Abstract:
Systems and methods for efficiently generating display driver timing signals are provided. In one example, display driver circuitry of an electronic display may provide a negative voltage from a negative voltage supply to display control circuitry during a first period and may provide a positive voltage from a positive voltage supply to the display control circuitry during a second period. After providing the negative voltage during the first period but before providing the positive voltage during the second period, the display driver circuitry may precharge the capacitance of the display control circuitry to ground. In this way, the positive voltage supply substantially does not supply charge to raise the voltage on the capacitance of the display control circuitry from the negative voltage to ground.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application No. 61/727,557, titled “DISPLAY DRIVER PRECHARGE CIRCUITRY” and filed 16 Nov. 2012, which is incorporated by reference herein in its entirety for all purposes. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to display driver circuitry to generate an efficient timing signal by precharging to ground before transitioning between periods of positive voltage and negative voltage. 
         [0003]    This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
         [0004]    Electronic displays appear in many electronic devices. One type of electronic display, known as a liquid crystal display (LCD), modulates light passing through pixels of various colors using a liquid crystal material. By programming the pixels to display different colors, images are generated on the display. To do so, display driver circuitry provides a gate clock signal to activate a row of pixels. While the pixels are activated, the display driver circuitry may program the pixels to display particular colors. Specifically, the display driver circuitry may receive multiplexed image data (e.g., a single signal of red, green, and blue (RGB) image data). Demultiplexers having switches clocked to demultiplexer timing signals then demultiplex the image data into separate signals of different colors (e.g., separate red, green, and blue signals). The display driver circuitry provides these demultiplexed image signals to the pixels while the pixels remain activated by the gate clock signal. After the pixels have been programmed, the gate clock signal deactivates the pixels. 
         [0005]    The various timing signals used by the display driver circuitry may have periods of positive voltage and periods of negative voltage. In general, the switches and/or gates that receive the timing signals may be activated during positive voltage periods and deactivated during negative voltage periods, though this arrangement may be reversed. In either case, a positive voltage supply provides positive charge to cause the timing signal to reach a positive voltage, and a negative voltage supply provides negative charge to cause the timing signal to reach a negative voltage. Repeatedly and alternatingly providing the positive and negative charges to generate the timing signals used by the display driver circuitry may consume a substantial amount of power. 
         [0006]    Moreover, the rise and fall transition time properties (e.g., slew rate) of these timing signals (e.g., the gate clock signals) may influence and affect channel charge distribution on the row of pixels being activated or deactivated. In some cases, a relatively rapid slew rate of the gate clock signals may cause certain visual artifacts, such as flicker, to occur more frequently and/or more severely. On the other hand, a slower slew rate may reduce some visual artifacts. As such, it may be desirable to design and provide an LCD display that can regulate the slew rate of gate clock signals, while also reducing the amount of power consumed by these signals. 
       SUMMARY 
       [0007]    A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
         [0008]    Embodiments of the present disclosure relate to systems and methods for efficiently generating display driver timing signals having periods of positive voltage and periods of negative voltage. Specifically, the amount of charge supplied by positive voltage supplies and negative voltage supplies may be reduced by precharging the timing signals to ground between periods of positive and negative voltage. Thus, in one example, display driver circuitry of an electronic display may provide a negative voltage from a negative voltage supply to display control circuitry during a first period and may provide a positive voltage from a positive voltage supply to the display control circuitry during a second period. After providing the negative voltage during the first period but before providing the positive voltage during the second period, the display driver circuitry may precharge the capacitance of the display control circuitry to ground. In this way, the positive voltage supply substantially does not supply charge to raise the voltage on the capacitance of the display control circuitry from the negative voltage to ground. Compared to timing circuitry that switches directly between the positive voltage and the negative voltage, the timing circuitry of this disclosure may offer substantial efficiencies. Indeed, the timing circuitry of this disclosure may consume roughly half the power consumed by timing circuitry that switches directly between the positive voltage and the negative voltage. 
