Patent Publication Number: US-8994756-B2

Title: Method for driving display device in which analog signal and digital signal are supplied to source driver

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for driving a display device. 
     2. Description of the Related Art 
     In recent years, a so-called self-luminous display device in which a pixel is formed using a light emitting element such as a light emitting diode (LED) has attracted attention. As a light emitting element used for such a self-luminous display device, an organic light emitting diode (also referred to as an OLED, an organic EL element, an electroluminescent (EL) element, or the like) has attracted attention, and have been used for an EL display (for example, an organic EL display or the like). A light emitting element such as an OLED is of self-luminous type; therefore, it has advantages such as higher pixel visibility, no backlight, and higher response speed compared to a liquid crystal display. The luminance of a light emitting element is controlled by the value of the current flowing therethrough. 
     Driving methods to control light emission gray scales of such a display device include a digital gray scale method and an analog gray scale method. In the digital gray scale method, a light emitting element is turned on/off by controlling in a digital manner to express a gray scale. On the other hand, in the analog gray scale method, there is a method of controlling the light emission intensity of a light emitting element in an analog manner and a method of controlling the light emission time of a light emitting element in an analog manner. 
     In the case of the digital gray scale method, there are only two states; a light emitting state and a non-light emitting state. So only two gray scale levels can be expressed. Therefore, multiple gray scale display is attempted by using another method in combination with the digital gray scale method. As the method for achieving multiple gray scale, a time gray scale method is often used (see Reference 1: Japanese Patent Laid-Open No. 2001-324958 and Reference 2: Japanese Patent Laid-Open No. 2001-343933). 
     Displays in which a display mode is controlled in a digital manner and a time gray scale method is combined to express a gray scale, include displays other than an organic EL display using a digital gray scale method. An example thereof is a plasma display. 
     The time gray scale method is a method for expressing a gray scale by controlling the length of a light emitting period or the number of light emissions. In other words, one frame is divided into a plurality of subframes, each of which is weighted by the number of light emissions, a light emitting period, or the like, and the total weight (the sum of the number of light emissions or the sum of the light emitting periods) is differentiated for each gray scale level, thereby expressing a gray scale. 
     Thus, there are an analog gray scale method and a digital gray scale method; however, both methods have advantages and disadvantages, and there has been no method that combines the advantages of both methods. Therefore, the method has to be limited to either of them. 
     For example, in the case of an analog gray scale method, a gray scale is displayed smoothly, whereas noise is also displayed together or contrast is decreased. 
     SUMMARY OF THE INVENTION 
     In view of such problems as described above, it is an object of the present invention to provide a display device which has the advantages of both an analog gray scale method and a digital gray scale method and which can perform high-contrast clear display. 
     The present invention provides a display device including a means that can perform display in a plurality of display modes. In other words, an analog signal and a digital signal are switched and input to a source driver. In addition, the display device includes a means that switches and outputs an analog signal and a digital signal. By using such means, the display device can have the advantages of both the analog gray scale method and the digital gray scale method, thereby achieving the above object. 
     A display device of the present invention is a display device in which a plurality of pixels is arranged in matrix. The display device includes a source driver and a gate driver, and at least two display modes, in which an analog signal is supplied to the source driver in a first display mode, and a digital signal is supplied to the source driver in a second display mode. 
     The present invention is a display device in which a plurality of pixels is arranged in matrix. The display device includes a source driver and a gate driver, and at least two display modes, in which an analog signal is supplied to the source driver and an analog signal is supplied to the pixel from the source driver in a first display mode, and a digital signal is supplied to the source driver and a digital signal is supplied to the pixel from the source driver in a second display mode. 
     In the present invention, a display device according to the above structure further includes a display mode-specific video signal generation circuit, in which the analog signal and the digital signal supplied to the source driver are output from the display mode-specific video signal generation circuit. 
     In the present invention, a display device according to the above structure further includes a display mode-specific video signal generation circuit including a binarization circuit, in which a video signal input to the display mode-specific video signal generation circuit is an analog signal, and a signal of the video signal used for the second display mode is converted into a digital signal using the binarization circuit. Alternatively, the display device may include a display mode-specific video signal generation circuit including a value multiplexing circuit, in which a video signal input to the display mode-specific video signal generation circuit is an analog signal, and a signal of the video signal used for the second display mode is converted into a digital signal using the value multiplexing circuit. Alternatively, the display device may include a display mode-specific video signal generation circuit including a digital analog converter circuit, in which a video signal input to the display mode-specific video signal generation circuit is a digital signal, and a signal of the video signal used for the first display mode is converted into an analog signal using the digital analog converter circuit. 
