Patent Publication Number: US-9851839-B2

Title: Display system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a display device. One embodiment of the present invention relates to a display system including a display device. One embodiment of the present invention relates to a method for driving a display device. 
     Note that one embodiment of the present invention is not limited to the above technical field. Examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them. 
     2. Description of the Related Art 
     Nowadays, information processing devices each provided with an input unit, a display unit, and an arithmetic portion are prevalent. 
     As the display unit, a variety of display units such as a liquid crystal display device, a display device including an organic electroluminescent (EL) element, or electronic paper is used. 
     A method is known in which an information processing device including a display unit and an input unit is driven in the following steps: a first step of acquiring an input signal with the input unit, a second step of starting the movement of an image displayed on the display unit in accordance with the input signal, a third step of reducing the luminance of the image, a fourth step of judging whether the image has reached predetermined coordinates or not, a fifth step of increasing the luminance of the image when it is determined that the image has reached the predetermined coordinates, and a sixth step of stopping the movement of the image. This method can reduce eye fatigue of a user and achieve eye-friendly display (Patent Document 1). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2014-115641 
     SUMMARY OF THE INVENTION 
     An object is to provide a novel display device or display system which is highly convenient or reliable. Another object is to achieve eye-friendly display that can reduce eye strain on a user. 
     Another object is to provide a novel display device, a novel display system, or a novel semiconductor device. 
     Note that the description of these objects does not disturb the existence of other objects. One embodiment of the present invention does not necessarily achieve all the objects. Other objects can be derived from the description of the specification and the like. 
     In a first step of a method or program of one embodiment of the present invention, the setting is initialized. 
     In a second step, interrupt processing is allowed. 
     In a third step, image information is displayed in a predetermined mode selected in the first step or in the interrupt processing. 
     In a fourth step, the next step is determined as follows: a fifth step is selected when a termination instruction has been supplied, whereas the third step is selected when the termination instruction has not been supplied. 
     In the fifth step, processing is terminated. 
     The interrupt processing includes the following sixth to eleventh steps. 
     In the sixth step, the processing proceeds to the seventh step when a predetermined event has been supplied, whereas the processing proceeds to the eleventh step when the predetermined event has not been supplied. 
     In the seventh step, the processing proceeds to the eighth step when image information to be displayed next has a predetermined contrast, whereas the processing proceeds to the tenth step when the image information to be displayed next does not have the predetermined contrast. 
     In the eighth step, the processing proceeds to the ninth step when the proportion of the area of a dark portion in the image information to be displayed next is higher than or equal to a predetermined proportion, whereas the processing proceeds to the tenth step when the proportion of the area of the dark portion is lower than the predetermined proportion. 
     In the ninth step, a first mode is selected. 
     In the tenth step, a second mode is selected. 
     In the eleventh step, the processing returns from the interrupt processing. 
     In this manner, eye strain on a user at the time of switching displayed image information in accordance with a predetermined event such as scrolling can be reduced, whereby eye-friendly display for the user can be achieved. Thus, a novel program which is highly convenient or reliable can be provided. 
     One embodiment of the present invention is a display system including a display portion, an input portion, and a control portion. The display portion is configured to display an image. The input portion is configured to sense an input from a user and output a signal to the control portion. The control portion is configured to execute a first mode and a second mode. In the first mode executed by the control portion, an image is displayed on the display portion by an interlace method. In the second mode executed by the control portion, an image is displayed on the display portion by a progressive method. The control portion is configured to switch between the first mode and the second mode in accordance with the signal. 
     In the above display system, it is preferable that the control portion execute the first mode when the input is a first input which corresponds to screen switching or screen scrolling and that the control portion execute the second mode when there is no input or the input is not the first input. 
     The above display system preferably has the following configuration. The control portion includes an arithmetic portion and a memory portion. The memory portion is configured to temporarily store image data every time the image data is input. The image data includes data of one frame including 2n lines (n is a natural number). In the second mode, the arithmetic portion is configured to sequentially read out data from first to 2n-th lines of first image data and output the data to the display portion in a first frame period, and sequentially read out data from first to 2n-th lines of second image data and output the data to the display portion in a second frame period following the first frame period. In the first mode, a third frame period includes a first field period and a second field period. The arithmetic portion is configured to sequentially read out data from odd-numbered lines of third image data and output the data to the display portion in the first field period, and sequentially read out data from even-numbered lines of fourth image data and output the data to the display portion in the second field period. 
     Alternatively, the above display system preferably has the following configuration. The control portion includes an arithmetic portion and a memory portion. The memory portion is configured to temporarily store image data every time the image data is input. The image data includes data of one frame including 2n lines (n is a natural number). In the second mode, the arithmetic portion is configured to sequentially read out data from first to 2n-th lines of first image data and output the data to the display portion in a first frame period, and sequentially read out data from first to 2n-th lines of second image data and output the data to the display portion in a second frame period following the first frame period. In the first mode, a third frame period includes a field periods (a is an integer of 2 or more and 2n or less). The arithmetic portion is configured to sequentially read out data from {a×(m−1)+k}-th lines (k is an integer of 1 or more and a or less; m is an integer of 1 or more and n or less) of k-th image data and output the data to the display portion in a k-th field period. 
     The above display system preferably has the following configuration. The display portion includes a liquid crystal element or a light-emitting element. The display portion includes a plurality of pixels. The pixels each include a transistor. A semiconductor of the transistor in which a channel is formed includes an oxide semiconductor. 
     Alternatively, the above display system preferably has the following configuration. The display portion includes a liquid crystal element or a light-emitting element. The display portion includes a plurality of pixels. The pixels each include a transistor. A semiconductor of the transistor in which a channel is formed includes silicon. In particular, the semiconductor is amorphous silicon or polycrystalline silicon. 
     The input portion preferably includes at least one of a keyboard, a hardware button, a pointing device, a touch sensor, an imaging device, an audio input device, a viewpoint input device, and a pose detection device. 
     The display portion and the input portion preferably form a touch panel. 
     Another embodiment of the present invention is a program including first to eleventh steps. In the first step, the setting is initialized. In the second step, interrupt processing is allowed. In the third step, image information is displayed in a predetermined mode selected in the first step or in the interrupt processing. In the fourth step, processing proceeds to the fifth step when a termination instruction has been supplied in the interrupt processing, whereas the processing proceeds to the third step when the termination instruction has not been supplied in the interrupt processing. In the fifth step, the processing is terminated. The interrupt processing includes the sixth to eleventh steps. In the sixth step, the processing proceeds to the seventh step when a predetermined event has been supplied, whereas the processing proceeds to the eleventh step the predetermined event has not been supplied. In the seventh step, the processing proceeds to the eighth step when image information to be displayed next has a predetermined contrast, whereas the processing proceeds to the tenth step when the image information to be displayed next does not have the predetermined contrast. In the eighth step, the processing proceeds to the ninth step when the proportion of the area of a dark portion in the image information to be displayed next is higher than or equal to a predetermined proportion, whereas the processing proceeds to the tenth step when the proportion of the area of the dark portion is lower than the predetermined proportion. In the ninth step, a first mode is selected. In the tenth step, a second mode is selected. In the eleventh step, the processing returns from the interrupt processing. In the first mode, the image information is displayed by an interlace method. In the second mode, the image information is displayed by a progressive method. 
     According to one embodiment of the present invention, a novel display device or display system which is highly convenient or reliable can be provided. Furthermore, eye-friendly display that can reduce eye strain on a user can be achieved. 
     Furthermore, a novel display device, a novel display system, or a novel semiconductor device can be provided. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects. Other effects can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a configuration example of a display system according to an embodiment. 
         FIGS. 2A and 2B  illustrate a configuration example of a display system according to an embodiment. 
         FIGS. 3A to 3C  illustrate data reading methods according to an embodiment. 
         FIGS. 4A to 4C  each illustrate image rewriting timing according to an embodiment. 
         FIGS. 5A and 5B  illustrate examples of image display according to an embodiment. 
         FIG. 6  illustrates an example of image display according to an embodiment. 
         FIGS. 7A to 7D  schematically illustrate an optic nerve and a transfer function according to an embodiment. 
         FIGS. 8A to 8D  schematically illustrate a visual transfer function according to an embodiment. 
         FIG. 9  is a flow chart illustrating a program according to an embodiment. 
         FIG. 10  is a flow chart illustrating a program according to an embodiment. 
         FIGS. 11A-1, 11A-2, 11B-1, 11B-2, 11C-1, and 11C-2  schematically illustrate examples of a scroll instruction according to an embodiment. 
         FIGS. 12A to 12C  schematically illustrate a configuration of image information according to an embodiment. 
         FIGS. 13A and 13B  illustrate a configuration example of a display device according to an embodiment. 
         FIG. 14  illustrates a structure example of a display device according to an embodiment. 
         FIG. 15  illustrates a structure example of a display device according to an embodiment. 
         FIG. 16  illustrates a structure example of a display device according to an embodiment. 
         FIGS. 17A and 17B  each illustrate a structure example of a display device according to an embodiment. 
         FIGS. 18A and 18B  each illustrate a structure example of a display device according to an embodiment. 
         FIG. 19  illustrates a structure example of a display device according to an embodiment. 
         FIG. 20  illustrates a structure example of a display device according to an embodiment. 
         FIG. 21  illustrates a structure example of a display device according to an embodiment. 
         FIG. 22  illustrates a structure example of a display device according to an embodiment. 
         FIG. 23  illustrates a structure example of a display device according to an embodiment. 
         FIG. 24  illustrates a structure example of a display device according to an embodiment. 
         FIG. 25  illustrates a structure example of a display device according to an embodiment. 
         FIG. 26  illustrates a structure example of a display device according to an embodiment. 
         FIG. 27  illustrates a structure example of a display device according to an embodiment. 
         FIG. 28  illustrates a structure example of a display device according to an embodiment. 
         FIG. 29  illustrates a structure example of a display device according to an embodiment. 
         FIG. 30  illustrates a structure example of a display device according to an embodiment. 
         FIG. 31  illustrates a structure example of a display device according to an embodiment. 
         FIGS. 32A and 32B  illustrate a structure example of a display device according to an embodiment. 
         FIGS. 33A and 33B  illustrate a structure example of a display device according to an embodiment. 
         FIGS. 34A to 34C  each illustrate a pixel structure according to an embodiment. 
         FIGS. 35A and 35B  are a block diagram and a timing chart, respectively, of a touch sensor according to an embodiment. 
         FIG. 36  is a circuit diagram of a touch sensor according to an embodiment. 
         FIGS. 37A and 37B  illustrate pixels provided with touch sensors according to an embodiment. 
         FIGS. 38A and 38B  illustrate the operation of touch sensors and pixels according to an embodiment. 
         FIG. 39  illustrates a display module according to an embodiment. 
         FIGS. 40A to 40G  each illustrate an electronic device according to an embodiment. 
         FIGS. 41A to 41C  show changes in display luminance according to Example. 
         FIGS. 42A to 42C  show the calculation results of changes in visual stimulation according to Example. 
         FIGS. 43A and 43B  show the measurement results of critical fusion frequencies according to Example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention includes, for example, a step of selecting a first mode or a second mode and a step of performing display in the selected mode. 
     For example, one embodiment of the present invention can include a step of selecting the first mode or the second mode when a scroll event occurs, in accordance with the contrast between a dark portion and a bright portion or the proportion of the area of the dark portion in image information to be displayed. 
     &lt;&lt;First Mode&gt;&gt; 
     In the case where the first mode is selected, image information to be displayed next is displayed by the following method to reduce visual stimulation. 
     For example, the image information to be displayed next is displayed such that an influence of lateral inhibition caused by presently displayed image information may be avoided. 
     In the first mode, for example, an image is displayed by an interlace method. 
     &lt;&lt;Second Mode&gt;&gt; 
     In the case where the second mode is selected, image information is displayed by the following method. 
     In the second mode, for example, an image is displayed by a progressive method. 
     In this manner, eye strain on a user at the time of switching displayed image information in accordance with a predetermined event such as scrolling can be reduced, whereby eye-friendly display for the user can be achieved. Thus, a novel program which is highly convenient or reliable can be provided. 
     &lt;Display Method in which Influence of Lateral Inhibition is Avoided&gt; 
     A display method in which an influence of lateral inhibition is avoided will be described with reference to  FIGS. 7A to 7D  and  FIGS. 8A to 8D . 
       FIGS. 7A to 7D  schematically illustrate an optic nerve and a visual transfer function.  FIGS. 7A and 7B  schematically illustrate a positional relation between a display device and a user of the display device.  FIG. 7C  schematically illustrates an example of stimuli applied to an optic nerve when image information is switched from one to another.  FIG. 7D  schematically illustrates responses to the applied stimuli which are transformed in accordance with the visual transfer function. Note that the vertical axis L in  FIG. 7C  represents the brightness, where the brightness to which the eyes are adapted is assumed to be 0. The vertical axis S in  FIG. 7D  represents the intensity of a response. 
       FIGS. 8A to 8D  schematically illustrate an optic nerve and a visual transfer function.  FIG. 8A  schematically illustrates an example of stimuli applied to an optic nerve when image information is switched from one to another.  FIG. 8B  schematically illustrates responses to the applied stimuli which are transformed in accordance with the visual transfer function.  FIGS. 8C and 8D  each schematically illustrate the display method of one embodiment of the present invention, in which amplification of responses to applied stimuli can be suppressed. 
     In the first mode, for example, image information is switched from one to another at a time interval of 100 msec or longer, preferably 150 msec or longer, whereby an influence of lateral inhibition can be avoided. Thus, amplification of responses to visual stimuli can be suppressed. 
     &lt;&lt;Lateral Inhibition&gt;&gt; 
     A neuron of a stimulated optic nerve is capable of inhibiting activities of adjacent other neurons. This phenomenon may cause transformation of responses to a pulsed visual stimulus. 
     For example, a bright image and a dark image are displayed in a pulsed manner in a region which is on a plane at a distance of 40 cm from the user&#39;s eye and has a diameter of 100 μm (see  FIG. 7C ). Note that the size of one photoreceptor cell (CELL) corresponds to that of a region which is on a plane at a distance of 40 cm from the user&#39;s eye and has a diameter of approximately 100 μm (see  FIGS. 7A and 7B ). 
     In some cases, a pulsed stimulus is transformed into wave-shaped responses in accordance with the visual transfer function (see  FIGS. 7C and 7D ). Specifically, a pulsed positive visual stimulus is transformed into a positive response accompanied with a negative response, whereas a pulsed negative visual stimulus is transformed into a negative response accompanied with a positive response (David C. Burr and M. Concetta Morrone, “Impulse-response functions for chromatic and achromatic stimuli,” Journal of Optical Society of America, 1993, Vol. 10, No. 8, p. 1706). 
     When a bright image and a dark image are sequentially displayed at a sufficiently short time interval, for example, a response to the preceding stimulus and a response to the following stimulus are both wave-shaped. Accordingly, these waves may be superimposed on each other to increase the amplitude. 
     For example, pulsed bright first image information is displayed, and 50 msec later, pulsed dark second image information is displayed. In this case, a negative response which follows a positive response to the displayed first image information may be superimposed on a negative response to the displayed second image information. Accordingly, a significantly amplified negative response may be formed (see  FIGS. 8A and 8B ). 