         [0009]    Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
           [0011]      FIG. 1  is a block diagram of an electronic device that employs efficient display timing circuitry by precharging to ground between positive and negative voltage periods of a timing signal, in accordance with an embodiment; 
           [0012]      FIG. 2  is a perspective view of the electronic device of  FIG. 1  in the form of a notebook computer, in accordance with an embodiment; 
           [0013]      FIG. 3  is a front view of the electronic device of  FIG. 1  in the form of a handheld device, in accordance with an embodiment; 
           [0014]      FIG. 4  is a circuit diagram of display driver circuitry of the display that uses the efficient timing circuitry, in accordance with an embodiment; 
           [0015]      FIG. 5  is a circuit diagram of the efficient timing circuitry, in accordance with an embodiment; 
           [0016]      FIG. 6  is a timing diagram for controlling the circuitry of  FIG. 5  to precharge to ground while transitioning between a higher voltage VGH and a lower voltage VGL, in accordance with an embodiment; 
           [0017]      FIG. 7  is a flowchart of a method for controlling the circuitry of  FIG. 5 , in accordance with an embodiment; 
           [0018]      FIG. 8  is a circuit diagram of the efficient timing circuitry that includes slew rate control circuitry, in accordance with an embodiment; 
           [0019]      FIG. 9  is a timing diagram illustrating a manner of controlling the circuitry of  FIG. 8  to provide a desired slew rate while precharging to ground, in accordance with an embodiment; 
           [0020]      FIG. 10  is a circuit diagram of another example of efficient timing circuitry that includes slew rate control circuitry, in accordance with an embodiment; and 
           [0021]      FIG. 11  is a flowchart of a method for controlling the timing circuitry of  FIG. 8  or  10 , in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0023]    When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
         [0024]    This disclosure relates to circuitry to efficiently provide timing signals in an electronic display. Specifically, display driver circuitry may provide timing signals to control a variety of different elements of an electronic display. For example, a gate clock signal may control when individual pixels of the electronic display are activated. In one example, a voltage level higher than ground may be used to activate gates of thin film transistors (TFTs) of a row of pixels to activate the pixels while they are programmed with image data signals. To deactivate the pixels, the gate clock signal may be reduced to a voltage level lower than ground. Rather than switch directly between the voltages level higher than ground (VGH) and the voltage level lower than ground (VGL), the gate clock signal may be precharged (also referred to as “discharged” in this disclosure) to ground between these transitions. As will be described below, precharging to ground rather than switching directly between the higher voltage VGH and the lower voltage VGL, and vice versa, may provide power savings of approximately 50% over circuits that do not precharge to ground between higher and lower voltages. 
         [0025]    Such efficient timing circuitry may also enable slew rate control. As used herein, slew rate refers to the transition time between the higher voltage VGH and the lower voltage VGL, and vice versa. Since the slew rate may impact the performance of the display (e.g., an especially high slew rate may result in flicker or other artifacts), the display driver circuitry of this disclosure may additionally include slew rate control circuitry. The slew rate control circuitry may operate in cooperation with the precharge circuitry. 
         [0026]    Finally, this disclosure tends to describe efficient timing circuitry for use with a liquid crystal display (LCD). However, the efficient timing circuitry may be employed using any suitable type of electronic display. For example, other electronic displays that employ a matrix of pixels, such as organic light emitting diode (OLED) displays, may also employ the efficient timing circuitry of this disclosure. 
         [0027]    Indeed, many suitable electronic devices may use electronic displays that include efficient timing circuitry. For example,  FIG. 1  is a block diagram depicting various components that may be present in an electronic device suitable for use with such a display.  FIGS. 2 and 3  respectively illustrate perspective and front views of suitable electronic devices. Specifically,  FIGS. 2 and 3  illustrate a notebook computer and a handheld electronic device, respectively. 
         [0028]    Turning first to  FIG. 1 , an electronic device  10  according to an embodiment of this disclosure may include, among other things, one or more processor(s)  12 , memory  14 , nonvolatile storage  16 , a display  18  that includes efficient timing circuitry  20 , input structures  22 , an input/output (I/O) interface  24 , network interfaces  26 , and/or a power source  28 . The various functional blocks shown in  FIG. 1  may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should be noted that  FIG. 1  is merely one example of a particular implementation and is intended to illustrate the types of components that may be present in the electronic device  10 . 
