Charge accumulation effects arise when non-black content is displayed. Black content and other content with low gray levels does not involve application of large electric fields to display  14  and therefore does not give rise to significant charge accumulation. Content with large gray levels (e.g., white content), however, is associated with large electric fields across layer  52  and therefore has the potential to lead to charge accumulation. In addition to being dependent on the gray level of displayed image frames, charge accumulation effects are also dependent on the amount of time that white content (high gray level content) is displayed for each polarity. 
     Charge accumulation can become excessive when the images that are displayed on display  14  do not contain content that is evenly divided between positive and negative frames. For example, excessive charge accumulation conditions may arise when more white content is displayed during positive frames than during negative frames. The likelihood that excessive charge accumulation conditions will arise may be exacerbated in displays that implement variable refresh rate schemes. With a variable refresh rate scheme, display  14  is sometimes operated with a relatively high frame rate and is sometimes operated with a relatively low frame rate. The high frame rate may be used to display rapidly moving content. The low frame rate may be used to conserve power when content is changing less rapidly. 
     A graph in which frame rate FR has been plotted as a function of time in an illustrative configuration in which display  14  has variable refresh rate capabilities is shown in  FIG. 7 . As shown in  FIG. 7 , display  14  may be operated at an elevated frame rate FRH when it is desired to display rapidly moving content on display  14  (e.g., video). Frame rate FRH may be, for example, 60 Hz, 30 Hz, or other relatively high frame rate. In the example of  FIG. 7 , display  14  uses frame rate FRH at times between t 0  and t 1 . At time t 1 , elevated frame rate FRH is no longer needed, so device  10  lowers frame rate FR for display  14  to lowered frame rate FRL (e.g., for times between t 1  and t 2 , before frame rate FR is returned to high frame rate FRH). Frame rate FRL may be, for example, a rate between 1 Hz and 10 Hz, less than 10 Hz, or other frame rate lower than frame rate FRH. Because the frame rate has been reduced, power consumption at times between times t 1  and t 2  may be reduced within display  14 . 
     The reduced frame rates that are involved in operating a display with variable refresh rate capabilities are associated with frames of potentially long duration (e.g., 1 s, etc.). Particularly in scenarios in which display  14  is operating with long frames, there is a potential for an undesirable interplay between the pattern of content being displayed on display  14  and the polarities of the frames that can lead to excessive charge accumulation. 
     To ensure that device  10  and display  14  operate satisfactorily, a charge accumulation tracker may be implemented that monitors for the occurrence of conditions that are likely associated with excess charge accumulation. When charge accumulation is detected, remedial actions may be taken. For example, in a display with variable refresh rate capabilities, variable refresh operations can be suspended (e.g., by returning device  10  to high refresh rate FRH for a given period of time or by at least elevating the frame rate for display  14  above desired low rate FRL for a given period of time). As another example, the polarity of the frames of image data being displayed on display  14  can be flipped (e.g., by inserting an extra positive frame between a positive frame and a negative frame). 
     The charge accumulation tracker can be spatially sensitive. For example, display  14  may be divided into multiple subregions (e.g., rectangular blocks), each of which may be monitored separately to determine whether excessive charge accumulation is present. The charge accumulation tracker may also take into account the gray level of displayed content, weighting higher gray levels (whiter content) more heavily than lower gray levels (darker content). The duration of positive and negative frames (which affects how long the content is displayed with each polarity) can also be taken into account. Based on these inputs and/or other information, the charge accumulation tracker may determine whether or not remedial actions are required. 
     If desired, the charge accumulation tracker may determine the average gray level for each frame (i.e., the charge accumulation tracker in this type of arrangement will not divide display  14  into an array of smaller blocks and will therefore not be spatially sensitive). The average gray level in each frame may be, for example, the mean gray level of the pixels in the frame or may be the median gray level of the pixels in the frame. Scenarios in which the charge accumulation tracker uses a fixed estimation of the average gray level of each frame (e.g., by assuming that frames include a worst-case gray level of 255 or include an average gray level of 127 or other suitable fixed value) may also be used by the charge accumulation tracker. Weighting factors may be applied to the computed average gray level to help determine an appropriate charge accumulation metric (which can then be compared against a predetermined threshold to determine whether charge accumulation is excessive and requires remediation). As an example, gray level weighting may be used to weight frames with higher average gray levels more heavily than frames with lower average gray levels and/or time-based weighting may be used to weight positive and negative frames by their respective durations (in addition to taking into account their average gray levels). 