     In the first mode, for example, displayed image information from one to another at a time interval of 100 msec or longer, preferably 150 msec or longer, whereby an influence of wave-shaped responses caused by the visual transfer function can be avoided. Thus, amplification of responses to visual stimuli can be suppressed (see  FIG. 8C ). 
     As another example, in the first mode, displayed image information is switched from one to another with intermediate image information displayed therebetween. Specifically, a gray image or an image with a gray level between that of the preceding image information and that of the following image information (a halftone image) can be used for the intermediate image information (see  FIG. 8D ). Thus, wave-shaped responses to the preceding stimulus can be canceled by wave-shaped responses to the following stimulus, thereby weakening in amplitude. 
     Alternatively, intermediate image information can be obtained by displaying images in such a manner that the preceding image information fades out while the following image information fades in (this technique is also referred to as cross-fade). 
     In this manner, an influence of lateral inhibition can be avoided. Thus, amplification of responses to visual stimuli can be suppressed. 
     As another example, a display element may be overdriven in the second mode, whereas the overdrive may be turned down or stopped in the first mode. Specifically, the overdrive of a liquid crystal element may be stopped in the first mode, whereas the liquid crystal element may be overdriven in the second mode. 
     &lt;Program Example&gt; 
     One embodiment of the present invention will be described using a program of one embodiment of the present invention with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9  is a flow chart illustrating main processing of the program of one embodiment of the present invention.  FIG. 10  is a flow chart illustrating interrupt processing of the program of one embodiment of the present invention. 
     The program of one embodiment of the present invention includes the following eleven steps (see  FIG. 9  and  FIG. 10 ). 
     In a first step (S 1 ), the setting is initialized. For example, the first mode or the second mode is set as initial setting, and a predetermined image is loaded. 
     In a second step (S 2 ), interrupt processing is allowed. Note that an arithmetic device allowed to execute the interrupt processing can perform the interrupt processing in parallel with the main processing. The arithmetic device which has returned from the interrupt processing to the main processing can reflect the results of the interrupt processing in the main processing. 
     The arithmetic device may execute the interrupt processing when a counter has an initial value, and the counter may be set at a value other than the initial value when the arithmetic device returns from the interrupt processing. Thus, the interrupt processing is ready to be executed after the program is started up. 
     In a third step (S 3 ), image information is displayed in a predetermined mode selected in the first step (S 1 ) or in the interrupt processing. 
     In a fourth step (S 4 ), the next step is determined as follows: a fifth step (S 5 ) is selected when a termination instruction has been supplied, whereas the third step (S 3 ) is selected when the termination instruction has not been supplied. 
     In the fifth step (S 5 ), processing is terminated. 
     The interrupt processing includes the following sixth to eleventh steps (T 6  to T 11 ) (see  FIG. 10 ). 
     In the sixth step (T 6 ), the processing proceeds to the seventh step (T 7 ) when a predetermined event has been supplied, whereas the processing proceeds to the eleventh step (T 11 ) when the predetermined event has not been supplied. 
     In the seventh step (T 7 ), the processing proceeds to the eighth step (T 8 ) when image information to be displayed next has a predetermined contrast, whereas the processing proceeds to the tenth step (T 10 ) when the image information to be displayed next does not have the predetermined contrast. 
     In the eighth step (T 8 ), the processing proceeds to the ninth step (T 9 ) when the proportion of the area of a dark portion in the image information to be displayed next is higher than or equal to a predetermined proportion, whereas the processing proceeds to the tenth step (T 10 ) when the proportion of the area of the dark portion is lower than the predetermined proportion. 
     In the ninth step (T 9 ), the first mode is selected. 
     In the tenth step (T 10 ), the second mode is selected. 
     In the eleventh step (T 11 ), the processing returns from the interrupt processing. 
     &lt;&lt;Predetermined Event&gt;&gt; 
     A variety of instructions can be associated with a variety of events. 
     The following instructions can be given as examples: “page-turning instruction” for switching displayed image information from one to another and “scroll instruction” for moving the display position of part of image information and displaying another part continuing from that part. 
     For example, the following events can be used: events supplied using a pointing device such as a mouse (e.g., “click” and “drag”) and events supplied to a touch panel with a finger or the like used as a pointer (e.g., “tap”, “drag” and “swipe”). 
     For example, the position of a thumb (also referred to as a handle or knob) of a scroll bar pointed by a pointer, the swipe speed, and the drag speed can be used as parameters assigned to various instructions. 
     Specifically, a parameter that determines the page-turning speed or the like can be used to execute the “page-turning instruction” and a parameter that determines the moving speed of the display position or the like can be used to execute the “scroll instruction.” 
     Furthermore, the display brightness or contrast may be changed in accordance with the page-turning speed and/or the scroll speed, for example. Specifically, in the case where the page-turning speed and/or the scroll speed are/is higher than the speed at which user&#39;s eyes can follow displayed images, the display brightness or contrast may be decreased in synchronization with the page-turning speed and/or the scroll speed. 
     &lt;&lt;Scroll Instruction&gt;&gt; 
     Examples of a scroll instruction for moving the display position of image information at various speeds will be described with reference to  FIGS. 11A-1, 11A-2 ,  11 B- 1 ,  11 B- 2 ,  11 C- 1 , and  11 C- 2 . In the scroll instruction, for example, the speed at which a touch panel is swiped can be used to determine the moving speed of the display position. 
       FIGS. 11A-1, 11B-1, and 11C-1  each schematically illustrate a scroll instruction for moving the display position of image information at a time-varying speed V. 
       FIG. 11A-2  illustrates a method for adjusting the brightness L of a bright portion of the image information whose display position is moved at the speed shown in  FIG. 11A-1 . 
       FIG. 11B-2  illustrates a method for adjusting the brightness L of the bright portion of the image information whose display position is moved at the speed shown in  FIG. 11B-1 . 
       FIG. 11C-2  illustrates a method for adjusting the brightness L of the bright portion of the image information whose display position is moved at the speed shown in  FIG. 11C-1 . 
     &lt;&lt;Example 1 of Scroll Instruction&gt;&gt; 
     Described will be an example of a scroll instruction in which the moving speed of the display position of the image information is increased from 0 to V1 in a period from Time T 11  to Time T 12  (see  FIGS. 11A-1 and 11A-2 ). 
     For example, in a period until Time T 11 , in which the display position of the image information does not change, the bright portion is displayed at Brightness L 1 . 
     In the period from Time T 11  to Time T 12 , in which the display position of the image information is moved at an increasing speed, the bright portion is displayed at a brightness changing between Brightness L 1  and Brightness L 3 , which is lower than Brightness L 1 . 
     In a period after Time T 12 , in which the display position of the image information is moved constantly at Speed V1, the bright portion is displayed at Brightness L 2 , which is lower than Brightness L 1  and higher than Brightness L 3 . 
     &lt;&lt;Example 2 of Scroll Instruction&gt;&gt; 
     Described will be an example of a scroll instruction in which the moving speed of the display position of the image information is decreased from V1 to 0 in a period from Time T 13  to Time T 14  (see  FIGS. 11B-1 and 11B-2 ). 
     For example, in a period until Time T 13 , in which the display position of the image information is moved constantly at Speed V1, the bright portion is displayed at Brightness L 2 . 
     In the period from Time T 13  to Time T 14 , in which the display position of the image information is moved at a decreasing speed, the bright portion is displayed at a brightness increasing from Brightness L 2 . 
     In a period from Time T 14  to Time T 15 , in which the display position of the image information is fixed, the bright portion is displayed at a brightness increasing to predetermined Brightness L 1 , which is higher than Brightness L 2 . Note that the length of the period from Time T 14  to Time T 15  is preferably 0 or longer. 
     &lt;&lt;Example 3 of Scroll Instruction&gt;&gt; 
     The following scroll instruction will be described as an example. The display position of the image information is moved at a speed increasing from 0 to V2 in a period from Time T 16  to Time T 17  and moved at Speed V2 in a period from Time T 17  to Time T 18 . Then, the display position of the image information is moved at a speed decreasing from V2 to V1 in a period from Time T 18  to Time T 19  and moved at Speed V1 in a period after Time T 19  (see  FIGS. 11C-1 and 11C-2 ). 
     For example, in a period until Time T 16 , in which the display position of the image information does not change, the bright portion is displayed at Brightness L 1 . 
     In the period from Time T 16  to Time T 17 , in which the display position of the image information is moved at an increasing speed, the bright portion is displayed at a brightness changing from Brightness L 1  to Brightness L 3 , which is lower than Brightness L 1 . 
     In the period from Time T 17  to Time T 18 , in which the display position of the image information is moved constantly at Speed V2, the bright portion is displayed at Brightness L 3 . 
     In the period from Time T 18  to Time T 19 , in which the display position of the image information is moved at a decreasing speed, the bright portion is displayed at a brightness increasing from Brightness L 3 . 
     In the period after Time T 19 , in which the display position of the image information is moved constantly at Speed V1, which is lower than Speed V2, the bright portion is displayed at Brightness L 2 , which is lower than Brightness L 1  and higher than Brightness L 3 . 
     &lt;&lt;Condition for Mode Selection&gt;&gt; 
     A method in which characteristics of image information to be displayed next are used as conditions for mode selection will be described with reference to  FIGS. 12A to 12C . 
       FIG. 12A  schematically illustrates image information including a dark portion and a bright portion. 
       FIG. 12B  schematically illustrates the area ratio in terms of brightness (or luminance, light intensity, or the like) in the image information to be displayed next. Note that the horizontal axis represents the normalized brightness, where the lowest brightness and the highest brightness of the display device are 0 and 1, respectively. 
       FIG. 12C  is a diagram (or a histogram) showing the results of determining the area ratio in terms of brightness in a general document in which, for example, texts are printed on white paper. Note that the horizontal axis represents the normalized brightness, where the brightness at which the proportion of the area of the bright portion peaks is 1. 
     Specifically, the case where the contrast or the proportion of the area of the dark portion in the image information to be displayed next is used as a condition for mode selection will be described. 
     &lt;&lt;Contrast&gt;&gt; 
     For example, the first mode can be selected depending on whether the contrast in the image information to be displayed next exceeds a predetermined value or not. 
     Specifically, in the image information, a region with a normalized brightness higher than or equal to 0 and lower than or equal to 0.3 is defined as a dark portion, and a region with a normalized brightness higher than or equal to 0.7 and lower than or equal to 1.0 is defined as a bright portion. The mode can be selected depending on whether the image information includes both the bright portion and the dark portion or not. 
     For example, image information including a region with a normalized brightness of 0.2 and a region with a normalized brightness of 0.95 satisfies the condition for mode selection (see  FIG. 12B ). 
     In the case where the contrast in the image information to be displayed next is lower than that in a general document in which, for example, texts are printed on white paper (see  FIG. 12C ), the second mode may be selected because only a little visual stimulation is caused by display change. 
     &lt;&lt;Proportion of Area of Dark Portion&gt;&gt; 
     As a condition for mode selection, for example, it is also possible to use the proportion of the area of the dark portion in the image information to be displayed next. 
     Specifically, the mode can be selected depending on whether the dark portion occupies 30% or more of the image information or not. 
     For example, image information in which the proportion of the area of a region with a normalized brightness of 0.2 is 35% satisfies the condition for mode selection (see  FIG. 12B ). 
     In the case where the proportion of the area of the dark portion in the image information to be displayed next is lower than that in a general document in which, for example, texts are printed on white paper (see  FIG. 12C ), the second mode may be selected because only a little visual stimulation is caused by display change. 
     Embodiments will be described in detail with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. 
     In the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated. Furthermore, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     In each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such scales. 
     In this specification and the like, ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the components numerically. 
     A transistor is a kind of semiconductor element and enables amplification of current or voltage, a switching operation for controlling conduction or non-conduction, or the like. A transistor in this specification includes an insulated-gate field-effect transistor (IGFET) and a thin film transistor (TFT). 
     Embodiment 1 
     In this embodiment, a display system of one embodiment of the present invention will be described with reference to drawings. 
     Configuration Example 
       FIG. 1A  is a block diagram illustrating the configuration of a display system  10  of one embodiment of the present invention. 
     The display system  10  includes a control portion  11 , an input/output device  12 , a sensor controller  13 , and the like. The input/output device  12  includes a display portion  21  and an input portion  22 . 
     The display portion  21  has a function of displaying an image. As the display portion  21 , typically, a display device including an optical element such as a liquid crystal element or a MEMS element, a display device including a light-emitting element such as an organic electroluminescent (EL) element, or the like can be used. 
     A device which detects an input from a user can be used as the input portion  22 . For example, an input device such as a keyboard, a hardware button, a pointing device, a touch sensor, an imaging device, an audio input device, a viewpoint input device, or a pose detection device can be used. The input portion  22  may include two or more of these input devices. With a touch sensor or the like, a variety of gestures (e.g., tap, drag, swipe, and pinch in) can be detected. When an acceleration sensor is used as a pose detection device, the pose (e.g., inclination, displacement, or deformation) of a device including the display system  10  can be detected as input motion. 
     In particular, a touch panel which includes the display portion  21  and the input portion  22  is preferably used as the input/output device  12 . For example, a touch panel in which a touch sensor is mounted on a display device (or a display panel) is preferably used. 
     The sensor controller  13  has functions of driving the input portion  22  and outputting an output signal of the input portion  22  to the control portion  11 . In the case where the input portion  22  has the above functions of the sensor controller  13 , the sensor controller  13  may be omitted and a signal may be directly output from the input portion  22  to the control portion  11 . 
     The control portion  11  has a function of generating an image to be displayed on the display portion  21 . The control portion  11  also has a function of outputting an image signal to the display portion  21 . The display portion  21  can display an image in accordance with the image signal. 
     Image signals may be sequentially input from an image output device to the control portion  11 . For example, a reproducing device or a recording/reproducing device for a memory medium such as a Blu-ray Disc, a digital versatile disc (DVD), a flash memory, or the like, or a recording/reproducing device including a memory device such as a hard disk drive (HDD) or a solid state drive (SSD) can be used as the image output device. As the image signal, for example, an output signal of a tuner which receives broadcasting or image data such as streaming images delivered via a network can be used. The control portion  11  can convert an input image signal into an image to be displayed on the display portion  21  and can output the converted image signal. 
     The control portion  11  has functions of analyzing a signal which is input from the input portion  22  through the sensor controller  13  and executing the first mode or the second mode which is switched in accordance with the signal. 
     In the first mode executed by the control portion  11 , an image is displayed on the display portion  21  by an interlace method. In the second mode executed by the control portion  11 , an image is displayed on the display portion  21  by a progressive method. 
       FIG. 1B  illustrates a specific configuration example of the control portion  11  and the display portion  21 . The control portion  11  includes an arithmetic portion  41  and a memory portion  42 . The display portion  21  includes a pixel portion  31  and a driver portion  32 . 
     The memory portion  42  has a function of temporarily storing data of an image (hereinafter also referred to as image data) generated by the arithmetic portion  41 . The arithmetic portion  41  temporarily stores the memory portion  42  with image data generated by rendering processing or the like, reads out the image data, and outputs it to the display portion  21 . In other words, the memory portion  42  has a buffer function. During the rendering processing or the like by the arithmetic portion  41 , the memory portion  42  may also be used to temporarily store data for the processing or data in processing. 