     Note that in the present invention, display modes are distinguished according to the number of gray scale levels; for example, the number of gray scale levels is different between the first display mode and the second display mode. 
     In the present invention, one pixel means one element that can control brightness. Therefore, one pixel means, for example, one color element, which expresses brightness. Accordingly, in the case of a color display device including color elements of R (Red), G (Green), and B (Blue), it is assumed that the smallest unit of an image is composed of three pixels of R, G. and B. Note that the color elements are not limited to three colors, and may be more than three colors, for example, RGBW (W is white). 
     Note that the phrase “pixels are arranged in matrix” in this specification includes the case where pixels are arranged in a so-called grid of a combination of vertical stripes and lateral stripes. It also includes the case where pixels of three color elements (for example, RGB) representing the smallest unit of one image are in so-called delta arrangement when full color display is performed with the three color elements. Furthermore, it also includes the case of Bayer arrangement. In addition, the color elements may each have different light emitting regions. 
     In the present invention, there is no limitation on the kind of applicable transistor. A thin film transistor (TFT) using a non-single crystal semiconductor film typified by an amorphous silicon film or a polycrystalline silicon film, a MOS transistor formed using a semiconductor substrate or an SOI substrate, a junction transistor, a bipolar transistor, a transistor using an organic semiconductor or a carbon nanotube, or another transistor can be used. Note that the non-single crystal semiconductor film may contain hydrogen or halogen. In addition, there is no limitation on the kind of substrate over which the transistor is to be located. The transistor can be located over a single-crystal substrate, an SOI substrate, a glass substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, or the like. Further, the transistor may be formed over one substrate, and later, may be transferred to and located over another substrate. 
     Note that as described above, the transistor in the present invention may be of any kind and may be formed over any substrate. Therefore, all circuits may be formed over a glass substrate, a plastic substrate, a single-crystal substrate, an SOI substrate, or any other type of substrate. By forming all circuits over one substrate, the number of components can be reduced to reduce cost and the number of connections with circuit components can be reduced to improve reliability and the like. Alternatively, part of circuits may be formed over one substrate and another part of circuits may be formed over another substrate. In other words, all circuits are not necessarily formed over one substrate. For example, part of circuits may be formed using transistors over a glass substrate and as another part of circuits, an IC chip formed over a single-crystal substrate or the like may be arranged over the glass substrate by connecting by COG using a (Chip On Glass) method. Alternatively, the IC chip may be connected to the glass substrate by using a TAB (Tape Automated Bonding) method or a printed wiring board. Thus, by forming part of circuits over one substrate, the number of components can be reduced to reduce cost and the number of connections with circuit components can be reduced to improve reliability and the like. In addition, a portion with high drive voltage and a portion with high drive frequency consume a large amount of power. Therefore, by preventing such portions from being formed over the same substrate, an increase in power consumption can be prevented. 
     Note that a switch to be described in this specification can be of various types, one example of which is an electric switch, a mechanical switch, or the like. In other words, any switch that can control current flow can be used, and there is no particular limitation. For example, the switch may be a transistor, a diode (such as a PN diode, a PIN diode, a Schottky diode, or a diode-connected transistor), or a logic circuit that is a combination thereof. In the case of using a transistor as the switch, the transistor operates as a mere switch. Therefore, the polarity (conductivity type) of the transistor is not particularly limited. However, it is desirable to use a transistor having a polarity with lower off-current. As the transistor with low off-current, a transistor provided with an LDD region, a transistor having a multi-gate structure, or the like can be used. In addition, it is desirable to use an n-channel transistor when a transistor to be operated as a switch operates in a state where a potential of a source electrode thereof is close to a lower potential side power source (such as Vss, GND, or 0 V), whereas it is desirable to use a p-channel transistor when a transistor operates in a state where a potential of a source electrode thereof is close to a higher potential side power source (such as Vdd). This is because the absolute value of a gate-source voltage can be increased, so that the transistor easily operates as a switch. Note that the switch may be of CMOS type using both an n-channel transistor and a p-channel transistor. If the switch is of CMOS type, it can operate appropriately because output voltage can be easily controlled with respect to various input voltages. 
     Note that in the present invention, the phrase “being connected” is synonymous with being electrically connected. Therefore, in the constitution disclosed in the present invention, another element (such as a switch, a transistor, a capacitor element, an inductor, a resistor element, or a diode) which enables electrical connection may be interposed in a predetermined connection. Naturally, components may be arranged without another element interposed therebetween, and the phrase “being electrically connected” includes the case of being directly connected. 
     According to the present invention, display can be performed while switching an analog gray scale method and a digital gray scale method. Consequently, display quality such as contrast can be improved, and power consumption can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram explaining a structure of a display device of the present invention. 