     Consider, as an example, the scenario of  FIG. 8 . In the example of  FIG. 8 , frames (F 1  . . . F 7 ) are being displayed in sequence on display  14  while a charge accumulation tracker is being used to evaluate the gray level of each frame and the duration of each frame (i.e., gray level weighting and frame duration weighting is linear in this example). The charge accumulation tracker computes an updated value for charge accumulation metric COUNT as each frame of image data is displayed on display  14 . The value of COUNT is compared to a threshold level (e.g., a threshold level TL of 20,000 in this example). So long as COUNT does not exceed TL, frames of image data may be displayed on display  14  normally (e.g., using a variable refresh rate scheme in combination with alternating positive and negative frame polarities P and N as in the example of  FIG. 8 ). If, however, the charge accumulation tracker determines that the value of COUNT has exceeded threshold value TL, appropriate remedial action may be taken to reduce charge accumulation. 
     As shown in  FIG. 8 , the average gray level (AGL) of frame F 1  is 100 and the duration of frame F 1  is 13 mS. The charge accumulation tracker can compute the product of the average gray level and frame duration to produce an initial count value of COUNT equal to 1300 (13*100). Frame F 2  has a negative polarity N (i.e., a polarity that is opposite to that of positive frame F 1 ), so in updating COUNT while displaying frame F 2 , the charge accumulation tracker may subtract the product of the average gray level of frame F 2  (110) and the duration of frame F 2  (13 mS) from the value of COUNT following frame F 1 . The resulting updated COUNT value following frame F 2  is −130. Frame F 3  has an average gray level of 160, a duration of 13 mS, and a positive polarity P, which brings the value of COUNT up to 1950. Frame F 4  has an average gray level of 140, a duration of 13 mS, and a negative polarity. The charge accumulation tracker therefore updates COUNT to have a value of 130 at frame F 4 . 
     In the  FIG. 8  example, display  14  is a variable refresh rate display. For frame F 5  and subsequent frames F 6  and F 7 , the refresh rate (frame rate) for display  14  is reduced to 10 Hz. As a result, each frame has a duration of 100 mS. The lengthened value of each frame is taken into account by the charge accumulation tracker when updating COUNT. As shown in  FIG. 8 , illustrative frame F 5  has an average gray level of 150, a duration of 100 mS, and a positive polarity, so COUNT rises to 15,130 at frame F 5 . The value of 15,130 is less than threshold value 20,000, so excessive charge accumulation is not present at frame F 5 . Frame F 6  of  FIG. 8  has an average gray level of 50. Frame F 6  has a duration of 100 mS and a negative polarity, so at frame F 6  the updated value of COUNT becomes 10,130 (which is also below threshold TL). Frame F 7  of  FIG. 8  has an average gray level of 150, a duration of 100 mS, and a positive polarity. When the charge accumulation tracker computes COUNT at frame F 7 , the updated value of COUNT is 25,130, which exceeds threshold TL. Because threshold TL is exceeded, the charge accumulation tracker recognizes that there is a potential for excess charge accumulation within display  14  and takes appropriate remedial action. As this example demonstrates, the charge accumulation tracker can determine whether excessive charge accumulation has occurred by taking into account factors such as average (mean or median) gray level for each frame, frame duration, and frame polarity and computing the value of a corresponding charge accumulation metric such as parameter COUNT, which can be compared to a predetermined threshold TL. If desired, look-up tables, mathematical functions, or other arrangements may be used to apply weights to the inputs of the charge accumulation tracker. Weighting functions may be linear or non-linear. 
       FIG. 9  is a schematic diagram of illustrative circuitry in device  10  that may be used in implementing display  14  and a charge accumulation tracker for monitoring charge accumulation conditions for display  14 . As shown in  FIG. 9 , device  10  may have control circuitry  110 . Control circuitry  110  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  110  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Control circuitry  110  may include a graphics processing unit such as graphics processing unit  116 . Graphics processing unit  116  may receive image frames for frame buffer  120  (e.g., frame buffer  120 A) from content generator  114 . Content generator  114  may be an application running on control circuitry  110  such as a game, a media playback application, an application that presents text to a user, an operating system function, or other code running on control circuitry  110  that generates image data to be displayed on display  14 . 
     Control circuitry  110  may be coupled to input-output circuitry such as input-output devices  112 . Input-output devices  112  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  112  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  112  and may receive status information and other output from device  10  using the output resources of input-output devices  112 . 