         [0029]    By way of example, the electronic device  10  may represent a block diagram of the notebook computer of  FIG. 2 , the handheld device of  FIG. 3 , or similar devices. In the electronic device  10  of  FIG. 1 , the processor(s)  12  and/or other data processing circuitry may be operably coupled with the memory  14  and the nonvolatile memory  16  to execute instructions. For instance, the processor(s)  12  may generate image data to be displayed on the display  18 . The display  18  may be a touch-screen liquid crystal display (LCD). In some embodiments, the electronic display  18  may be a Multi-Touch™ display that can detect multiple touches at once. The display  18  may use the efficient timing circuitry  20  to control the display while reducing the amount of power consumed by the display  18 . 
         [0030]    The input structures  22  of the electronic device  10  may enable a user to interact with the electronic device  10  (e.g., pressing a button to increase or decrease a volume level). The I/O interface  24  may enable electronic device  10  to interface with various other electronic devices, as may the network interfaces  26 . The network interfaces  26  may include, for example, interfaces for a personal area network (PAN), such as a Bluetooth network, for a local area network (LAN), such as an 802.11x Wi-Fi network, and/or for a wide area network (WAN), such as a  3 G or  4 G cellular network. The power source  28  of the electronic device  10  may be any suitable source of power, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. 
         [0031]    The electronic device  10  may take the form of a computer or other suitable type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. By way of example, the electronic device  10 , taking the form of a notebook computer  32 , is illustrated in  FIG. 2  in accordance with one embodiment of this disclosure. The depicted computer  32  may include a housing  34 , a display  18 , input structures  22 , and ports of an I/O interface  24 . In one embodiment, the input structures  22  (such as a keyboard and/or touchpad) may be used to interact with the computer  32 , such as to start, control, or operate a GUI or applications running on computer  32 . The display  18  may use the efficient timing circuitry  20  to generate internal timing signals. The efficient timing circuitry  20  of the display  18  reduces power consumption by precharging the timing signal to ground between higher voltage and lower voltage periods. 
         [0032]      FIG. 3  depicts a front view of a handheld device  36 , which represents one embodiment of the electronic device  10 . The handheld device  36  may represent, for example, a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. By way of example, the handheld device  36  may be a model of an iPod® or iPhone® available from Apple Inc. of Cupertino, Calif. In other embodiments, the handheld device  36  may be a tablet-sized embodiment of the electronic device  10 , which may be, for example, a model of an iPad® available from Apple Inc. 
         [0033]    The handheld device  36  may include an enclosure  38  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  38  may surround the display  18 . The I/O interfaces  24  may open through the enclosure  38  and may include, for example, a proprietary I/O port from Apple Inc. to connect to external devices. User input structures  40 ,  42 ,  44 , and  46 , in combination with the display  18 , may allow a user to control the handheld device  36 . For example, the input structure  40  may activate or deactivate the handheld device  36 , the input structure  42  may navigate a user interface to a home screen, a user-configurable application screen, and/or activate a voice-recognition feature of the handheld device  36 , the input structures  44  may provide volume control, and the input structure  46  may toggle between vibrate and ring modes. A microphone  48  may obtain a user&#39;s voice for various voice-related features, and a speaker  50  may enable audio playback and/or certain phone capabilities. A headphone input  52  may provide a connection to external speakers and/or headphones. The display  18  of the handheld device  36  may also use the efficient timing circuitry  20  to generate internal timing signals. The efficient timing circuitry  20  of the display  18  reduces power consumption by precharging the timing signal to ground between higher voltage and lower voltage periods. 