         FIG. 2  is a diagram explaining a structure of a display device of the present invention. 
         FIG. 3  is a diagram explaining a structure of a part of a display device of the present invention. 
         FIGS. 4A to 4C  are diagrams explaining methods for driving display devices of the present invention. 
         FIG. 5  is a diagram explaining a structure of a part of a display device of the present invention. 
         FIGS. 6A and 6B  are diagrams explaining a part of a structure of a display device of the present invention. 
         FIG. 7  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 8  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 9  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 10  is a diagram explaining a display condition of a display device of the present invention. 
         FIG. 11  is a diagram explaining a display condition of a display device of the present invention. 
         FIG. 12  is a diagram explaining a display condition of a display device of the present invention. 
         FIG. 13  is a diagram explaining a display condition of a display device of the present invention. 
         FIGS. 14A to 14D  are diagrams explaining switches of the present invention. 
         FIG. 15  is a diagram explaining a structure of a pixel in a display device of the present invention. 
         FIG. 16A  is a diagram explaining a part of a structure of a display device of the present invention, and  FIG. 16B  is a graph explaining a driving method thereof. 
         FIG. 17  is a diagram explaining a structure of a pixel in a display device of the present invention. 
         FIG. 18  is a diagram explaining a structure of a display device of the present invention. 
         FIG. 19  is a diagram explaining an electronic device to which the present invention is applied. 
         FIGS. 20A and 20B  are diagrams explaining structures of display devices of the present invention. 
         FIG. 21  is a diagram explaining a structure of a display device of the present invention. 
         FIG. 22  is a diagram explaining a structure of a display device of the present invention. 
         FIGS. 23A to 23H  are diagrams explaining electronic devices to which the present invention is applied. 
         FIG. 24  is a diagram explaining a structure of a display device of the present invention. 
         FIG. 25  is a diagram explaining a structure of a part of a display device of the present invention. 
         FIG. 26  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 27  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 28  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 29  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 30  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 31  is a diagram explaining a part of a structure of a display device of the present invention. 
         FIG. 32  is a diagram explaining a structure of a pixel in a display device of the present invention. 
         FIG. 33  is a diagram explaining a structure of a pixel in a display device of the present invention. 
         FIG. 34  is a diagram explaining a structure of a pixel in a display device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiment modes of the present invention are explained with reference to the drawings. However, the present invention is not limited to the following description. As is easily understood to a person skilled in the art, the mode and the detail of the invention can be variously changed without departing from the spirit and the scope of the present invention. Thus, the present invention is not interpreted while limiting to the following description of the embodiment modes. 
     (Embodiment Mode 1) 
     An overall configuration diagram is shown in  FIG. 1 . A source driver  102  and a gate driver  103  are provided to drive a pixel array  101 . A video signal is input to the source driver  102 . Note that a plurality of source drivers  102  and gate drivers  103  may each be provided. 
     Note that the source driver or part thereof may be formed using, for example, an external IC chip without existing over the same substrate as the pixel array  101 . 
     Note that the transistor in the present invention may be of any type and may be formed over any substrate as described above. Therefore, such circuits as shown in  FIG. 1  may all be formed over a glass substrate, a plastic substrate, a single crystalline substrate, an SOI substrate, or any other substrate. Alternatively, part of circuits shown in  FIG. 1  or the like may be formed over a certain substrate and another part thereof shown in  FIG. 1  or the like may be formed over another substrate. In other words, all circuits shown in  FIG. 1  or the like are not necessarily formed over one substrate. For example, in  FIG. 1  or the like, the pixel array  101  and the gate driver  103  may be formed using TFTs over a glass substrate, the source driver  102  (or part thereof) may be formed over a single crystalline substrate, and an IC chip thereof may be connected by using a COG (Chip On Glass) method and provided over the glass substrate. Alternatively, the IC chip may be connected to the glass substrate by using a TAB (Tape Automated Bonding) method or a printed circuit board. 
     The video signal input to the source driver  102  is generated in a display mode-specific video signal generation circuit  106  in accordance with each display mode. The display mode-specific video signal generation circuit  106  is controlled using a controller (control circuit)  107 . An original video signal is input to the display mode-specific video signal generation circuit  106 . Then, the video signal in accordance with each display mode is generated in the display mode-specific video signal generation circuit  106  using the original video signal and is output to the source driver  102 . 
     The display modes can be roughly divided into an analog mode and a digital mode. In the analog mode, a video signal input to a pixel is an analog value. On the other hand, in the digital mode, a video signal input to a pixel is a digital value. 