     As the memory portion  42 , typically, a video random access memory (VRAM) can be used. Alternatively, part of a main memory included in the control portion  11  may be used as the memory portion  42 . For example, a static RAM (SRAM) or a dynamic RAM (DRAM) is preferably used as the memory portion  42  because a high-speed operation can be achieved. 
     As the memory portion  42 , it is also possible to use a memory device including a memory element in which data is written and held as follows: a potential is supplied to a capacitor through a transistor including an oxide semiconductor in a semiconductor layer in which a channel is formed, and then, the transistor is turned off, so that the potential is held. Such a transistor in the memory element has extremely low leakage current; accordingly, leakage current during the data holding period can be suppressed, the frequency of refresh operations can be very low, or storage capacity can be reduced. Therefore, power consumption can be extremely low compared with the case of using an SRAM or a DRAM. 
     A plurality of pixels is arranged in a matrix in the pixel portion  31  of the display portion  21 . The driver portion  32  has a function of driving the pixels in the pixel portion  31  in accordance with signals from the control portion  11 . In other words, the driver portion  32  has functions of converting a signal input from the control portion  11  into a signal for driving the pixel portion  31  and outputting the converted signal to the pixel portion  31 . 
     For the driver portion  32 , for example, a scan line driver circuit and a signal line driver circuit can be used. The driver portion  32  may further include a buffer circuit, an amplifier circuit, or the like. The amplifier circuit has a function of amplifying a signal from the control portion  11 . 
     The arithmetic portion  41  can analyze a signal which is input from the input portion  22  through the sensor controller  13  and execute the first mode in which an image is displayed on the display portion  21  by an interlace method or the second mode in which an image is displayed by a progressive method, which is switched in accordance with the analysis result. Furthermore, the arithmetic portion  41  can read out image data stored in the memory portion  42  in different manners corresponding to these two methods. Note that specific methods for reading out image data will be described later. 
     In the case where a user&#39;s input to the input portion  22  corresponds to screen switching motion or screen scrolling motion, for example, the control portion  11  executes the first mode. In the case where there is no user&#39;s input or the user&#39;s input does not correspond to the motion given above, the control portion  11  executes the second mode. The screen switching or screen scrolling is often accompanied with an abrupt change in luminance; thus, such an operation of the control portion  11  can achieve eye-friendly display in which an image including such an abrupt change in luminance is not displayed on the display portion  21 . 
     As illustrated in  FIGS. 2A and 2B , the control portion  11  may include two arithmetic portions (the arithmetic portion  41  and an arithmetic portion  43 ). The arithmetic portion  41  and the memory portion  42  form a display controller  14 . The display controller  14  in the control portion  11  is specialized for processing relating to image display, such as image processing or output of image data. Such a configuration can reduce the processing load on the arithmetic portion  43  and achieve a higher operating speed than a configuration including one arithmetic portion. In addition, the control portion  11  preferably includes, besides the memory portion  42 , a memory portion which can be used for processing of the arithmetic portion  43 . 
     [Method for Reading Out Image Data] 
     Next, the operation of the arithmetic portion  41  in the first mode and the second mode will be described with reference to  FIGS. 3A to 3C . 
       FIG. 3A  schematically illustrates image data  50  which is temporarily stored in the memory portion  42 . Note that the image data  50  corresponds to image data of one frame. 
     As illustrated in  FIG. 3A , the image data  50  includes 2n lines (n is an integer), namely, a first line (line (1)) to a 2n-th line (line (2n)). For example, one line includes data for pixels in one scan line of the pixel portion  31 . 
     [Second Mode (Progressive Method)] 
     First, a method by which the arithmetic portion  41  reads out data from the memory portion  42  in the second mode will be described with reference to  FIG. 3B . 
     Here, the image data  50  to be displayed first at a certain point in time is referred to as first image data  50 ( 1 ), and the image data  50  to be displayed next is referred to as second image data  50 ( 2 ). 
     The arithmetic portion  41  sequentially reads out data from the first line (line (1)) to the 2n-th line (line (2n)) of the first image data  50 ( 1 ) stored in the memory portion  42  at a certain point in time. After reading out data from one line, the arithmetic portion  41  outputs the data read out from the line to the display portion  21 . 
     Completion of the output of data read out from the 2n-th line (line (2n)) of the first image data  50 ( 1 ) of one frame is followed by reading of the second image data  50 ( 2 ) stored in the memory portion  42 . The second image data  50 ( 2 ) is sequentially read out in a manner similar to that described above. 
     Through the above-described operation, reading of the image data of two frames (the image data  50 ( 1 ) and the image data  50 ( 2 )) is completed. 
     In the second mode, the arithmetic portion  41  repeats such a reading operation. 
     This reading method enables the arithmetic portion  41  to output image data of one frame to the display portion  21  in one frame period. That is, rewriting of the screen is completed in one frame period. Such an operation can provide smooth moving images, for example. 
     Note that the arithmetic portion  41  does not necessarily need to perform the reading operation and the output operation alternately and may perform these operations concurrently or in parallel. 
     At a certain point in time, the image data  50  of at least one frame is stored in the memory portion  42 . When the memory portion  42  has a capacity enough to store image data which is larger than that of one frame at a certain point in time, processing speed can be increased. In a line of the image data  50  which has been subjected to the reading, data may be deleted or overwritten with part of the next image data  50 . When the reading operation and the writing operation of image data are performed concurrently or in parallel, processing speed can be increased. Alternatively, the memory portion  42  may include two regions each capable of storing image data of one frame, and while image data stored in one of the regions is read out, image data stored in the other region may be updated (rewritten). 
     [First Mode (Interlace Method)] 
     Next, a method by which the arithmetic portion  41  reads out data from the memory portion  42  in the first mode will be described with reference to  FIG. 3C . 
     The arithmetic portion  41  sequentially reads out data from the odd-numbered lines of the first image data  50 ( 1 ) stored in the memory portion  42  at a certain point in time. Specifically, data is read out from the (2m−1)-th lines (m is an integer of 1 or more and n or less) of the first image data  50 ( 1 ) in ascending order of m. 
     Completion of the reading of data from the (2n−1)-th line (line (2n−1)) of the first image data  50 ( 1 ) of one frame is followed by reading of the second image data  50 ( 2 ) stored in the memory portion  42 . 
     The arithmetic portion  41  sequentially reads out data from the even-numbered lines of the second image data  50 ( 2 ). Specifically, data is read out from the 2m-th lines in ascending order of m. 
     A set of data which includes data read out from a plurality of lines of the first image data  50 ( 1 ) or the second image data  50 ( 2 ) can be referred to as data of one field. In some cases, the data extracted from the first image data  50 ( 1 ) and the data extracted from the second image data  50 ( 2 ) are referred to as first field data and second field data, respectively. 
     Through the above-described operation, reading of the image data of two fields from the image data  50 ( 1 ) and the image data  50 ( 2 ) is completed. 
     In the first mode, the arithmetic portion  41  repeats such a reading operation. That is, reading of data from odd-numbered lines of image data stored at a certain point in time and reading of data from even-numbered lines of different image data stored at the next point in time are repeated. 
     This reading method enables the arithmetic portion  41  to output image data of one field to the display portion  21  in one field period. That is, a half of the screen can be rewritten in one field period, and rewriting of the entire screen can be completed in the next one field period. Consequently, the length of one frame period can be twice or more of that in the second mode. Such an operation can suppress an abrupt change in luminance caused by image switching, thereby achieving eye-friendly display for a user. 
     In the above description, the odd-numbered lines are subjected to the reading first; however, the even-numbered lines may be subjected to the reading first. 
     Modification Example 
     In the above-described example, one frame period includes two field periods; however, provision of more than two field periods can further suppress an abrupt change in luminance caused by image switching, thereby achieving eye-friendly display for a user. 
     Note that the interlace method in this specification and the like includes, besides the display method in which one frame period includes two field periods, a display method in which one frame period includes three or more field periods. 
     The case where one frame period includes a field periods (a is an integer of 2 or more and 2n or less) will be described. Note that the previously described example corresponds to the case where a is 2. 
     When k-th image data  50 ( k ) (k is an integer of 1 or more and a or less) to be displayed for the k-th place is stored in the memory portion  42 , the arithmetic portion  41  reads out data from {a×(m−1)+k}-th lines of the k-th image data  50 ( k ) in ascending order of m. Note that the maximum value of m, that is, the number of lines included in one field data is 2n/a. 
     When a is 3, for example, field data of the first image data  50 ( 1 ) is extracted from the first line (line (1)), the fourth line (line (4)), the seventh line (line (7)), . . . , and the (2n−2)-th line (line (2n−2)) in this order. Subsequently, field data of the second image data  50 ( 2 ) is extracted from the second line (line (2)), the fifth line (line (5)), the eighth line (line (8)), . . . , and the (2n−1)-th line (line (2n−1)) in this order. Subsequently, field data of third image data  50 ( 3 ) is extracted from the third line (line (3)), the sixth line (line (6)), the ninth (line (9)), . . . , and the 2n-th line (line (2n)) in this order. 
     When a is 4, for example, field data of the first image data  50 ( 1 ) is extracted from the first line (line (1)), the fifth line (line (5)), the ninth line (line (9)), . . . , and the (2n−3)-th line (line (2n−3)) in this order. Subsequently, field data of the second image data  50 ( 2 ) is extracted from the second line (line (2)), the sixth line (line (6)), the tenth line (line (10)), . . . , and the (2n−2)-th line (line (2n−2)) in this order. Subsequently, field data of the third image data  50 ( 3 ) is extracted from the third line (line (3)), the seventh line (line (7)), the eleventh line (line (11)), . . . , and the (2n−1)-th line (line (2n−1)) in this order. Subsequently, field data of fourth image data  50 ( 4 ) is extracted from the fourth line (line (4)), the eighth line (line (8)), the twelfth line (line (12)), . . . , and the 2n-th line (line (2n)) in this order. 
     The above is the description of the modification example. 
     [Image Rewriting] 
     Next, methods by which the driver portion  32  drives the pixels in the pixel portion  31  in accordance with signals from the control portion  11  will be described with reference to  FIGS. 4A to 4C . 
     [Second Mode] 
       FIG. 4A  is a timing chart in the second mode. For simplicity, an example in which the pixel portion  31  includes four scan lines (scan lines G 1  to G 4 ) will be described. A clock signal CLK in  FIG. 4A  is a signal for specifying the timing at which a transition to data writing to the next scan line is made (writing is started). 
     In the second mode, the driver portion  32  sequentially selects the scan lines G 1  to G 4  to write data. In the second mode, a second frame period FL 2  corresponds to a period from the time at which data writing to the scan line G 1  is started to the time at which next data writing to the scan line G 1  is started. That is, in the second mode, rewriting of the screen is completed in the second frame period FL 2 . 
     [First Mode] 
       FIG. 4B  is a timing chart in the first mode. Here, the case where one frame period includes two field periods (i.e., a=2) will be described. 
     A first frame period FL 1  includes two field periods (a first field period F 1  and a second field period F 2 ). 
     In the first field period F 1 , data writing to the first scan line G 1  and data writing to the third scan line G 3  are performed in this order. 
     In the following second field period F 2 , data writing to the second scan line G 2  and data writing to the fourth scan line G 4  are performed in this order. 
     At this point in time, rewriting of the screen is completed. 
     In the example described here, a period corresponding to one cycle of the clock signal CLK is provided between data writing to a scan line and the following data writing to another scan line; as illustrated in  FIG. 4C , data writing to a scan line and the following data writing to another scan line may be consecutively performed. The latter method can shorten the image rewriting period in one field period; accordingly, even in a long frame period, display can be performed without causing a feeling of strangeness. 
     [Display Operation] 
     Next, an image displayed on the pixel portion  31  of the display portion  21  driven by the above-described method will be described. 
     [Second Mode] 
       FIG. 5A  schematically illustrates the pixel portion  31 . Here, the pixel portion  31  includes a plurality of pixels  35  arranged in a matrix of 12×12. In the drawing, the horizontal direction corresponds to the scan line direction, and the vertical direction corresponds to the signal line direction. 
       FIG. 5A  illustrates an image displayed at Time T 1  and an image displayed at Time T 2  when one frame period has passed since Time T 1 . A hatching pattern is applied to pixels which have not been subjected to data rewriting and not to pixels which have been subjected to data rewriting. 
     In the second mode, data of all the pixels  35  can be rewritten in one frame period as illustrated in  FIG. 5A . Therefore, moving images can displayed smoothly. 
     [First Mode] 
       FIG. 5B  schematically illustrates the pixel portion  31  at Time T 1 , Time T 2 , and Time T 3 . At Time T 2 , the first field period F 1  has passed since Time T 1 . At Time T 3 , the second field period F 2  has passed since Time T 2 .  FIG. 5B  illustrates the case where one frame period includes two field periods (i.e., a=2). 
     As illustrated in  FIG. 5B , data rewriting of the pixels  35  in the odd-numbered scan lines is completed at Time T 2 . At this time, the pixels  35  in the even-numbered scan lines are not subjected to data rewriting and display the same image as at Time T 1 . 
     Subsequently, data rewriting of the pixels  35  in the even-numbered scan lines is completed at Time T 3 . At this time, the pixels  35  in the odd-numbered scan lines are not subjected to data rewriting and display the same image as at Time T 2 . At Time T 3 , data rewriting of all the pixels is completed. 
       FIG. 6  illustrates an example in which one frame period includes three field periods (i.e., a=3). 
     As illustrated in  FIG. 6 , data rewriting of the pixels  35  in the first, fourth, seventh, and tenth scan lines is completed at Time T 2  when the first field period F 1  has passed since Time  1 . Subsequently, data rewriting of the pixels  35  in the second, fifth, eighth, and eleventh scan lines is completed at Time T 3  when the second field period F 2  has passed since Time T 2 . Then, data rewriting of the pixels  35  in the third, sixth, ninth, and twelfth scan lines is completed at Time T 4  when a third field period F 3  has passed since Time T 3 . At Time T 4 , data rewriting of all the pixels is completed. 
     Thus, in the case where the frame frequency is the same, the time for rewriting the screen in the first mode can be made longer than that in the second mode. 
     An example in which the display is switched from an image with a uniformly low luminance to an image with a uniformly high luminance will be described. In the case where two field periods are provided as in  FIG. 5B , the luminance of the entire screen at Time T 2  is about a half of the luminance of the entire screen at Time T 3 , at which rewriting of the screen is completed. In the case where three field periods are provided as in  FIG. 6 , the luminance of the entire screen at Time T 2  is about one third of the luminance at Time T 4 , at which rewriting of the screen is completed. The luminance of the entire screen at Time T 3  is about two thirds of the luminance at Time T 4 . 
     In this manner, the first mode is employed particularly when screen switching may cause an abrupt change in luminance, whereby the luminance can be gradually changed and thus eye-friendly display can be achieved. 
     It is particularly preferable that the screen switching in the first mode take 100 msec or longer, further preferably 150 msec or longer, because an influence of lateral inhibition can be avoided. For example, a plurality of field periods may be provided such that the length of the frame period FL 1  in the first mode is 100 msec or longer, preferably 150 msec or longer. The number of field periods may be determined in accordance with the field frequency. 