         [0034]    The display  18  may operate by activating and programming a number of picture elements, or pixels. These pixels may be generally arranged in a pixel array  100 , as shown in  FIG. 4 . The pixel array  100  of the display  18  may include a number of unit pixels  102  disposed in a pixel array or matrix. In the pixel array  100 , each unit pixel  102  may be defined by an intersection of gate lines  104  (also referred to as scanning lines) and source lines  106  (also referred to as data lines). Although only six unit pixels  102  are shown ( 102 A- 102 F), it should be understood that in an actual implementation, the pixel array  100  may include hundreds, thousands, or millions of such unit pixels  102 . Each of the unit pixels  102  may represent one of three subpixels that respectively filter only one color (e.g., red, blue, or green) of light. For purposes of this disclosure, the terms “pixel,” “subpixel,” and “unit pixel” may be used largely interchangeably. 
         [0035]    In the example of  FIG. 4 , each unit pixel  102  includes a thin film transistor (TFT)  108  for switching a data signal supplied to a respective pixel electrode  110 . The potential stored on the pixel electrode  110  relative to a potential of a common electrode  112  may generate an electrical field sufficient to alter the arrangement of a liquid crystal layer of the display  18 . When the arrangement of the liquid crystal layer changes, the amount of light passing through the pixel  102  also changes. A source  114  of each TFT  108  may connect to a source line  106  and a gate  116  of each TFT  108  may connect to a gate line  104 . A drain  118  of each TFT  108  may connect to a respective pixel electrode  110 . Each TFT  108  may serve as a switching element that may be activated and deactivated by a scanning or activation signal on the gate lines  104 . 
         [0036]    When activated, a TFT  108  may pass the data signal from its source line  106  onto its pixel electrode  110 . As noted above, the data signal stored by the pixel electrode  110  may be used to generate an electrical field between the respective pixel electrode  110  and a common electrode  112 . This electrical field may align the liquid crystal molecules within the liquid crystal layer to modulate light transmission through the pixel  102 . Thus, as the electrical field changes, the amount of light passing through the pixel  102  may increase or decrease. In general, light may pass through the unit pixel  102  at an intensity corresponding to the applied voltage from the source line  106 . 
         [0037]    These signals and other operating parameters of the display  18  may be controlled by integrated circuits (ICs) of the display  18 . These driver ICs of the display  18  may include a processor, microcontroller, or application specific integrated circuit (ASIC). The driver ICs may be chip-on-glass (COG) components on a TFT glass substrate, components of a display flexible printed circuit (FPC), and/or components of a printed circuit board (PCB) that is connected to the TFT glass substrate via the display FPC. Further, the driver ICs of the display  18  may include any suitable article of manufacture having one or more tangible, computer-readable media for storing instructions that may be executed by the driver ICs. 
         [0038]    For instance, a source driver integrated circuit (IC)  120  may receive image data  122  from the processor(s)  12  and send corresponding image signals to the unit pixels  102  of the pixel array  100 . The source driver  120  may also couple to a gate driver integrated circuit (IC)  124  that may activate or deactivate rows of unit pixels  102  via the gate lines  104 . As such, the source driver  120  may provide some timing signals  126  to the gate driver  124  to facilitate the activation and/or deactivation of individual lines (e.g., rows or columns) of pixels  102 . In other embodiments, timing information may be provided to the gate driver  124  in some other manner. In the illustrated example, the gate driver  124  uses efficient timing circuitry  20  to generate the gate clock signals provided over the gate lines  104 . 
         [0039]    The source driver IC  120  may also use the efficient timing circuitry  20  to generate timing signals for demultiplexers  128 . The source driver IC  120  may include a vast number of demultiplexers  128 , which may demultiplex the image data  122  that is subsequently provided to the source lines  106 . The number of demultiplexers  128  of the display  18  may be equal to the number of groupings of subpixels used to collectively generate a particular color in each line (e.g., several hundred or several thousand). The demultiplexers  128  may demultiplex RGB image data  122 , for example, into three separate demultiplexed components of red, green, and blue, each demultiplexer  128  doing so using three respective switches. As such, though not expressly pictured in  FIG. 4 , the efficient timing circuitry  20  may provide three separate timing signals to all of the demultiplexers  128  of the display  18 , one timing signal for each of the three switches of the demultiplexers  128 . 