     Control circuitry  110  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  110  (e.g., content generator  114 ) may display images on display  14  using pixels  90  of pixel array  92 . Display  14  may include display driver circuitry such as display driver circuitry  122  (see, e.g., circuitry  62 A and  62 B of  FIG. 5 ) that receives image data from graphics processing unit  116 . The display driver circuitry of display  14  may include one or more display driver integrated circuits (e.g., a timing controller integrated circuit or other display driver circuitry such as display driver circuitry  122  of  FIG. 9 ) and gate driver circuitry  124 . Gate driver circuitry  124  may be implemented using thin-film transistor circuitry on a display substrate and/or may be implemented using one or more integrated circuits. Array  92  may have display driver circuitry such as circuitry  124  that is located on the left and right edges of array  92 , on only the left edge or only the right edge of array  92 , or that is located elsewhere in display  14 . 
     Image frames to be displayed on array  92  by the display driver circuitry may be stored in frame buffer  120  (e.g., frame buffer  120 B). Charge accumulation tracker  118  may be implemented using resources in graphics processing unit  116  (see, e.g., charge accumulation tracker  118 A) and/or using resources in display driver circuitry of display  14  (see, e.g., charge accumulation tracker  118 B). Charge accumulation tracker  118  may use information on frame durations (e.g., the durations for which image frames in frame buffer circuitry  120  are displayed on array  92 ) in evaluating the values of charge accumulation metrics for the image frames displayed on the pixels of display  14 . In arrangements in which frame duration information is not available in graphics processing unit  116 , frame duration information may be provided by display driver circuitry  122  (e.g., charge accumulation tracker  118  may be implemented on circuitry  122  as illustrated by tracker  118 B of  FIG. 9 ). Charge accumulation tracker  118  may analyze gray levels in the content being displayed on array  92  by processing image frames in buffer circuitry  120 . Image frames may be processed in their entirety (e.g., to compute an average gray level for each frame) or image frames may be broken into multiple subregions (e.g., to compute an average gray level or other image parameter related to charge accumulation for each individual subregion). 
     When subregions of each image frame are evaluated, charge accumulation scenarios that affect only a portion of display  14  can be detected. If, for example, a small portion of display  14  is white for all positive frames and black for all negative frames, whereas the remainder of the display has a relatively constant low gray level across positive and negative frames, there is a risk that a global gray level evaluation technique of the type described in connection with computation of the global COUNT value of  FIG. 7  may not recognize the risk of charge accumulation in the small affected portion of display  14 . In contrast, a charge accumulation tracker that evaluates subregions of display  14  (e.g., blocks with edges that are 3-8 mm long, more than 1 mm long, less than 1 cm long, or other suitable size), can recognize local charge accumulation problems and can take appropriate remedial action before display flickering and other visible artifacts are noticed by a viewer. 
     When using charge accumulation tracker  118  to evaluate charge accumulation risk in subregions of display  14  (or for entire image frames), the charge accumulation tracker may use look-up tables or mathematical equations to apply weighting functions to inputs such as measured average gray level and image frame duration. Curve  126  of the graph of  FIG. 10  represents an illustrative non-linear weighting function that may be applied to measured gray levels in part of an image frame (or an entire image frame). Curve  128  of the graph of  FIG. 11  represents an illustrative non-linear weighting function that may be applied to all or part of an image frame based on the duration of that frame (or frame portion). Curves  126  and  128  may be the same for positive and negative polarities or may be different. The weighting functions may be determined by empirical measurements on sample displays (i.e., measurements that evaluate the amount of charge accumulation that is produced at various gray levels and frame polarities for various amounts of time) and/or may be modeled theoretically. 
     The operation of a display with a configuration in which charge accumulation tracker  118  evaluates image frames on a block-by-block basis (i.e., in which charge accumulation tracker  118  is a block-based charge accumulation tracker) is illustrated in  FIG. 12 . In the example of  FIG. 12 , display  14  is displaying image frames FA, FB, FC, FD, and FE. There are four subregions (sometimes referred to as blocks or subareas) of display  14  in the  FIG. 12  example. These blocks (blocks B 1 , B 2 , B 3 , and B 4 ) each have varying gray levels. The image frames that containing the blocks have positive polarity P or negative polarity N. All of the frames in the  FIG. 12  scenario have the same duration (e.g., 13 mS or other suitable value). As the gray level of the blocks vary from frame to frame, charge accumulation tracker  118  computes charge accumulation metric C 1  for block B 1 , C 2  for block B 2 , C 3  for block B 3 , and C 4  for block B 4 . If the value of any of these metrics (C 1 , C 2 , C 3 , or C 4 ) exceeds predetermined threshold TH (which is 15 in this example), there is an excessive risk for charge accumulation and remedial action can be taken. 