     The field period in the first mode may be longer than the frame period in the second mode. That is, the frame frequency in the second mode may be made different from the field frequency in the first mode by varying the cycle of the clock signal CLK output from the arithmetic portion  41 , for example. In this manner, the screen can be switched over a long time even when the number of field periods is smaller. Moreover, processing load on the arithmetic portion  41  can be reduced. 
     The above is the description of the display operation. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Embodiment 2 
     In this embodiment, a structural example of a display device that can be used for the display system described in the above embodiment will be described. A display device  200  will be described below with reference to  FIGS. 13A and 13B ,  FIG. 14 ,  FIG. 15 ,  FIG. 16 ,  FIGS. 17A and 17B ,  FIGS. 18A and 18B ,  FIG. 19 ,  FIG. 20 ,  FIG. 21 ,  FIG. 22 ,  FIG. 23 ,  FIG. 24 ,  FIG. 25 ,  FIG. 26 ,  FIG. 27 ,  FIG. 28 ,  FIG. 29 ,  FIG. 30 , and  FIG. 31 . 
     The display device  200  in  FIG. 13A  includes a pixel portion  271 , a scan line driver circuit  274 , a signal line driver circuit  276 , m scan lines  277  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the scan line driver circuit  274 , and n signal lines  279  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the signal line driver circuit  276 . Furthermore, the pixel portion  271  includes a plurality of pixels  270  arranged in a matrix. Furthermore, common lines  275  which are arranged parallel or substantially parallel to each other are provided along the signal lines  279 . The scan line driver circuit  274  and the signal line driver circuit  276  are collectively referred to as a driver circuit portion in some cases. 
     Each scan line  277  is electrically connected to n pixels  270  in the corresponding row among the pixels  270  arranged in m rows and n columns in the pixel portion  271 . Each signal line  279  is electrically connected to m pixels  270  in the corresponding column among the pixels  270  arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each common line  275  is electrically connected to m pixels  270  in the corresponding column among the pixels  270  arranged in m rows and n columns. 
       FIG. 13B  illustrates an example of the circuit configuration that can be used for the pixel  270  of the display device  200  in  FIG. 13A . 
     The pixel  270  in  FIG. 13B  includes a liquid crystal element  251 , a transistor  252 , and a capacitor  255 . 
     One of a pair of electrodes of the liquid crystal element  251  is connected to the transistor  252 , and the potential of the electrode is set as appropriate in accordance with the specifications of the pixel  270 . The other of the pair of electrodes of the liquid crystal element  251  is connected to the common line  275  and supplied with a common potential. The alignment of liquid crystal included in the liquid crystal element  251  is controlled by data written to the transistor  252 . 
     The liquid crystal element  251  is an element which controls transmission or non-transmission of light by an optical modulation action of liquid crystal. Note that the optical modulation action of liquid crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and an oblique electric field). As the liquid crystal used for the liquid crystal element  251 , thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     In the case where a horizontal electric field mode is employed, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase when the temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for a liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy. In addition, the liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material does not need alignment treatment and has a small viewing angle dependence. An alignment film does not need to be provided, and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented, and defects and damage of a liquid crystal display device in the manufacturing process can be reduced. 
     As a method for driving the display device  200  including the liquid crystal element  251 , a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     Furthermore, the display device  200  may be a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode. As the vertical alignment mode, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an Advanced Super View (ASV) mode, or the like can be used. 
     In this embodiment, horizontal electric field modes typified by an FFS mode and an IPS mode are mainly described. 
     In the configuration of the pixel  270  in  FIG. 13B , one of a source electrode and a drain electrode of the transistor  252  is electrically connected to the signal line  279 , and the other electrode is electrically connected to the one of the pair of electrodes of the liquid crystal element  251 . A gate electrode of the transistor  252  is electrically connected to the scan line  277 . The transistor  252  has a function of controlling writing of a data signal. 
     In the pixel  270  in  FIG. 13B , one of a pair of electrodes of the capacitor  255  is connected to the other of the source electrode and the drain electrode of the transistor  252 . The other of the pair of electrodes of the capacitor  255  is electrically connected to the common line  275 . The potential of the common line  275  is set as appropriate in accordance with the specifications of the pixel  270 . The capacitor  255  functions as a storage capacitor for storing written data. In the display device  200  driven in the FFS mode, the one of the pair of electrodes of the capacitor  255  corresponds to part or the whole of the one of the pair of electrodes of the liquid crystal element  251 , and the other of the pair of electrodes of the capacitor  255  corresponds to part or the whole of the other of the pair of electrodes of the liquid crystal element  251 . 
     &lt;Structure Example of Pixel&gt; 
     Next, a specific structure of the pixel included in the display device  200  will be described.  FIG. 14  is a top view illustrating pixels  270   a ,  270   b , and  270   c  included in the display device  200  driven in the FFS mode. 
     In  FIG. 14 , a conductive film  213  functioning as a scan line extends substantially perpendicularly to a conductive film  221   a  (in the horizontal direction in the drawing). The conductive film  221   a  functioning as a signal line extends substantially perpendicularly to the conductive film  213  (in the vertical direction in the drawing). Note that the conductive film  213  functioning as a scan line is electrically connected to the scan line driver circuit  274 , and the conductive film  221   a  functioning as a signal line is electrically connected to the signal line driver circuit  276  (see  FIG. 13A ). 
     The transistor  252  is provided near the intersection of the conductive film  213  and the conductive film  221   a . The transistor  252  includes the conductive film  213  which also functions as a gate electrode, a gate insulating film (not illustrated in  FIG. 14 ), a semiconductor film  219  which is formed over the gate insulating film and in which a channel region is formed, and the conductive film  221   a  and a conductive film  221   b  which also function as a source electrode and a drain electrode. In the conductive film  213 , a region overlapping with the semiconductor film  219  functions as the gate electrode of the transistor  252 . In the conductive film  221   a , a region overlapping with the semiconductor film  219  functions as the source electrode or the drain electrode of the transistor  252 . In  FIG. 14 , an end portion of the conductive film  213  is on the outer side of an end portion of the semiconductor film  219  when seen from above. Thus, the conductive film  213  functions as a light-blocking film for blocking light from a light source such as a backlight. For this reason, the semiconductor film  219  included in the transistor  252  is not irradiated with light, so that a change in the electrical characteristics of the transistor can be reduced. 
     The conductive film  221   b  is electrically connected to a conductive film  220  functioning as a pixel electrode. A conductive film  229  is provided over the conductive film  220  with an insulating film (not illustrated in  FIG. 14 ) positioned therebetween. 
     The conductive film  229  functions as a common electrode, for example. The conductive film  229  has stripe regions extending in the direction intersecting with the conductive film  221   a . The stripe regions are connected to a region extending in the direction parallel or substantially parallel to the conductive film  221   a . Therefore, in the plurality of pixels in the display device  200 , the stripe regions of the conductive film  229  have the same potential. 
     The capacitor  255  is formed in a region in which the conductive film  220  and the conductive film  229  overlap with each other. The conductive film  220  and the conductive film  229  transmit light. That is, the capacitor  255  transmits light. 
     Owing to the light-transmitting property of the capacitor  255 , the capacitor  255  can be formed large (in a large area) in the pixel  270 . Thus, a display device with increased capacitance and an increased aperture ratio of typically 50% or higher, preferably 60% or higher can be obtained. For example, in a high-resolution display device such as a high-resolution liquid crystal display device, the area of a pixel is small, and accordingly, the area of a capacitor is also small. For this reason, the amount of charge accumulated in the capacitor is small in the high-resolution display device. However, when the capacitor  255  of this embodiment, which transmits light, is provided in each pixel, enough capacitance can be obtained in the pixel and the aperture ratio can be increased. Typically, the capacitor  255  can be favorably used for a high-resolution display device with a pixel density of 200 pixels per inch (ppi) or higher, 300 ppi or higher, or 500 ppi or higher. 
     In a liquid crystal display device, an increase in the capacitance of a capacitor can increase the period in which the alignment of liquid crystal molecules in a liquid crystal element can be kept constant while an electric field is applied. That period can be made longer when a still image is displayed; accordingly, the number of times of rewriting image data can be reduced, leading to a reduction in power consumption. Furthermore, the structure of this embodiment can increase the aperture ratio even in a high-resolution display device, which makes it possible to use light from a light source such as a backlight efficiently, so that the power consumption of the display device can be reduced. 
       FIG. 15  shows cross-sectional views taken along dashed-dotted line Q 1 -R 1  and dashed-dotted line S 1 -T 1  in  FIG. 14 . The transistor  252  in  FIG. 15  is a channel-etched transistor. Note that the transistor  252  in the channel length direction and the capacitor  255  are illustrated in the cross-sectional view taken along dashed-dotted line Q 1 -R 1 , and the transistor  252  in the channel width direction is illustrated in the cross-sectional view taken along dashed-dotted line S 1 -T 1 . 
     The transistor  252  in  FIG. 15  has a single-gate structure and includes the conductive film  213  which is provided over a substrate  211  and functions as a gate electrode. In addition, the transistor  252  includes an insulating film  215  which is formed over the substrate  211  and the conductive film  213  functioning as a gate electrode, an insulating film  217  which is formed over the insulating film  215 , the semiconductor film  219  which overlaps with the conductive film  213  functioning as a gate electrode with the insulating film  215  and the insulating film  217  positioned therebetween, and the conductive films  221   a  and  221   b  which are in contact with the semiconductor film  219  and function as a source electrode and a drain electrode. Moreover, an insulating film  223  is formed over the insulating film  217 , the semiconductor film  219 , and the conductive films  221   a  and  221   b  functioning as a source electrode and a drain electrode, and an insulating film  225  is formed over the insulating film  223 . The conductive film  220  is formed over the insulating film  225 . The conductive film  220  is electrically connected to one of the conductive films  221   a  and  221   b  functioning as a source electrode and a drain electrode (here, the conductive film  221   b ) through an opening in the insulating film  223  and the insulating film  225 . An insulating film  227  is formed over the insulating film  225  and the conductive film  220 . The conductive film  229  is formed over the insulating film  227 . 
       FIG. 15  illustrates the case where a liquid crystal layer  250  is positioned between a substrate  241  and the substrate  211 . A light-blocking film  261  functioning as a black matrix, a color film  262  functioning as a color filter, and the like are provided on a surface of the substrate  241  facing the substrate  211 . 
     The conductive film  220  may be provided over the insulating film  225  so as to overlap with the semiconductor film  219 , in which case the transistor  252  has a double-gate structure in which the conductive film  220  is used as a second gate electrode. 
     A region in which the conductive film  220 , the insulating film  227 , and the conductive film  229  overlap with one another functions as the capacitor  255 . 
     Note that a cross-sectional view of one embodiment of the present invention is not limited to  FIG. 15 . The display device can have a variety of different structures. For example, the conductive film  220  may have a slit. Alternatively, the conductive film  220  may have a comb-like shape. 
     As illustrated in  FIG. 16 , the conductive film  229  may be provided over an insulating film  228  over the insulating film  227 . The insulating film  228  functions as a planarization film. 
     In  FIG. 15 , for example, the thickness of part of the semiconductor film  219  in which the top surface is exposed is reduced by etching; as illustrated in  FIG. 17A , the thickness is not necessarily reduced. As illustrated in  FIG. 17B , an insulating film  245  which protects the semiconductor film  219  may also be provided. 
       FIGS. 18A and 18B  each illustrate an example in which a conductive film  243  functioning as a second gate electrode is provided. The second gate electrode can be used to control the threshold voltage of the transistor. Furthermore, when the same potential is supplied to the conductive film  243  and the conductive film  213 , the field-effect mobility of the transistor can be increased. 
     &lt;Modification Example of Pixel Structure&gt; 
       FIG. 19  is a top view illustrating pixels  270   d ,  270   e , and  270   f  which are included in the display device  200  and are different from the pixels in  FIG. 14 . 
     In  FIG. 19 , the conductive film  213  functioning as a scan line extends in the horizontal direction in the drawing. The conductive film  221   a  functioning as a signal line extends substantially perpendicularly to the conductive film  213  (in the vertical direction in the drawing) and has a dogleg shape (V-like shape). Note that the conductive film  213  functioning as a scan line is electrically connected to the scan line driver circuit  274 , and the conductive film  221   a  functioning as a signal line is electrically connected to the signal line driver circuit  276 . 
     The transistor  252  is provided near the intersection of the conductive film  213  and the conductive film  221   a . The transistor  252  includes the conductive film  213  which also functions as a gate electrode, a gate insulating film (not illustrated in  FIG. 19 ), the semiconductor film  219  which is formed over the gate insulating film and in which a channel region is formed, and the conductive films  221   a  and  221   b  which also function as a source electrode and a drain electrode. In the conductive film  213 , a region overlapping with the semiconductor film  219  functions as the gate electrode of the transistor  252 . In the conductive film  221   a , a region overlapping with the semiconductor film  219  functions as the source electrode or the drain electrode of the transistor  252 . In  FIG. 19 , an end portion of the conductive film  213  is on the outer side of an end portion of the semiconductor film  219  when seen from above. Thus, the conductive film  213  functions as a light-blocking film for blocking light from a light source such as a backlight. For this reason, the semiconductor film  219  included in the transistor is not irradiated with light, so that a change in the electrical characteristics of the transistor can be reduced. 
     The conductive film  221   b  is electrically connected to the conductive film  220  functioning of a pixel electrode. The conductive film  220  is formed in a comb-like shape. An insulating film (not illustrated in  FIG. 19 ) is provided over the conductive film  220 , and the conductive film  229  is provided over the insulating film. The conductive film  229  is formed in a comb-like shape to partly overlap and engage with the conductive film  220  when seen from the above. Tooth regions of the comb-like conductive film  229  are connected to each other in a region extending in the direction parallel or substantially parallel to the conductive film  213 . Therefore, in the plurality of pixels in the display device  200 , the tooth regions of the conductive film  229  have the same potential. Note that the conductive film  220  and the conductive film  229  each have a dogleg shape (V-like shape) bent along the signal line (the conductive film  221   a ). 
     The capacitor  255  is formed in a region in which the conductive film  220  and the conductive film  229  overlap with each other. The conductive film  220  and the conductive film  229  transmit light. That is, the capacitor  255  transmits light. 
       FIG. 20  shows cross-sectional views taken along dashed-dotted line Q 2 -R 2  and dashed-dotted line S 2 -T 2  in  FIG. 19 . The transistor  252  in  FIG. 20  is a channel-etched transistor. Note that the transistor  252  in the channel length direction and the capacitor  255  are illustrated in the cross-sectional view taken along dashed-dotted line Q 2 -R 2 , and the transistor  252  in the channel width direction is illustrated in the cross-sectional view taken along dashed-dotted line S 2 -T 2 . 