         [0040]    One example of the efficient timing circuitry  20  appears in  FIG. 5 . In the example of  FIG. 5 , the timing signal is equal to the output voltage Vout. The output voltage Vout may be understood to be provided to display control circuitry of the display panel pixel array  100  (e.g., the gates  116  of the pixels  102 ) or of the display driver circuitry (e.g., the switches of the demultiplexers  128 ). Such display control circuitry is represented in  FIG. 5  as a capacitance C. The capacitance C may vary depending on the characteristics of the display control circuitry to which the output voltage Vout is provided. 
         [0041]    At any one time, the output voltage of the efficient timing circuitry  20  is either a higher voltage VGH, a lower voltage VGL, or ground GND. The higher voltage VGH is higher than ground GND and the lower voltage VGL is lower than ground GND. The efficient timing circuitry  20  includes a transistor M 1  that provides the higher voltage VGH to the output voltage Vout, a transistor M 2  that provides the lower voltage VGL to the output voltage Vout, and a transistor M 3  that provides the ground GND to the output voltage Vout. In the example of  FIG. 5 , the transistor M 1  is a PMOS transistor while the transistors M 2  and M 3  are NMOS transistors. In other embodiments, however, the transistors M 1 , M 2 , and M 3  may be any other suitable switches. 
         [0042]    Voltage control logic  140  may control the output voltage Vout of the efficient timing circuitry  20  by selectively activating and deactivating the transistors M 1 , M 2 , and M 3 . These are shown to be a first signal HI to control the transistor M 1 , a second signal LOW to control the transistor M 2 , and a third signal PRE to control the third transistor M 3 . The voltage control logic  140  may represent any suitable logic to generate the timing signals on the output voltage Vout a discussed in this disclosure. In some examples, the voltage control logic  140  may represent instructions executed on the processor(s)  12  of the electronic device  10  in which the electronic display  18  is installed. In other examples, the voltage control logic  140  may represent instructions executed on a processor (e.g., a microcontroller) of the electronic display  18 . In still other embodiments, the voltage control logic  18  may represent logic coded in hardware, such as a component of an applicant-specific integrated circuit (ASIC). 
         [0043]    The voltage control logic  140  may vary the output of the HI, LOW, and PRE control signals to efficiently alternate the voltage provided to the output voltage Vout between the high voltage VGH and the low voltage VGL, as illustrated in  FIGS. 6 and 7 . Both  FIGS. 6 and 7  will be described together for ease of explanation.  FIG. 6  represents a timing diagram  150 , in which a plot  152  shows the output voltage Vout, a plot  154  shows the HI control signal, a plot  156  shows the LOW signal, a plot  158  shows the PRE signal, and a timeline  160  illustrates time.  FIG. 7  represents a flowchart  162  that describes how the voltage control logic  140  may generate the output voltage Vout as shown in the timing diagram  150 . 
         [0044]    Specifically, the voltage control logic  140  may cause the lower voltage VGL supply to display control circuitry (e.g., gates  118  of pixels  102  or), represented as the capacitance C (block  164  of  FIG. 7 ). Indeed, as seen at the start of the timing diagram  150  of  FIG. 6 , the output voltage Vout is at the lower voltage VGL from time t 0  to until time t 1 . Thus, during this lower voltage period, the LOW signal is at a logic high, causing the NMOS transistor M 2  to be activated and connecting the capacitance C to the lower voltage VGL supply. Meanwhile, the HI control signal is at a logic high to cause the PMOS transistor M 1  to be deactivated. Likewise, the PRE control signal is at a logic low, causing the NMOS transistor M 3  to be deactivated as well. Thus, from the time t 0  to the time t 1 , substantially only the transistor M 2  is activated. 
         [0045]    The voltage control logic  140  next may disconnect the lower voltage VGL supply and precharge the capacitance C to ground (block  166  of  FIG. 7 ). For example, as seen in the timing diagram  150  of  FIG. 6 , the NMOS transistor M 2  is deactivated (LOW is set to a logic low) and the NMOS transistor M 3  is activated (PRE is set to a logic high) between time t 1  and time t 2 . This allows the capacitance C to precharge to ground GND. 