     Frame FA is a positive frame, so charge accumulation parameters C 1 , C 2 , C 3 , and C 4  acquire the values of the gray levels in blocks B 1 , B 2 , B 3 , and B 4 , respectively. Frame FB is a negative frame, so the value of B 1  in frame FB is subtracted from C 1  of frame FA, etc. The gray levels of each block in frame FC likewise are added to the respective parameters C 1 , C 2 , C 3 , and C 4  and the gray levels of each block in frame FD are subtracted from parameters C 1 , C 2 , C 3 , and C 4 . As content is being provided to display  14  from content generator  114 , there is a potential for the gray levels of blocks B 1 , B 2 , B 3 , and B 4  to vary significantly between frames in a pattern that gives rise to charge accumulation in at least one of the blocks. This is illustrated by positive frame FE, in which the value of the charge accumulation metric C 3  that has been computed by charge accumulation tracker  118  for block B 3  in frame FE exceeds threshold TH. When charge accumulation tracker  118  produces a charge accumulation parameter value for a given one of the blocks that exceeds threshold TH, charge accumulation tracker  118  can conclude that there is a risk of excessive charge accumulation for at least that one subregion of display  14  and can take appropriate remedial action. 
     A flow chart of illustrative operations involved in using charge accumulation tracker  118  to monitor for the occurrence of charge accumulation conditions in display  14  is shown in  FIG. 13 . During the charge accumulation operations of  FIG. 13 , charge accumulation tracker  118  may monitor image frames globally (e.g., by computing an average gray value for each frame in its entirety) or may monitor charge accumulation in each of multiple subregions such as blocks B 1 , B 2 , B 3 , and B 4  of  FIG. 12 . Configurations in which charge accumulation is evaluated on a block-by-block basis are sometimes described as an example. 
     At step  130 , as content generator  114  provides charge accumulation tracker  118  with image data to display on array  92  of display  14  (e.g., as image frames are provided to the frame buffer circuitry), charge accumulation tracker  118  computes the value of a charge accumulation metric (e.g., C 1  . . . C 4 , etc.) for each subregion of interest in display  14 . There may be any suitable number of regions of display  14  that are evaluated by tracker  118  (e.g., one region, two regions, four or more regions, 10 or more regions, 10-100 regions, 100-10000 regions, fewer than 1000 regions, fewer than 100 regions, or other suitable number of subregions). In computing the charge accumulation metric values, tracker  118  may use data stored in look-up tables or other stored data such as weighting data (based on gray level, duration, polarity, etc.) and/or may use mathematical weighting functions to weight raw image data. The computed charge accumulation metric value in each subregion may be compared to a suitable threshold value to determine whether there is a risk of excessive charge accumulation in that subregion. 
     So long as the computed charge accumulation values do not exceed the charge accumulation threshold, no remedial actions need be taken and processing may loop back to step  130  so that charge accumulation tracker  118  can continue to evaluate the frames of image data being displayed on display  14 . 
     If the charge accumulation threshold is exceeded by the charge accumulation metric that has been computed for any of the subregions of display  14 , tracker  118  can initiate appropriate remedial actions (step  134 ). Processing may then loop back to step  130 , as indicated by line  136 . 
     The remedial actions that are performed at step  134  may be performed using graphics processing unit  116  and/or display driver circuitry such as display driver circuitry  122 . These actions may include, for example, temporarily suspending variable refresh rate operations (e.g., by restoring the frame rate of display  14  to a relatively high rate such as 30 Hz or 60 Hz or other non-reduced refresh rate rather than allowing a reduced rate of 1-10 Hz to be used), flipping the polarity of the image frames being displayed (e.g., by changing from a scheme in which odd frames are positive and even frames are negative to a scheme in which odd frames are negative and even frames are positive), lengthening the duration of a particular frame (e.g., a positive frame when more positive polarity operations are needed to reduce charge accumulation, etc.), by inserting a remedial frame with a duration and polarity that reduces charge accumulation, or other suitable actions. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.