     The transistor  252  in  FIG. 20  has a single-gate structure and includes the conductive film  213  which is provided over the substrate  211  and functions as a gate electrode. In addition, the transistor  252  includes the insulating film  215  which is formed over the substrate  211  and the conductive film  213  functioning as a gate electrode, the insulating film  217  which is formed over the insulating film  215 , the semiconductor film  219  which overlaps with the conductive film  213  functioning as a gate electrode with the insulating film  215  and the insulating film  217  positioned therebetween, and the conductive films  221   a  and  221   b  which are in contact with the semiconductor film  219  and function as a source electrode and a drain electrode. Moreover, the insulating film  223  is formed over the insulating film  217 , the semiconductor film  219 , and the conductive films  221   a  and  221   b  functioning as a source electrode and a drain electrode, and the insulating film  225  is formed over the insulating film  223 . The conductive film  220  is formed over the insulating film  225 . The conductive film  220  is electrically connected to one of the conductive films  221   a  and  221   b  functioning as a source electrode and a drain electrode (here, the conductive film  221   b ) through an opening in the insulating film  223  and the insulating film  225 . The insulating film  227  is formed over the insulating film  225  and the conductive film  220 . The conductive film  229  is formed over the insulating film  227 . 
     The conductive film  220  may be provided over the insulating film  225  so as to overlap with the semiconductor film  219 , in which case the transistor  252  has a double-gate structure in which the conductive film  220  is used as a second gate electrode. 
     A region in which the conductive film  220 , the insulating film  227 , and the conductive film  229  overlap with one another functions as the capacitor  255 . 
     In the liquid crystal display device in  FIG. 19  and  FIG. 20 , a capacitor in a pixel is formed in a region in which an end portion of the conductive film  220  and an end portion of the conductive film  229  overlap with each other. With this structure, a capacitor with a suitable size, not a too large size, can be formed in a large liquid crystal display device. 
     As illustrated in  FIG. 21 , the conductive film  229  may be provided over the insulating film  228  over the insulating film  227 . 
     As illustrated in  FIG. 22  and  FIG. 23 , a structure in which the conductive film  220  and the conductive film  229  do not overlap with each other may be employed. The positions of the conductive film  220  and the conductive film  229  can be set as appropriate depending on the size of the capacitor in accordance with the resolution and driving method of the display device. As in the display device illustrated in  FIG. 24 , the conductive film  229  may be provided over the insulating film  228  functioning as a planarization film. 
     In the liquid crystal display device in  FIG. 19  and  FIG. 20 , a width (d 1 ) of a region of the conductive film  220  extending in the direction parallel or substantially parallel to the signal line (the conductive film  221   a ) is smaller than a width (d 2 ) of a region of the conductive film  229  extending in the direction parallel or substantially parallel to the signal line, but the widths are not limited to this relation. As illustrated in  FIG. 25  and  FIG. 26 , the width d 1  may be larger than the width d 2 . Alternatively, the width d 1  may be equal to the width d 2 . Further alternatively, in a pixel (e.g., the pixel  270   d ), the widths of a plurality of regions of the conductive film  220  and/or the conductive film  229  extending in the direction parallel or substantially parallel to the signal line may be different from one another. 
     As illustrated in  FIG. 27 , a structure in which the insulating film  228  over the insulating film  227  is partially removed such that an insulating film  228   a  under the conductive film  229  is left may be employed. In this case, the insulating film  228  can be etched using the conductive film  229  as a mask. Unevenness of the conductive film  229  over the insulating film  228  functioning as a planarization film can be suppressed, and the insulating film  228   a  can have a gentle side surface from an end portion of the conductive film  229  to the insulating film  227 . As illustrated in  FIG. 28 , a structure in which part of a surface of an insulating film  228   b  parallel to the substrate  211  is not covered with the conductive film  229  may be employed. 
     As illustrated in  FIG. 29  and  FIG. 30 , the conductive film  229  and the conductive film  220  may be formed over the same layer, that is, over the insulating film  225 . The conductive film  229  in  FIG. 29  and  FIG. 30  can be formed with the same material at the same time as the conductive film  220 . 
     As illustrated in  FIG. 31 , the conductive film  229  may be positioned between the insulating film  215  and the substrate  211 . In this case, as illustrated in  FIG. 31 , a conductive film  247  which is formed by processing the same film as the conductive film  213  functioning as the gate electrode of the transistor  252  is preferably used as a wiring. 
     The display device  200  can employ various modes and include various display elements. Examples of the display element include a liquid crystal element, an electroluminescent (EL) element (an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element) including an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, an electrophoretic element, a display element using micro electro mechanical systems (MEMS) such as a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS) element, a MIRASOL (registered trademark) display, an interferometric modulator display (IMOD) element, or a piezoelectric ceramic display, and an electrowetting element. In addition, a display medium whose contrast, luminance, reflectance, transmittance, or the like is changed by electrical or magnetic effect may be included. Alternatively, quantum dots may be used as the display element. Examples of a display device including a liquid crystal element include a liquid crystal display (a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including an EL element is an EL display. Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). An example of a display device including quantum dots is a quantum dot display. An example of a display device including electronic ink or an electrophoretic element is electronic paper. In a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes. Thus, power consumption can be further reduced. 
     As a display method of the display device  200 , a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in pixels at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of an R pixel, a G pixel, a B pixel, and a W (white) pixel may be used. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ between color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Note that the size of a display region may differ between dots of color elements. Embodiments of the disclosed invention are not limited to a color display device; the disclosed invention can also be applied to a monochrome display device. 
     Color films (also referred to as color filters) may be used to obtain a full-color display device in which white light (W) for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) is used. For example, a red (R) color film, a green (G) color film, a blue (B) color film, and a yellow (Y) color film may be combined as appropriate. With the use of the color film, higher color reproducibility can be obtained than in the case without the color film. Here, by providing a region with the color film and a region without the color film, white light in the region without the color film may be directly utilized for display. By partly providing the region without the color film, a decrease in the luminance of a bright image due to the color film can be suppressed, and approximately 20% to 30% of power consumption can be reduced in some cases. In the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light in their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption may be further reduced compared with the case of using the color film. 
     &lt;Substrate&gt; 
     There is no particular limitation on a material and the like of the substrate  211  as long as the material has heat resistance high enough to withstand at least heat treatment performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate  211 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used. Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate  211 . In the case where a glass substrate is used as the substrate  211 , a large substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large display device can be manufactured. Alternatively, a flexible substrate may be used as the substrate  211 , and the transistor, the capacitor, and the like may be formed directly on the flexible substrate. 
     Other than the above, a transistor can be formed using various substrates as the substrate  211 . The type of the substrate is not limited to a certain type. As the substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper containing a fibrous material, or a base film can be used, for example. As examples of the glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate can be given. For the flexible substrate, a flexible synthetic resin such as plastic typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES) or acrylic can be used, for example. An example of the attachment film is a film formed using polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or the like. An example of the base film is a film formed using polyester, polyamide, polyimide, an inorganic vapor deposition film, paper, or the like. In particular, a transistor formed using a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like can have little variation in characteristics, size, shape, or the like, high current supply capability, and a small size. Such a transistor can achieve lower power consumption or higher integration of a circuit. 
     A transistor may be formed using one substrate, and then, the transistor may be transferred to another substrate. Examples of a substrate to which a transistor is transferred include, in addition to the above substrates over which the transistor can be formed, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, and a rubber substrate. When such a substrate is used, a transistor with excellent characteristics or a transistor with low power consumption can be formed, a device with high durability can be manufactured, high heat resistance can be provided, or a reduction in weight or thickness can be achieved. 
     &lt;Semiconductor Film&gt; 
     Examples of a material that can be used for the semiconductor film  219  include silicon, germanium, an organic semiconductor, and an oxide semiconductor. 
     There is no particular limitation on the crystallinity of a semiconductor material used for the transistor, and an amorphous semiconductor or a semiconductor having crystallinity (a microcrystalline semiconductor, a polycrystalline semiconductor, a single crystal semiconductor, or a semiconductor partly including crystal regions) may be used. A semiconductor having crystallinity is preferably used, in which case deterioration of transistor characteristics can be suppressed. 
     In particular, the semiconductor film  219  preferably contains silicon. As silicon, for example, amorphous silicon or silicon having crystallinity is preferably used. As silicon having crystallinity, for example, microcrystalline silicon, polycrystalline silicon, or single crystal silicon is preferably used. In particular, polycrystalline silicon can be formed at a lower temperature than single crystal silicon and has higher field-effect mobility and higher reliability than amorphous silicon. When such a polycrystalline semiconductor is used for a pixel, the aperture ratio of the pixel can be improved. Even in an extremely high-resolution display device, a gate driver circuit and a source driver circuit can be formed over a substrate over which pixels are formed, and the number of components of an electronic device can be reduced. 
     Alternatively, the semiconductor film  219  preferably includes a film of an In-M-Zn oxide which contains at least indium, zinc, and M (metal such as aluminum, titanium, gallium, yttrium, zirconium, lanthanum, cerium, tin, or hafnium). In order to reduce variation in the electrical characteristics of the transistor including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer. 
     Examples of the stabilizer, including metals that can be used as M, are gallium, tin, hafnium, aluminum, and zirconium. As another stabilizer, lanthanoid such as lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium can be given. 
     As an oxide semiconductor included in the semiconductor film  219 , for example, the following oxide can be used: an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide. 
     Note that here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components, and there is no limitation on the ratio of In to Ga and Zn. The “In—Ga—Zn-based oxide” may contain another metal element in addition to In, Ga, and Zn. 
     The semiconductor film  219  and the conductive film  220  may contain the same metal element selected from metal elements contained in the above oxides. The use of the same metal element for the semiconductor film  219  and the conductive film  220  can reduce the manufacturing cost. For example, when metal oxide targets with the same metal composition are used, the manufacturing cost can be reduced, and the same etching gas or the same etchant can be used in processing the semiconductor film  219  and the conductive film  220 . Even when the semiconductor film  219  and the conductive film  220  contain the same metal element, they have different compositions in some cases. For example, a metal element in a film may be released in the manufacturing process of the transistor and the capacitor, which results in different metal compositions. 
     In the case where the semiconductor film  219  includes an In-M-Zn oxide, the proportions of In and M, the sum of which is assumed to be 100 atomic %, are preferably as follows: the proportion of In is higher than 25 atomic % and the proportion of M is lower than 75 atomic %, further preferably, the proportion of In is higher than 34 atomic % and the proportion of M is lower than 66 atomic %. 
     The energy gap of the semiconductor film  219  is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor  252  can be reduced. 
     The thickness of the semiconductor film  219  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     In the case where the semiconductor film  219  includes an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd), the atomic ratio of metal elements in a sputtering target used for depositing the In-M-Zn oxide preferably satisfies In≧M and Zn≧M. As the atomic ratio of metal elements in such a sputtering target, InM:Zn=1:1:1, 1:1:1.2, or 3:1:2 is preferable. Note that the proportion of each metal element in the atomic ratio of the deposited semiconductor film  219  varies within a range of error of ±40% of that in the above atomic ratio of the sputtering target. 
     An oxide semiconductor film with low carrier density is used as the semiconductor film  219 . For example, an oxide semiconductor film whose carrier density is 1×10 17 /cm 3  or lower, preferably 1×10 15 /cm 3  or lower, further preferably 1×10 13 /cm 3  or lower, still further preferably 1×10 11 /cm 3  or lower is used as the semiconductor film  219 . 
     Without being limited to the above examples, a material with an appropriate composition may be used in accordance with required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. To obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the semiconductor film  219  be set as appropriate. 
     If silicon or carbon, which are elements belonging to Group 14, is contained in the semiconductor film  219 , the number of oxygen vacancies is increased in the semiconductor film  219 , and the semiconductor film  219  becomes an n-type film. To prevent this, the concentration of silicon or carbon (the concentration is measured by secondary ion mass spectrometry (SIMS)) in the semiconductor film  219  is set to 2×10 18  atoms/cm 3  or lower, preferably 2×10 17  atoms/cm 3  or lower. 
     Furthermore, the concentration of alkali metal or alkaline earth metal in the semiconductor film  219 , which is measured by SIMS, is set to 1×10 18  atoms/cm 3  or lower, preferably 2×10 16  atoms/cm 3  or lower. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might increase. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the semiconductor film  219 . 
     If nitrogen is contained in the semiconductor film  219 , electrons serving as carriers are generated and the carrier density increases, so that the semiconductor film  219  easily becomes an n-type film. Thus, a transistor formed using an oxide semiconductor containing nitrogen is likely to be normally on. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the nitrogen concentration measured by SIMS is preferably, for example, 5×10 18  atoms/cm 3  or lower. 
     The semiconductor film  219  may have a non-single-crystal structure, for example. Examples of the non-single-crystal structure include a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure which is described later, and an amorphous structure. Among the non-single-crystal structures, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states. 
     The semiconductor film  219  may have an amorphous structure, for example. An oxide semiconductor film with an amorphous structure has disordered atomic arrangement and no crystalline component, for example. Alternatively, an amorphous oxide film has, for example, an absolutely amorphous structure and no crystal part. 
     Note that the semiconductor film  219  may be a mixed film including two or more of the following regions: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single crystal structure. The mixed film includes, for example, two or more of the following regions in some cases: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single crystal structure. Furthermore, for example, the mixed film has a stacked-layer structure of two or more of the following regions in some cases: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single crystal structure. 
     The bottom-gate transistor described in this embodiment is preferable because the number of manufacturing steps can be reduced. When an oxide semiconductor or amorphous silicon, which can be formed at a lower temperature than polycrystalline silicon, is used for the semiconductor film  219 , materials with low heat resistance can be used for an electrode and a substrate below the semiconductor film  219 , so that the range of choices of materials can be widened. For example, the above-mentioned large glass substrate can be favorably used. 
     &lt;Insulating Film&gt; 
     As each of the insulating films  215  and  217  functioning as a gate insulating film of the transistor  252 , an insulating film including at least one of the following films formed by a plasma-enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Instead of the stacked insulating films  215  and  217 , a single-layer insulating film selected from the above films may be used. 
     In the case where an oxide semiconductor is used for the semiconductor film  219 , the insulating film  215  functions as a blocking film that inhibits permeation of oxygen. For example, in the case where excess oxygen is supplied to the insulating films  217 , the insulating film  223 , the insulating film  225 , and/or the semiconductor film  219 , oxygen can be prevented from permeating the insulating film  215 . 
     In the case where an oxide semiconductor is used for the semiconductor film  219 , the insulating film  217  that is in contact with the semiconductor film  219  functioning as a channel region of the transistor  252  is preferably an oxide insulating film and further preferably includes a region containing oxygen in excess of that in the stoichiometric composition (oxygen-excess region). In other words, the insulating film  217  is an insulating film capable of releasing oxygen. To provide the oxygen-excess region in the insulating film  217 , the insulating film  217  may be formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the deposited insulating film  217 . As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be employed. 
     In the case where hafnium oxide is used for the insulating films  215  and  217 , the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, the thickness of the insulating films can be made large compared with the case where silicon oxide is used; as a result, leakage current due to tunnel current can be reduced. That is, it is possible to provide a transistor with low off-state current. Moreover, hafnium oxide with a crystalline structure has a higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to these examples. 
     In this embodiment, a silicon nitride film is formed as the insulating film  215 , and a silicon oxide film is formed as the insulating film  217 . A silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the gate insulating film of the transistor  252  includes a silicon nitride film, the physical thickness of the gate insulating film can be increased. This makes it possible to suppress a decrease in the withstand voltage of the transistor  252  and rather to increase the withstand voltage, thereby inhibiting electrostatic discharge damage of the transistor  252 . 