         [0046]    After this precharge period, the voltage control logic  140  may disconnect ground and connect the higher voltage VGH supply to the output voltage Vout for a subsequent higher voltage period (block  168  of  FIG. 7 ). The higher voltage period may begin at time t 2  of the timing diagram  150  of  FIG. 6 , in which the NMOS transistor M 3  is deactivated by the PRE control signal and the PMOS transistor M 1  become activated as the HI control signal becomes a logic LOW. Thus, from time t 2  to time t 3 , substantially only the PMOS transistor M 1  is activated and the resulting output voltage Vout is the higher voltage VGH. 
         [0047]    The voltage control logic  140  next may cause the efficient timing circuitry  20  to perform a discharging process over a discharge period, by disconnecting the higher voltage VGH supply and reconnecting to ground GND (block  170  of  FIG. 7 ). In the timing diagram  150  of  FIG. 6 , the discharge period may occur from time t 3  to time t 4 . Over this time period, the PMOS transistor M 1  is deactivated and the NMOS transistor M 3  activated once more. From the time t 3  to time t 4 , substantially only the NMOS transistor M 3  may be activated and the capacitance C may be discharged to ground GND. The process may repeat as the voltage control logic disconnects the output voltage Vout from ground GND, connecting instead the lower voltage VGL supply to begin another lower voltage period (block  172  of  FIG. 7 ). The subsequent lower voltage period is shown in the timing diagram  150  of  FIG. 6  as beginning at time t 4 . As can be seen, the NMOS transistor M 3  may be deactivated and the NMOS transistor M 2  activated once again, bringing the output voltage Vout back to VGL. 
         [0048]    By precharging and/or discharging the capacitance C to ground between transitions between the lower voltage VGL and the higher voltage VGH, a substantial amount of power may be saved. First, this disclosure will consider the case in which the capacitance C is not precharged or discharged to ground GND between periods of higher voltage VGH and lower voltage VGL. The total power consumed under these circumstances may be described by equations 1-4 below. Without first precharging to ground, the amount of charge Q 1  sourced from the higher voltage VGH supply, per cycle, could be described as follows: 
         [0000]        Q 1 =C (VGH−VGL)  (1).
 
         [0049]    Without first discharging to ground, the amount of charge Q 2  sunk into the lower voltage VGL supply, per cycle, could be described as follows: 
         [0000]        Q 2 =C (VGH−VGL)  (2).
 
         [0050]    As such, without precharging and discharging to ground between outputting the higher voltage VGH and lower voltage VGL, the total amount of energy consumed per cycle W could be described as follows: 
         [0000]        W=Q 1*VGH+ Q 2*(−VGL)= C *(VGH−VGL)̂2  (3).
 
         [0051]    In an example in which the magnitude of the lower voltage VGL is the same as the higher voltage VGH, the energy consumed per cycle W without precharging or discharging to ground could be described as follows: 
         [0000]        W= 4 *C *VGĤ2  (4).
 
         [0052]    Precharging and/or discharging to ground as described in this disclosure, however, may offer substantial power savings. When the efficient timing circuitry  20  precharges and discharges to ground between transitions, the total amount of charge Q 1  sourced from the higher voltage VGH supply per cycle may be lower: 
         [0000]        Q 1 =C *(VGH−0)  (5).
 
         [0053]    The total amount of charge Q 2  sunk into the lower voltage VGL supply per cycle may also be lower: 
         [0000]        Q 2 =C *(0−VGL)  (6).
 
         [0054]    As such, the amount of energy per cycle W consumed by the higher voltage VGH supply and the lower voltage VGL supply may be: 
         [0000]        W=Q 1*VGH+ Q 2*(−VGL)= C *(VGĤ2+VGL̂2)  (7).
 
         [0055]    Thus, when the magnitude of the lower voltage VGL supply is the same as the higher voltage VGH supply, the total amount of energy consumed per cycle W may be: 
         [0000]        W =2 *C *VGH ̂2  (8).
 
         [0056]    By comparing equation 8 to equation 4, it can be seen that the efficient timing circuitry  20  of  FIG. 5  may offer power savings of up to 50% in some examples. 