     The insulating film  228  can be formed using a heat-resistant organic material such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin, for example. For example, the insulating film  228  is formed by forming an organic resin film over the insulating film  227 , patterning the organic resin film into a desired region, and etching the patterned organic resin film to remove unnecessary regions. 
     &lt;Gate Electrode, Source Electrode, and Drain Electrode&gt; 
     The conductive film  213 , the conductive film  221   a , and the conductive film  221   b  can each have a single-layer structure or a stacked-layer structure formed using a metal such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, or tungsten, or an alloy containing the metal as its main component. For example, a two-layer structure in which a titanium film is stacked over an aluminum film; a two-layer structure in which a titanium film is stacked over a tungsten film; a two-layer structure in which a copper film is stacked over a molybdenum film; a two-layer structure in which a copper film is stacked over an alloy film containing molybdenum and tungsten; a two-layer structure in which a copper film is stacked over an alloy film containing copper, magnesium, and aluminum; a three-layer structure in which titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order; or a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order can be employed. In the case where the conductive film  221   a  and the conductive film  221   b  each have a three-layer structure, it is preferable that a film of titanium, titanium nitride, molybdenum, tungsten, an alloy containing molybdenum and tungsten, an alloy containing molybdenum and zirconium, or molybdenum nitride be formed as each of the first and third layers and that a film of a low-resistance material such as copper, aluminum, gold, silver, or an alloy containing copper and manganese be formed as the second layer. Note that it is also possible to use a light-transmitting conductive material such as indium thin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. The materials that can be used for the conductive film  213 , the conductive film  221   a , and the conductive film  221   b  can be deposited by a sputtering method, for example. 
     &lt;Conductive Film&gt; 
     The conductive film  229  functions as a common electrode. A material which transmits visible light may be used for the conductive film  229 , for example. Specifically, a material containing one of indium (In), zinc (Zn), and tin (Sn) is preferably used. For the conductive film  229 , for example, a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added can be used. The conductive film  229  can be formed by a sputtering method, for example. 
     The conductive film  220  functions as a pixel electrode. For the conductive film  220 , a material similar to that of the conductive film  229  can be used. 
     Alternatively, for the conductive film  220 , an oxide semiconductor similar to that of the semiconductor film  219  is preferably used. In this case, it is preferable that the conductive film  220  be formed to have a lower electrical resistance than a region of the semiconductor film  219  in which a channel is formed. 
     &lt;Method for Controlling Resistivity of Oxide Semiconductor&gt; 
     Oxide semiconductor films which can be used as the semiconductor film  219  and the conductive film  220  include a semiconductor material whose resistivity can be controlled by the number of oxygen vacancies in the film and/or the concentration of impurities such as hydrogen or water in the film. Thus, to control the resistivity of each of the oxide semiconductor films, treatment to be performed on the semiconductor film  219  and the conductive film  220  is selected from treatment for increasing oxygen vacancies and/or impurity concentration and treatment for reducing oxygen vacancies and/or impurity concentration. 
     Specifically, plasma treatment is performed on the oxide semiconductor film that is used as the conductive film  220  functioning as an electrode of the capacitor  255  to increase oxygen vacancies and/or impurities such as hydrogen or water in the oxide semiconductor film; thus, the oxide semiconductor film can have a high carrier density and a low resistivity. Furthermore, an insulating film containing hydrogen is formed, for example, as the insulating film  227  in contact with the oxide semiconductor film so that hydrogen may be diffused from the insulating film containing hydrogen into the oxide semiconductor film; thus, the oxide semiconductor film can have a high carrier density and a low resistivity. 
     The semiconductor film  219  functioning as a channel region of the transistor  252  is not in contact with the insulating films  215  and  227  containing hydrogen because the insulating films  217 ,  223 , and  225  are provided. With the use of an insulating film containing oxygen, that is, an insulating film capable of releasing oxygen for at least one of the insulating films  217 ,  223 , and  225 , oxygen can be supplied to the semiconductor film  219 . The semiconductor film  219  to which oxygen is supplied has a high resistivity because oxygen vacancies in the film or at the interface are filled. As the insulating film capable of releasing oxygen, a silicon oxide film or a silicon oxynitride film can be used, for example. 
     To obtain an oxide semiconductor film with a low resistivity, hydrogen, boron, phosphorus, or nitrogen may be introduced into the oxide semiconductor film by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. 
     In order to reduce the resistivity of the oxide semiconductor film, plasma treatment may be performed on the oxide semiconductor film. A typical example of the plasma treatment is plasma treatment using a gas containing at least one of a rare gas (He, Ne, Ar, Kr, or Xe), hydrogen, and nitrogen. Specifically, plasma treatment in an argon atmosphere, plasma treatment in a mixed gas atmosphere of argon and hydrogen, plasma treatment in an ammonia atmosphere, plasma treatment in a mixed gas atmosphere of argon and ammonia, plasma treatment in a nitrogen atmosphere, or the like can be employed. 
     In the oxide semiconductor film subjected to the plasma treatment, an oxygen vacancy is formed in a lattice from which oxygen is released (or in a portion from which oxygen is released). This oxygen vacancy can cause carrier generation. Furthermore, hydrogen supplied from an insulating film in the vicinity of the oxide semiconductor film, specifically, an insulating film in contact with the lower surface or the upper surface of the oxide semiconductor film might be bonded to the oxygen vacancy, so that an electron serving as a carrier might be generated. 
     The oxide semiconductor film in which oxygen vacancies are filled and the hydrogen concentration is reduced can be referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film. The term “substantially intrinsic” refers to the state in which an oxide semiconductor film has a carrier density lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , further preferably lower than 1×10 10 /cm 3 . A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources and thus can have a low carrier density. The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly can have a low density of trap states. 
     Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length L of 10 μm, the off-state current can be lower than or equal to the measurement limit of a semiconductor parameter analyzer, that is, lower than or equal to 1×10 −13  A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. Accordingly, the transistor  252  in which a channel region is formed in the semiconductor film  219  that is a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. 
     For example, an insulating film containing hydrogen, that is, an insulating film capable of releasing hydrogen, typically, a silicon nitride film, is used as the insulating film  227 , whereby hydrogen can be supplied to the conductive film  220 . The insulating film capable of releasing hydrogen preferably has a hydrogen concentration of 1×10 22  atoms/cm 3  or higher. Such an insulating film is formed in contact with the conductive film  220 , whereby hydrogen can be effectively contained in the conductive film  220 . Thus, the resistivity of the oxide semiconductor film can be controlled by changing the structure of insulating films in contact with the semiconductor film  219  and the conductive film  220 . Note that a material of the insulating film  215  may be similar to the material of the insulating film  227 . When silicon nitride is used for the insulating film  215 , oxygen released from the insulating film  217  can be prevented from being supplied to the conductive film  213 , so that oxidation of the conductive film  213  can be inhibited. 
     Hydrogen contained in the oxide semiconductor film reacts with oxygen bonded to a metal atom to form water and also causes an oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Entry of hydrogen into the oxygen vacancy may generate an electron serving as a carrier. In some cases, bonding of part of hydrogen to oxygen bonded to a metal element generates an electron serving as a carrier. Accordingly, the conductive film  220  provided in contact with the insulating film containing hydrogen is an oxide semiconductor film that has a higher carrier density than the semiconductor film  219 . 
     Hydrogen in the semiconductor film  219  of the transistor  252  in which a channel region is formed is preferably reduced as much as possible. Specifically, the hydrogen concentration in the semiconductor film  219 , which is measured by SIMS, is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 , lower than 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , still further preferably lower than or equal to 5×10 17  atoms/cm 3 , still further preferably lower than or equal to 1×10 16  atoms/cm 3 . 
     The conductive film  220  functioning as an electrode of the capacitor  255  is an oxide semiconductor film that has a higher hydrogen concentration and/or a larger number of oxygen vacancies (i.e., a lower resistivity) than the semiconductor film  219 . The concentration of hydrogen contained in the conductive film  220  is higher than or equal to 8×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 20  atoms/cm 3 , further preferably higher than or equal to 5×10 20  atoms/cm 3 . The concentration of hydrogen contained in the conductive film  220  is twice or more, preferably 10 times or more that in the semiconductor film  219 . In addition, the resistivity of the conductive film  220  is preferably greater than or equal to 1×10 −8  times and less than 1×10 −1  times the resistivity of the semiconductor film  219 . The resistivity of the conductive film  220  is typically higher than or equal to 1×10 −3  Ωcm and lower than 1×10 4  Ωcm, preferably higher than or equal to 1×10 −3  Ωcm and lower than 1×10 −1  Ωcm. 
     &lt;Protective Insulating Film&gt; 
     As each of the insulating films  223 ,  225  and  227  functioning as a protective insulating film of the transistor  252 , an insulating film including at least one of the following films formed by a PECVD method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. 
     In the case where an oxide semiconductor is used for the semiconductor film  219 , the insulating film  223  that is in contact with the semiconductor film  219  functioning as a channel region of the transistor  252  is preferably an oxide insulating film capable of releasing oxygen. In other words, the insulating film capable of releasing oxygen is an insulating film which includes a region containing oxygen in excess of that in the stoichiometric composition (oxygen-excess region). To provide the oxygen-excess region in the insulating film  223 , the insulating film  223  may be formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the deposited insulating film  223 . As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be employed. 
     The use of the insulating film capable of releasing oxygen as the insulating film  223  enables oxygen to move to the semiconductor film  219  functioning as a channel region of the transistor  252 , so that the number of oxygen vacancies in the semiconductor film  219  can be reduced. For example, the number of oxygen vacancies in the semiconductor film  219  can be reduced by using an insulating film having the following feature: the number of oxygen molecules released from the insulating film at a film surface temperature higher than or equal to 100° C. and lower or equal to 700° C. or higher than or equal to 100° C. and lower than or equal to 500° C. is greater than or equal to 1.0×10 18  molecules/cm 3  when measured by thermal desorption spectroscopy (hereinafter referred to as TDS). 
     In addition, the number of defects in the insulating film  223  is preferably small; typically, in ESR measurement, the spin density of a signal that appears at g=2.001 due to a dangling bond of silicon is preferably lower than or equal to 3×10 17  spins/cm 3 . This is because a high density of defects in the insulating film  223  causes oxygen to be bonded to the defects and decreases the amount of oxygen that permeates the insulating film  223 . Furthermore, the number of defects at the interface between the insulating film  223  and the semiconductor film  219  is preferably small; typically, in ESR measurement, the spin density of a signal that appears at g=1.89 or more and 1.96 or less due to a defect in the semiconductor film  219  is preferably lower than or equal to 1×10 17  spins/cm 3 , further preferably lower than or equal to the lower limit of detection. 
     Note that all oxygen having entered the insulating film  223  from the outside moves to the outside of the insulating film  223  in some cases. Alternatively, part of oxygen having entered the insulating film  223  from the outside remains in the insulating film  223  in some cases. In some cases, movement of oxygen occurs in the insulating film  223  in such a manner that oxygen enters the insulating film  223  from the outside and oxygen contained in the insulating film  223  moves to the outside of the insulating film  223 . When an oxide insulating film which is permeable to oxygen is formed as the insulating film  223 , oxygen released from the insulating film  225  over the insulating film  223  can be moved to the semiconductor film  219  through the insulating film  223 . 
     The insulating film  223  can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (Ec_os) and the energy of the conduction band minimum (Ec_os) of the oxide semiconductor film. A silicon oxynitride film that releases only a small amount of nitrogen oxide, an aluminum oxynitride film that releases only a small amount of nitrogen oxide, or the like can be used as the above oxide insulating film. 
     Note that a silicon oxynitride film that releases only a small amount of nitrogen oxide is a film which releases more ammonia than nitrogen oxide in TDS; the number of ammonia molecules released from the silicon oxynitride film is typically greater than or equal to 1×10 18  molecules/cm 3  and less than or equal to 5×10 19  molecules/cm 3 . Note that the amount of released ammonia is the amount of ammonia released by heat treatment at a film surface temperature higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C. 
     Nitrogen oxide (NO x ; x is more than 0 and 2 or less, preferably 1 or more and 2 or less), typically NO 2  or NO, forms states in the insulating film  223 , for example. The state is positioned in the energy gap of the semiconductor film  219 . Therefore, when nitrogen oxide is diffused into the interface between the insulating film  223  and the semiconductor film  219 , an electron is trapped by the state on the insulating film  223  side in some cases. As a result, the trapped electron remains in the vicinity of the interface between the insulating film  223  and the semiconductor film  219 ; thus, the threshold voltage of the transistor shifts in the positive direction. 
     Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide contained in the insulating film  223  reacts with ammonia contained in the insulating film  225  in heat treatment, the amount of nitrogen oxide contained in the insulating film  223  is reduced. Therefore, an electron is hardly trapped at the interface between the insulating film  223  and the semiconductor film  219 . 
     By using the above oxide insulating film as the insulating film  223 , a shift of the threshold voltage of the transistor can be reduced, which leads to a reduced change in the electrical characteristics of the transistor. 
     In an ESR spectrum at 100 K or lower of the insulating film  223  subjected to heat treatment in a manufacturing process of the transistor, typically heat treatment at a temperature lower than 400° C. or lower than 375° C. (preferably higher than or equal to 340° C. and lower than or equal to 360° C.), a first signal that appears at a g-factor greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals which are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10 18  spins/cm 3 , typically higher than or equal to 1×10 17  spins/cm 3  and lower than 1×10 18  spins/cm 3 . 
     In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO x ; x is more than 0 and 2 or less, preferably 1 or more and 2 or less). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. Accordingly, the smaller the sum of the spin densities of the first signal that appears at a g-factor greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is. 
     The nitrogen concentration in the above oxide insulating film measured by SIMS is lower than or equal to 6×10 20  atoms/cm 3 . 
     The above oxide insulating film is formed by a PECVD method at a substrate temperature higher than or equal to 220° C. and lower than or equal to 350° C. with the use of silane and dinitrogen monoxide, whereby a dense and hard film can be formed. 
     The insulating film  225  in contact with the insulating film  223  is formed using an oxide insulating film whose oxygen content is higher than that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film whose oxygen content is higher than that in the stoichiometric composition. The oxide insulating film whose oxygen content is higher than that in the stoichiometric composition is an oxide insulating film in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 19  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in TDS. Note that the film surface temperature in TDS is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. 
     In addition, the number of defects in the insulating film  225  is preferably small; typically, in ESR measurement, the spin density of a signal that appears at g=2.001 due to a dangling bond of silicon is preferably lower than 1.5×10 18  spins/cm 3 , further preferably lower than or equal to 1×10 18  spins/cm 3 . Note that the insulating film  225  is provided more apart from the semiconductor film  219  than the insulating film  223 ; thus, the insulating film  225  may have a higher density of defects than the insulating film  223 . 
     The thickness of the insulating film  223  can be greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, further preferably greater than or equal to 10 nm and less than or equal to 30 nm. The thickness of the insulating film  225  can be greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 150 nm and less than or equal to 400 nm. 
     Furthermore, the insulating films  223  and  225  can be formed using the same kinds of materials; thus, an interface between the insulating films  223  and  225  cannot be clearly observed in some cases. Therefore, in this embodiment, the interface between the insulating films  223  and  225  is shown by a dashed line. Although the two-layer structure of the insulating films  223  and  225  is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, a single-layer structure of the insulating film  223 , a single-layer structure of the insulating film  225 , or a stacked-layer structure including three or more layers may be used. 