         [0057]    Different examples of the efficient timing circuitry  20  may also control the slew rate—the rise and fall times—of the timing signal provided as the output voltage Vout. For instance, as seen in  FIG. 8 , the efficient timing circuitry  20  may additionally include first slew rate control circuitry  180  and second slew rate control circuitry  182 . The first slew rate control circuitry  180  may couple an output of the PMOS transistor M 1  to the output voltage Vout. The second slew rate control circuitry  182  may couple the outputs of the transistors M 2  and M 3  to the output voltage Vout. Both the first slew rate control circuitry  180  and the second slew rate control circuitry  182  may include a number of slew rate control switches coupled in parallel (e.g., MH 1 , MH 2 , . . . , MHN, and ML 1 , ML 2 , . . . , MLN) and a number of resistors (e.g., RH 1 , RH 2 , . . . , RHN−1, and RL 1 , RL 2 , . . . , RLN−1). Each of the switches of the slew rate control circuitry  180  and  182  are coupled so as provided to different resistances when different switches are activated. In the first slew rate control circuitry  180 , the slew rate control switches MH 1 , MH 2 , . . . , MHN are PMOS transistors, and in the slew rate control circuitry  182 , the slew rate control switches ML 1 , ML 2 , . . . , MLN are NMOS transistors, but other embodiments may employ any other suitable switches. Any suitable number of switches and resistors may be used to achieve a desired slew rate resolution. 
         [0058]    Upper slew rate control logic  184  provides control signals SH 1 , SH 2 , . . . , SHN to respectively control the slew rate control switches MH 1 , MH 2 , . . . , MHN. Lower slew rate control logic  186  provides control signals SL 1 , SL 2 , . . . , SLN to respectively control the slew rate control switches ML 1 , ML 2 , . . . , MLN. The upper slew rate control logic  184  and the lower slew rate control logic  186  may control the rise and fall times of the output voltage Vout by gradually reducing the amount of resistance to the output voltage Vout. 
         [0059]    An example of controlling the slew rate using the upper slew rate control logic  184  and the lower slew rate control logic  186  appears in a timing diagram  190  of  FIG. 9 . In the timing diagram  190 , a plot  192  represents the output voltage Vout, a plot  194  represents the voltage control signal HI, a plot  196  represents the voltage control signal LOW, a plot  198  represents the voltage control signal PRE, a plot  200  represents the slew rate control signal SL 1 , a plot  202  represents the slew rate control signal SL 2 , a plot  204  represents the slew rate control signal SLN, a plot  206  represents the slew rate control signal SH 1 , a plot  208  represents the slew rate control signal SH 2 , and a plot  210  represents the slew rate control signal SHN. 
         [0060]    As seen in the timing diagram  190 , when the output voltage Vout is to be precharged to ground, the lower slew rate control logic  186  may control the rise time of the output voltage Vout. Specifically, once the PRE signal goes high (at time t 1 ) and activates the NMOS transistor M 3 , the lower slew rate control logic  186  may progressively cause the slew rate control signals SLN, SL 2 , and SL 1  to become high (at times t 1 , t 2 , and t 3 ), thereby activating the corresponding slew rate control switches MLN, ML 2 , and ML 1 , and gradually reducing the number of resistors between the ground voltage GND and the output voltage Vout. As this resistance decreases, the output voltage Vout increases accordingly. At time t 4 , the PRE signal may go low, as may the slew rate control signals SLN, SL 2 , and SL 1 . 
         [0061]    The upper slew rate control circuitry  184  may thereafter control the rise time of the output voltage Vout. Namely, at time t 4 , the voltage control signal HI may go low, activating the PMOS transistor M 1 . The upper slew rate control logic  184  may progressively cause the slew rate control signals SHN, SH 2 , and SH 1  to become low (at times t 4 , t 5 , and t 6 ), thereby activating the corresponding slew rate control switches MHN, MH 2 , and MH 1 , and gradually reducing the number of resistors between the higher voltage VGH and the output voltage Vout. As this resistance decreases, the output voltage Vout increases accordingly. At time t 7 , the slew rate control signals other than SH 1  may be switched to a logic high, thereby deactivating the all but the transistor MH 1  for the remaining duration of the higher voltage period. It should be appreciated that a similar process may take place to control the fall time when transitioning from the higher voltage period to the ground discharge period to the lower voltage period, except that only the lower slew rate control circuitry  182  may be used. 