     The insulating film  227  functioning as a dielectric film of the capacitor  255  is preferably a nitride insulating film. In particular, a silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when a silicon nitride film is provided as the insulating film  227  functioning as a dielectric film of the capacitor  255 , the physical thickness of the insulating film can be increased. This makes it possible to suppress a decrease in the withstand voltage of the capacitor  255  and rather to increase the withstand voltage, thereby inhibiting electrostatic discharge damage of the capacitor  255 . Note that the insulating film  227  also has a function of decreasing the resistivity of the conductive film  220  that functions as an electrode of the capacitor  255 . 
     The insulating film  227  also has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like. The insulating film  227  can prevent outward diffusion of oxygen from the semiconductor film  219 , outward diffusion of oxygen contained in the insulating films  223  and  225 , and entry of hydrogen, water, and the like into the semiconductor film  219  from the outside. Instead of the nitride insulating film having a blocking effect against oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like, an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like may be provided. As examples of the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, and a hafnium oxynitride film can be given. 
     At least part of this embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Embodiment 3 
     In this embodiment, other structural examples of a display device that can be used for the display system described in the above embodiment will be described. 
       FIG. 32A  is a schematic top view of a display device  300 .  FIG. 32B  is a schematic cross-sectional view taken along lines A 1 -A 2 , A 3 -A 4 , and A 5 -A 6  in  FIG. 32A . Note that some components are not illustrated in  FIG. 32A  for clarity. 
     The display device  300  includes, over a top surface of a substrate  301 , a display portion  302 , a signal line driver circuit  303 , a scan line driver circuit  304 , and an external connection terminal  305 . A flexible printed circuit (FPC)  355  is connected to the external connection terminal  305  through a connection layer  356 . 
     The display portion  302  includes a liquid crystal element  314 . In the liquid crystal element  314 , the alignment of liquid crystal is controlled by an electric field generated in a direction parallel to the substrate surface. 
     The display device  300  includes the substrate  301 , a substrate  321 , an insulating layer  332 , an insulating layer  334 , an insulating layer  338 , an insulating layer  341 , an insulating layer  342 , a transistor  311 , a transistor  312 , the liquid crystal element  314 , a first electrode  343 , a second electrode  352 , a liquid crystal  353 , a color filter  327 , a light-blocking layer  328 , and the like. The substrate  301  and the substrate  321  are attached to each other with an adhesive layer  354 . 
     A pixel includes at least one switching transistor  312  and a storage capacitor (not illustrated). The comb-shaped second electrode  352  and the comb-shaped first electrode  343  which is electrically connected to one of a source electrode and a drain electrode of the transistor  312  are provided apart from each other over the insulating layer  341 . 
     For the first electrode  343  and/or the second electrode  352 , a light-transmitting conductive material is used. It is preferable to use a light-transmitting conductive material for both of the electrodes because the aperture ratio of the pixel can be increased. 
     The color filter  327  is provided in a position overlapping with the first electrode  343  and the second electrode  352 . The light-blocking layer  328  is provided to cover a side surface of the color filter  327 . Although being provided on the substrate  321  in  FIG. 32B , the position of the color filter  327  is not limited to this position. 
     The liquid crystal  353  is provided between the substrate  301  and the substrate  321 . An image can be displayed in the following manner: voltage is applied between the first electrode  343  and the second electrode  352  to generate an electric field substantially in the horizontal direction, the alignment of the liquid crystal  353  is controlled by the electric field, and polarization of light from a backlight provided outside the display device is controlled in each pixel. 
     Alignment films for controlling the alignment of the liquid crystal  353  are preferably provided on surfaces in contact with the liquid crystal  353 . A light-transmitting material is used for the alignment films. Although not illustrated here, polarizing plates are provided on an outer surface of the substrate  321  and an outer surface of the substrate  301  with respect to the liquid crystal element  314 . 
     As the liquid crystal  353 , thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or the like can be used. Moreover, liquid crystal exhibiting a blue phase is preferably used because an alignment film is not needed and a wide viewing angle is obtained. 
     A high-viscosity and low-fluidity material is preferably used for the liquid crystal  353 . 
     Although an IPS mode is applied to the liquid crystal element  314  in this structural example, the mode of the liquid crystal element is not limited to this, and a TN mode, an FFS mode, an ASM mode, an OCB mode, an FLC mode, an AFLC mode, or the like can be used. 
     The transistors (e.g., the transistor  311  and the transistor  312 ) provided in the display device  300  are top-gate transistors. Each of the transistors includes a semiconductor layer  335 , the insulating layer  334  functioning as a gate insulating layer, and a gate electrode  333 . In addition, the insulating layer  338  is provided to cover the gate electrode  333 . A pair of electrodes  336  is provided in contact with the semiconductor layer  335  through openings in the insulating layer  334  and the insulating layer  338 . 
     Here, an oxide semiconductor is preferably used for the semiconductor layer  335 . As the oxide semiconductor, for example, the oxide semiconductor described in the above embodiment can be used. 
     The semiconductor layer  335  may include a region functioning as a source region or a drain region which has lower resistance than a region functioning as a channel. For example, the source region and the drain region can be provided such that the source region and the drain region are in contact with the pair of electrodes  336  or that the region functioning as a channel is positioned between the source region and the drain region. For example, the source region and the drain region may be regions whose resistivity is controlled by the method described in the above embodiment. 
     Transistors with little variation can be formed at a low temperature in a large area by using an oxide semiconductor for the semiconductor layer  335  compared with the case of using polycrystalline silicon, for example. 
       FIGS. 33A and 33B  illustrate an example in which the semiconductor layer  335  includes a source region and a drain region. 
     Each transistor includes the semiconductor layer  335  having impurity regions  337  functioning as a source region and a drain region, the insulating layer  334  functioning as a gate insulating layer, and the gate electrode  333 . In addition, the insulating layer  338  is provided to cover the gate electrode  333 . The pair of electrodes  336  is provided in contact with the source region and the drain region of the semiconductor layer  335  through openings in the insulating layer  334  and the insulating layer  338 . 
     Here, silicon is preferably used for the semiconductor layer  335 . Silicon is preferably used also for semiconductor devices such as transistors used for pixels in display regions or driver circuits of the display device  300 . 
     Although amorphous silicon may be used as silicon, silicon having crystallinity is particularly preferable. For example, microcrystalline silicon, polycrystalline silicon, or single crystal silicon is preferably used. In particular, polycrystalline silicon can be formed at a lower temperature than single crystal silicon and has higher field-effect mobility and higher reliability than amorphous silicon. When such a polycrystalline semiconductor is used for a pixel, the aperture ratio of the pixel can be improved. Even in an extremely high-resolution display device, a gate driver circuit and a source driver circuit can be formed over a substrate over which pixels are formed, and the number of components of an electronic device can be reduced. 
     In particular, when polycrystalline silicon or single crystal silicon transferred to an insulating layer is used for a semiconductor layer, a top-gate transistor is preferably employed. In this case, a low heat-resistance material can be used for a wiring or an electrode over the semiconductor layer, and a range of choices of the material can be widened. In the case where a high heat-resistance material is used for the gate electrode or in the case where polycrystalline silicon is formed at a very low temperature (e.g., lower than 450° C.), the bottom-gate structure described in the above embodiment is preferably employed because the number of manufacturing steps can be reduced. 
     In one embodiment of the present invention, an active matrix method in which a pixel includes an active element or a passive matrix method in which a pixel does not include an active element can be used. 
     As an active element (a non-linear element) in an active matrix method, not only a transistor but also various active elements (non-linear elements) can be used. For example, a metal insulator metal (MIM) or a thin film diode (TFD) can also be used. Since such an element can be manufactured with a small number of steps, manufacturing cost can be reduced or yield can be improved. Since the size of the element is small, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved. 
     Besides the active matrix method, a passive matrix method in which no active element (non-linear element) is used may be employed. Since no active element (non-linear element) is used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Since no active element (non-linear element) is used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example. 
     At least part of this embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Embodiment 4 
     In this embodiment, the configuration of a pixel that can be used for a display device applicable to the display system of one embodiment of the present invention will be described with reference to  FIGS. 34A to 34C . 
       FIG. 34A  is a circuit diagram illustrating an example of a pixel circuit P(i,j) for a pixel including a liquid crystal display element. 
       FIG. 34B  is a circuit diagram illustrating an example of a pixel circuit PB(i,j) that has a configuration different from that of the pixel circuit P(i,j) in  FIG. 34A .  FIG. 34C  is a top view illustrating an example of the layout of pixel circuits PB(i,j) in  FIG. 34B . 
     &lt;Configuration Example 1 of Pixel Circuit&gt; 
     The pixel circuit P(i,j) is electrically connected to a control line GL(i), a signal line SL(j), and a wiring VCOM and includes a transistor SW, a liquid crystal element LC, and a capacitor C (see  FIG. 34A ). 
     A gate of the transistor SW is electrically connected to the control line GL(i), and a first electrode of the transistor SW is electrically connected to the signal line SL(j). 
     A first electrode of the liquid crystal element LC is electrically connected to a second electrode of the transistor SW, and a second electrode of the liquid crystal element LC is electrically connected to the wiring VCOM. 
     A first electrode of the capacitor C is electrically connected to the second electrode of the transistor SW, and a second electrode of the capacitor C is electrically connected to the wiring VCOM. 
     The pixel circuit P(i,j) is provided over a substrate and includes the substrate, a second conductive film E 2 , and a first conductive film E 1  between the substrate and the second conductive film E 2 . 
     For example, a light-transmitting conductive film can be used as the first conductive film and/or the second conductive film. 
     For example, the first conductive film E 1  can be used for the first electrode of the liquid crystal element LC, and the second conductive film E 2  can be used for the second electrode of the liquid crystal element LC. 
     For example, the first conductive film E 1  can be used for the first electrode of the capacitor C, and the second conductive film E 2  can be used for the second electrode of the capacitor C. 
     &lt;Configuration Example 2 of Pixel Circuit&gt; 
     The pixel circuit PB(i,j) is different from the pixel circuit P(i,j) in  FIG. 34A  in that a liquid crystal element LC 1  and a liquid crystal element LC 2  connected in parallel are provided instead of the liquid crystal element LC (see  FIG. 34B ). Different structures will be described in detail below, and the above description is referred to for other similar structures. 
     A first electrode of the liquid crystal element LC 1  is electrically connected to the second electrode of the transistor SW, and a second electrode of the liquid crystal element LC 1  is electrically connected to the wiring VCOM. 
     A second electrode of the liquid crystal element LC 2  is electrically connected to the second electrode of the transistor SW, and a first electrode of the liquid crystal element LC 2  is electrically connected to the wiring VCOM. 
     For example, the first conductive film E 1  can be used for the first electrode of the liquid crystal element LC 1 , and the second conductive film E 2  can be used for the second electrode of the liquid crystal element LC 1 . In addition, the first conductive film E 1  can be used for the first electrode of the liquid crystal element LC 2 , and the second conductive film E 2  can be used for the second electrode of the liquid crystal element LC 2  (see  FIG. 34C ). 
     The pixel circuit PB(i,j) includes the liquid crystal element LC 1  and the liquid crystal element LC 2 . The first electrode of the liquid crystal element LC 1  includes the first conductive film E 1  connected to the second electrode of the transistor SW, and the second electrode of the liquid crystal element LC 1  includes the second conductive film E 2  electrically connected to the wiring VCOM. The second electrode of the liquid crystal element LC 2  includes the second conductive film E 2  connected to the second electrode of the transistor SW, and the first electrode of the liquid crystal element LC 2  includes the first conductive film E 1  electrically connected to the wiring VCOM. 
     The liquid crystal element LC 1  and the liquid crystal element LC 2  are connected in parallel as described above. Accordingly, characteristics of the liquid crystal elements can be prevented from being asymmetric due to the positions of the first conductive film E 1  and the second conductive film E 2  even in the case where the liquid crystal elements are driven with the applied voltage inverted. 
     At least part of this embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Embodiment 5 
     In this embodiment, examples of a method for driving an input device or input/output device of one embodiment of the present invention will be described with reference to drawings. 
     [Example of Sensing Method of Sensor] 
       FIG. 35A  is a block diagram illustrating the configuration of a mutual capacitive touch sensor.  FIG. 35A  illustrates a pulse voltage output circuit  601  and a current sensing circuit  602 . In  FIG. 35A , six wirings X 1  to X 6  represent electrodes  621  to which pulse voltage is applied, and six wirings Y 1  to Y 6  represent electrodes  622  which sense changes in current.  FIG. 35A  also illustrates a capacitor  603  in which the electrode  621  and the electrode  622  overlap with each other. Note that functional replacement between the electrode  621  and the electrode  622  is possible. 
     The pulse voltage output circuit  601  is a circuit for sequentially applying pulse voltage to the wirings X 1  to X 6 . By application of pulse voltage to one of the wirings X 1  to X 6 , an electric field is generated between the electrode  621  and the electrode  622  of the capacitor  603 . For example, when the electric field between the electrodes is shielded, a change occurs in the capacitance of the capacitor  603 . The approach or contact of an object can be detected by utilizing this change. 
     The current sensing circuit  602  is a circuit for sensing changes in current flowing through the wirings Y 1  to Y 6  which are caused by the capacitance change in the capacitor  603 . No change in current value is sensed in the wirings Y 1  to Y 6  when there is no approach or contact of an object, whereas a decrease in current value is sensed when capacitance decreases owing to the approach or contact of an object. Note that an integrator circuit or the like may be used for sensing current. 
       FIG. 35B  is a timing chart illustrating input and output waveforms in the mutual capacitive touch sensor in  FIG. 35A . In  FIG. 35B , detection of an object is performed in all the rows and columns in one frame period.  FIG. 35B  illustrates a period in which no object is detected (without touch) and a period in which an object is detected (with touch). Current values sensed in the wirings Y 1  to Y 6  are illustrated as waveforms of voltage values. 
     Pulse voltage is sequentially applied to the wirings X 1  to X 6 , and waveforms of the wirings Y 1  to Y 6  change in accordance with the pulse voltage. When there is no approach or contact of an object, the waveforms of the wirings Y 1  to Y 6  uniformly change in accordance with changes in the voltage of the wirings X 1  to X 6 . As indicated by arrows in  FIG. 35B , the current value decreases at the point of the approach or contact of the object; accordingly, the waveform of the voltage value also changes. 
     By sensing a capacitance change in this manner, the approach or contact of an object can be detected. 
     It is preferable that the pulse voltage output circuit  601  and the current sensing circuit  602  be mounted on a substrate in a housing of an electronic device or on a touch panel in the form of an IC. In the case where the touch panel has flexibility, parasitic capacitance might increase in a bent portion of the touch panel, and the influence of noise might increase. Therefore, it is preferable to use an IC driven by a method which is less likely to be influenced by noise. For example, it is preferable to use an IC driven by a method capable of increasing the signal-noise ratio (S/N ratio). 
     Although  FIG. 35A  illustrates a passive matrix touch sensor in which only the capacitor  603  is provided at the intersection of wirings as a touch sensor, an active matrix touch sensor including a transistor and a capacitor may also be used.  FIG. 36  illustrates an example of a sensor circuit included in an active matrix touch sensor. 
     The sensor circuit includes the capacitor  603  and transistors  611 ,  612 , and  613 . A signal G 2  is input to a gate of the transistor  613 . Voltage VRES is applied to one of a source and a drain of the transistor  613 , and one electrode of the capacitor  603  and a gate of the transistor  611  are electrically connected to the other of the source and the drain of the transistor  613 . One of a source and a drain of the transistor  611  is electrically connected to one of a source and a drain of the transistor  612 , and voltage VSS is applied to the other of the source and the drain of the transistor  611 . A signal G 1  is input to a gate of the transistor  612 , and a wiring ML is electrically connected to the other of the source and the drain of the transistor  612 . The voltage VSS is applied to the other electrode of the capacitor  603 . 
     Next, the operation of the sensor circuit will be described. First, a potential for turning on the transistor  613  is supplied as the signal G 2 , and a potential corresponding to the voltage VRES is supplied to a node n connected to the gate of the transistor  611 . Then, a potential for turning off the transistor  613  is supplied as the signal G 2 , whereby the potential of the node n is held. 
     Then, the capacitance of the capacitor  603  changes owing to the approach or contact of an object such as a finger, and accordingly, the potential of the node n changes from VRES. 
     In a reading operation, a potential for turning on the transistor  612  is supplied as the signal G 1 . Current flowing through the transistor  611 , that is, current flowing through the wiring ML changes in accordance with the potential of the node n. By sensing this current, the approach or contact of an object can be detected. 
     It is preferable that the transistors  611 ,  612 , and  613  each include an oxide semiconductor in a semiconductor layer in which a channel is formed. In particular, with the transistor  613  having such a structure, the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced. 
     [Configuration Example of in-Cell Touch Panel] 
     In the above-described examples, the electrodes in the touch sensor are formed over a substrate different from a substrate over which the display element and the like are provided; however, one or both of the pair of electrodes in the touch sensor may be provided over the substrate over which the display element and the like are provided. 
     A configuration example of a touch panel in which a touch sensor is incorporated in a display portion including a plurality of pixels will be described below. In an example shown here, a liquid crystal element is used as a display element provided in the pixel. 
       FIG. 37A  is an equivalent circuit diagram of part of a pixel circuit provided in the display portion of the touch panel described in this configuration example. 
     Each pixel includes at least a transistor  3503  and a liquid crystal element  3504 . A gate of the transistor  3503  is electrically connected to a wiring  3501 , and one of a source and a drain of the transistor  3503  is electrically connected to a wiring  3502 . 
     The pixel circuit includes a plurality of wirings extending in the X direction (e.g., a wiring  3510 _ 1  and a wiring  3510 _ 2 ) and a plurality of wirings extending in the Y direction (e.g., a wiring  3511 ). These wirings intersect with each other, so that capacitance is formed therebetween. 
     Some adjacent pixels of the pixels provided in the pixel circuit form a block as follows: one electrode of the liquid crystal element in a pixel is electrically connected to one electrode of the liquid crystal element in an adjacent pixel. The block is classified into two types: an island-shaped block (e.g., a block  3515 _ 1  or a block  3515 _ 2 ) and a linear block (e.g., a block  3516 ) extending in the Y direction. Note that only part of the pixel circuit is illustrated in  FIG. 37A ; actually, these two kinds of blocks are repeatedly arranged in the X direction and the Y direction. 
     The wiring  3510 _ 1  (or  3510 _ 2 ) extending in the X direction is electrically connected to the island-shaped block  3515 _ 1  (or  35152 ). Although not illustrated, the wiring  3510 _ 1  extending in the X direction is electrically connected to a plurality of island-shaped blocks  3515 _ 1  which are provided discontinuously along the X direction with the linear blocks positioned therebetween. The wiring  3511  extending in the Y direction is electrically connected to the linear block  3516 . 
       FIG. 37B  is an equivalent circuit diagram illustrating the connection between a plurality of wirings  3510  extending in the X direction and the plurality of wirings  3511  extending in the Y direction. Input voltage or a common potential can be input to each of the wirings  3510  extending in the X direction. Furthermore, a ground potential can be input to each of the wirings  3511  extending in the Y direction, or the wirings  3511  can be electrically connected to a sensing circuit. 
     The operation of the above-described touch panel will be described below with reference to  FIGS. 38A and 38B . 
     Here, one frame period is divided into a writing period and a sensing period. In the writing period, in which image data is written to a pixel, the wirings  3501  (also referred to as gate lines or scan lines) in  FIG. 37A  are sequentially selected. On the other hand, in the sensing period, in which sensing is performed by a touch sensor, the wirings  3510  extending in the X direction are sequentially selected and input voltage is input. 
       FIG. 38A  is an equivalent circuit diagram in the writing period. In the writing period, a common potential is input to both the wiring  3510  extending in the X direction and the wiring  3511  extending in the Y direction. 
       FIG. 38B  is an equivalent circuit diagram at a certain point in time in the sensing period. In the sensing period, each of the wirings  3511  extending in the Y direction is electrically connected to the sensing circuit. Input voltage is input to selected one of the wirings  3510  extending in the X direction, and a common potential is input to the others of the wirings  3510 . 
     Note that the driving method described here can be applied to not only an in-cell touch panel but also the above-described touch panels and can be used in combination with the driving method described above as an example. 
     It is preferable that a period in which an image is written and a period in which sensing is performed by a touch sensor be separately provided as described above. This configuration can suppress a decrease in sensitivity of the touch sensor caused by noise generated when an image is written to a pixel. 
     At least part of this embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Embodiment 6 
     In this embodiment, a display module and electronic devices which include a display device or display system of one embodiment of the present invention will be described with reference to  FIG. 39  and  FIGS. 40A to 40G . 
     In a display module  8000  illustrated in  FIG. 39 , a touch panel  8004  connected to an FPC  8003 , a display panel  8006  connected to an FPC  8005 , a backlight  8007 , a frame  8009 , a printed board  8010 , and a battery  8011  are provided between an upper cover  8001  and a lower cover  8002 . 
     The display device of one embodiment of the present invention can be used for, for example, the display panel  8006 . 
     The shapes and sizes of the upper cover  8001  and the lower cover  8002  can be changed as appropriate in accordance with the sizes of the touch panel  8004  and the display panel  8006 . 
     The touch panel  8004  can be a resistive touch panel or a capacitive touch panel and overlap with the display panel  8006 . Alternatively, a counter substrate (sealing substrate) of the display panel  8006  can have a touch panel function. Further alternatively, a photosensor can be provided in each pixel of the display panel  8006  to form an optical touch panel. 
     The backlight  8007  includes light sources  8008 . Although the light sources  8008  are provided over the backlight  8007  in  FIG. 39 , one embodiment of the present invention is not limited to this structure. For example, a structure in which the light source  8008  is provided at an end portion of the backlight  8007  and a light diffusion plate is further used may be employed. Note that the backlight  8007  need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed. 
     The frame  8009  protects the display panel  8006  and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board  8010 . The frame  8009  can also function as a radiator plate. 
     The printed board  8010  is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the separate battery  8011  may be used. The battery  8011  can be omitted in the case where a commercial power source is used. 
     The display module  8000  may be provided with an additional member such as a polarizing plate, a retardation plate, or a prism sheet. 
       FIGS. 40A to 40G  illustrate electronic devices. These electronic devices can include a housing  5000 , a display portion  5001 , a speaker  5003 , an LED lamp  5004 , operation keys  5005  (including a power switch and an operation switch), a connection terminal  5006 , a sensor  5007  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone  5008 , and the like. 
       FIG. 40A  illustrates a mobile computer which can include a switch  5009 , an infrared port  5010 , and the like in addition to the above components.  FIG. 40B  illustrates a portable image reproducing device (e.g., a DVD reproducing device) provided with a recording medium, and the portable image reproducing device can include a second display portion  5002 , a recording medium reading portion  5011 , and the like in addition to the above components.  FIG. 40C  illustrates a goggle-type display which can include the second display portion  5002 , a support portion  5012 , an earphone  5013 , and the like in addition to the above components.  FIG. 40D  illustrates a portable game console which can include the recording medium reading portion  5011  and the like in addition to the above components.  FIG. 40E  illustrates a digital camera with a television reception function, and the digital camera can include an antenna  5014 , a shutter button  5015 , an image receiving portion  5016 , and the like in addition to the above components.  FIG. 40F  illustrates a portable game console which can include the second display portion  5002 , the recording medium reading portion  5011 , and the like in addition to the above components.  FIG. 40G  illustrates a portable television receiver which can include a charger  5017  capable of transmitting and receiving signals, and the like in addition to the above components. 
     The electronic devices in  FIGS. 40A to 40G  can have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a recording medium and displaying it on the display portion. Furthermore, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information mainly on another display portion, a function of displaying a three-dimensional image by displaying images on a plurality of display portions with a parallax taken into account, or the like. Furthermore, the electronic device including an image receiving portion can have a function of shooting a still image, a function of taking moving images, a function of automatically or manually correcting a shot image, a function of storing a shot image in a recording medium (an external recording medium or a recording medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions of the electronic devices in  FIGS. 40A to 40G  are not limited thereto, and the electronic devices can have a variety of functions. 
     Electronic devices described in this embodiment are characterized by a display portion for displaying some sort of information. The display device described in the above embodiment can be used for the display portion. 
     At least part of this embodiment can be implemented in appropriate combination with any of the other embodiments described in this specification. 
     Example 
     In this example, a display system of one embodiment of the present invention will be described with reference to  FIGS. 41A to 41C ,  FIGS. 42A to 42C , and  FIGS. 43A and 43B . 
       FIGS. 41A to 41C  show the measurement results of luminance changes in a  100 -μm-diameter region of a display device. Note that a text image was displayed on the display device while being scrolled. The text image includes 25 lines per page. Each line includes 49 letters with a font size of 20 points. 
       FIG. 41A  shows a change in luminance observed when the text image was displayed while being scrolled at a speed of 2.5 pages/sec. 
       FIG. 41B  shows a change in luminance observed when the letters in the text image were displayed with a higher gray level than those in  FIG. 41A  (specifically, the luminance of the letters was approximately 50% of that of the background image) while the text image was scrolled at a speed of 5 pages/sec. 
       FIG. 41C  shows a change in luminance observed when the letters in the text image were displayed with the same gray level as those in  FIG. 41A  while the text image was scrolled at a speed of 5 pages/sec. 
       FIGS. 42A to 42C  show the calculation results of changes in visual stimulation based on the luminance changes shown in  FIGS. 41A to 41C . The calculation was performed using the Barten model, which agrees well with results of previous sensitivity evaluation. The Barten model is expressed by the following equation (1). 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the equation, u and w are a parameter of the frequency of spatial modulation and a parameter of the frequency of temporal modulation, respectively. In addition, k represents a signal-noise ratio, T represents visual integration time, X 0  represents the size of an object, X max  represents the upper limit of integration, N max  represents the maximum number of integration cycles of bright and dark, η represents quantum efficiency, p represents a quantum conversion factor, E represents retinal illuminance, Φ 0  represents the spectral density of neural noise, and u 0  represents the spatial frequency of lateral inhibition. 
     In the equation (1), M opt (u) represents a visual transfer function relating to spatial luminance modulation and is expressed by the following equation (2). In the equation (2), σ depends on the pupil diameter as a parameter and corresponds to the standard deviation of a line-spread function, where the structures of visual organs such as the ocular media and the retina are taken into consideration.
 
[Formula 2]
 
 M   opt ( u )= e   −2π     2     σ     2     u     2     (2)
 
     In the equation (1), H 1 (w) and H 2 (w) each represent a transfer function relating to temporal modulation and are expressed by the following equation (3), where τ represents a time constant. The solution of the equation (1) agrees with the results of sensitivity evaluation in the case where 7 and 4 are substituted for n in H 1 (w) and H 2 (w), respectively. 
     
       
         
           
             
               
                 
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     In addition, F(u) in the equation (1) represents a function of lateral inhibition and is expressed by the following equation (4). In the equation (4), u 0  represents the spatial frequency of lateral inhibition.
 
[Formula 4]
 
 F ( u )=1−√{square root over (1 −e   −(u/u     0     )     2   )}  (4)
 
       FIG. 42A  shows the calculation result of the change in visual stimulation based on the luminance change shown in  FIG. 41A , which was obtained using the Barten model. 
       FIG. 42B  shows the calculation result of the change in visual stimulation based on the luminance change shown in  FIG. 41B , which was obtained using the Barten model. 
       FIG. 42C  shows the calculation result of the change in visual stimulation based on the luminance change shown in  FIG. 41C , which was obtained using the Barten model. 
       FIGS. 43A and 43B  show the measurement results of the critical fusion frequencies (CFF) of six subjects (A to F) who observed the text images of  FIGS. 41A to 41C . Specifically, the text image was observed for a minute while being scrolled, and then, the CFF was measured ten times, and the measurement values were averaged. This process was repeated five times, and added time was counted as time of stressing. 
       FIG. 43A  shows the measurement results of the CFFs of the six subjects who observed the text image of  FIG. 41B . 
       FIG. 43B  shows the measurement results of the CFFs of the six subjects who observed the text image of  FIG. 41C . 
     For the measurement, AQUOS PAD SH-06F produced by Sharp Corporation was used. The screen diagonal of the display panel was 7.0 inches, the pixel density was 323 ppi, and each pixel includes a VA-mode liquid crystal element and a transistor including an oxide semiconductor. 
     For the CFF measurement, a Roken-type digital flicker value tester, model RDF-1 produced by SIBATA SCIENTIFIC TECHNOLOGY LTD. was used. 
     &lt;Result&gt; 
     When compared in the same period, a luminance change at a low scroll speed ( FIG. 41A  and  FIG. 42A ) was smaller than that at a high scroll speed ( FIG. 41C  and  FIG. 42C ); accordingly, visual stimulation was suppressed when the scroll speed was low. 
     Comparison between luminance changes at a high scroll speed in the same period ( FIGS. 41B and 41C  and  FIGS. 42B and 42C ) showed that a luminance change in the text image displaying letters with a high gray level (i.e., the contrast is low) ( FIG. 41B  and  FIG. 42B ) was smaller, and thus, visual stimulation was suppressed. 
     In addition, decreases in the CFFs of the subjects who repeatedly observed the text image scrolled at a high speed were suppressed when the letters in the text image were displayed with a high gray level (i.e., when the contrast was low) (see  FIGS. 43A and 43B ). 
     Therefore, eye strain on the subject accumulated by high-speed scrolling can be reduced by displaying letters with a high gray level (i.e., displaying a low-contrast text image). 
     Specifically, when the letters with a high gray level (i.e., the low-contrast text image) were displayed, no decrease was observed in the CFFs of the subjects (see  FIG. 43A ). 
     On the other hand, when the gray level of the letters in the text image was not changed (i.e., the contrast was high), the CFFs of the subject A, the subject C, the subject D, the subject F were decreased (see  FIG. 43B ). 
     This application is based on Japanese Patent Application serial no. 2015-040869 filed with Japan Patent Office on Mar. 3, 2015, and Japanese Patent Application serial no. 2015-040870 filed with Japan Patent Office on Mar. 3, 2015, the entire contents of which are hereby incorporated by reference.