         [0062]    The efficient timing circuitry  20  may, additionally or alternatively, employ shared slew rate control circuitry  220  as shown in  FIG. 10 . In the example of  FIG. 10 , the voltage control logic  140  controls the voltage control transistors M 1 , M 2 , and M 3  using the HI, LOW, and PRE signals, respectively, as described above. The voltage control transistors M 1 , M 2 , and M 3  share a common output that connects to shared slew rate circuitry  220 . The shared slew rate control circuitry  220  may include any suitable number of pass-gate switches  222 A,  222 B, . . . ,  222 N, which may receive slew rate control signals S 1 , S 2 , . . . , SN, respectively (and/or the inverse of these signals). The shared slew rate control circuitry  220  also includes any suitable number and values of resistors R 1 , R 2 , . . . , RN−1). Each of the switches of the shared slew rate control circuitry  220  are coupled so as provided to different resistances when different switches are activated. 
         [0063]    Shared slew rate control logic  224  may provide the slew rate control signals S 1 , S 2 , . . . , SN. The shared slew rate control logic  224  may be implemented as instructions running on the processor(s)  12  or a microcontroller of the electronic display  18 , and/or which may be hardware (e.g., a component of an application specific integrated circuit (ASIC)). The shared slew rate control logic  224  may control the slew rate of the timing signal output as the output voltage Vout in any suitable way, including as generally described above with reference to the timing diagram  190  of  FIG. 9 . 
         [0064]    Whether the efficient timing circuitry  20  includes the first and second slew rate control circuitry  180  and  182 , or the shared slew rate control circuitry  220 , the efficient timing circuitry  20  may provide a timing signal having a desired slew rate. One method of doing so appears as a flowchart  230  of  FIG. 11 . The flowchart  230  may begin when the lower voltage VGL supply is currently being provided as the output voltage Vout over a lower voltage period (block  232 ). The lower voltage VGL supply next may be disconnected and the capacitance C may be precharged to ground via the second slew rate control circuitry  182  or the shared slew rate control circuitry  220  (block  234 ). During this precharge period, the second slew rate control circuitry  182  or the shared slew rate control circuitry  220  may control a rise time from the lower voltage VGL to ground GND. Ground then may be disconnected and the higher voltage VGH supply connected via the first slew rate control circuitry  180  or the shared slew rate control circuitry  220  (block  236 ). The first slew rate control circuitry  180  or the shared slew rate control circuitry  220  may control a rise time from ground GND to the higher voltage VGH as desired, before the higher voltage VGH is maintained for the duration of a higher voltage period of the timing signal. 
         [0065]    After the higher voltage period, the higher voltage VGH supply may be disconnected, and ground GND may be connected via the second slew rate control circuitry  180  or the shared slew rate control circuitry  220  during a discharge period (block  238 ). The second slew rate control circuitry  180  or the shared slew rate control circuitry  220  may control a fall time from the higher voltage VGH to ground GND. Following the discharge period, ground GND may be disconnected and the lower voltage VGL supply may be connected via the second slew rate control circuitry  180  or the shared slew rate control circuitry  220  (block  240 ). The second slew rate control circuitry  180  or the shared slew rate control circuitry  220  may control a fall time from ground GND to the lower voltage VGL. Thereafter, the output voltage Vout may be maintained at the lower voltage VGL for the duration of this subsequent lower voltage period. 
         [0066]    Technical effects of this disclosure include a substantial power savings by precharging and/or discharging a timing signal to ground between periods of output voltages higher than ground and output voltages lower than ground. In addition, by controlling the slew rate of the output voltage, certain display panel properties may be controlled despite the reduction in power consumption. In some instances, power consumption due to certain timing signals may be reduced by up to half. 
         [0067]    The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.