     Subsequently, details of a circuit are described.  FIG. 2  shows the structure of the source driver  102  and so on. A shift register  231  is a circuit which outputs a sequentially selecting signal (a so-called sampling pulse). Therefore, the circuit is not limited to the shift register as long as a circuit that performs a similar function is used. For example, a decoder circuit may be used. 
     The sampling pulse output from the shift register is input to analog switches  201  to  203 . Then, video signals are sequentially input to a video signal line  221 ; the analog switches  201  to  203  are sequentially turned on in accordance with the sampling pulse; and the video signals are input to the pixel array  101 . In the pixel array  101 , pixels  211  are arranged in matrix. 
     Note that  FIG. 2  shows the case where two rows and three columns of pixels  211  are arranged, but the invention is not limited thereto. Any number of pixels can be arranged. 
       FIG. 15  shows an example of one pixel  220 . A selection transistor  404  is controlled using a gate signal line  401 . If the selection transistor  404  is turned on, a video signal is input to a storage capacitor  405  from a source signal line  402 . Then, a driver transistor  406  is turned on or off in accordance with the video signal, and current flows to an opposite electrode  408  from a power supply line  403  through a light emitting element  407 . 
     Note that a pixel structure is not limited to that shown in  FIG. 15 . For example, a structure which corrects variations in the driver transistors may be employed. 
     The pixel structures which correct variations can be roughly divided into two types: one is that corrects variations in threshold voltage and the other is that inputs current as a video signal. 
       FIG. 31  shows the pixel structure which corrects variations in threshold voltage. A threshold voltage of a driver transistor  3101  is stored in a capacitor element  3104  by controlling a switch  3107  using a gate signal line  3115 . A switch  3103  controlled by a gate signal line  3114  serves to initialize the gate potential of the driver transistor  3101 . Then, a video signal is input from a source signal line  3111  through a switch  3102 . Note that the selection transistor  404  in  FIG. 15  corresponds to the switch  3102  in  FIG. 31 ; the storage capacitor  405 , to a capacitor element  3105 ; and the driver transistor  406 , to the driver transistor  3101 . In addition, the gate signal line  401  corresponds to a gate signal line  3113 ; the source signal line  402 , to the source signal line  3111 ; and the power supply line  403 , to a power supply line  3116 . 
     In  FIG. 31 , a wire  3112  for initializing the gate potential of the driver transistor  3101  is necessary. A structure where the wire  3112  is removed from  FIG. 31  is shown in  FIG. 32 . A gate of the driver transistor  3101  is connected to a drain of the driver transistor  3101  through a switch  3203 . 
     Note that there are various pixel structures which correct variations in threshold voltage, and the invention is not limited to the structures shown in  FIGS. 31 and 32 . By using the pixel structure which corrects variations in threshold voltage as described above, variations of current flowing to the light emitting elements can be reduced. In particular, luminance can be equalized in an analog mode. Therefore, the pixel structure which corrects variations in threshold voltage is more suitable. 
     Next, the pixel structure to which current is input as a video signal is shown in  FIG. 33 . A current in accordance with a video signal is supplied to a source signal line  3330 . Then, the current flows to a drain of a driver transistor  3301  through a switch  3302  and to a gate of the driver transistor  3301  through a switch  3304 ; accordingly, a gate-source voltage is generated. The gate-source voltage is stored in a capacitor element  3305 ; thereafter, a current is supplied to a light emitting element through a switch  3306 . Note that the switch  3302 , the switch  3304 , and the switch  3306  are controlled by a gate signal line  3333 , a gate signal line  3334 , and a gate signal line  3335 , respectively. Note that reference numeral  3336  denotes a power supply line. Note that a transistor to which a signal current is supplied is identical with a transistor for supplying current to a light emitting element in  FIG. 33 ; however, they may be different from each other. That case is shown in  FIG. 34 . A transistor  3401  to which a signal current is supplied and a transistor  3421  for supplying current to a light emitting element are different. Note that  3411  denotes a source signal line;  3413  and  3414 , gate signal lines;  3402 , a selection switch;  3404 , a switch;  3405 , a capacitor element;  3416 , a power supply line in  FIG. 34 . 
     Note that there are various types of pixel structures which correct variations by inputting current, and the invention is not limited to the structures shown in  FIGS. 33 and 34 . By using the pixel structure which corrects variations by inputting current as described above, variations in current flowing to the light emitting element can be reduced. In particular, luminance can be equalized in an analog mode. Therefore, the pixel structure which corrects variations by inputting a current is more suitable. 
     Note that a light emitting element arranged in a pixel is not limited to a specific one. As the light emitting element arranged in a pixel, a display medium in which contrast varies by an electromagnetic action can be used, such as an EL (electroluminescent) element (such as an organic light emitting diode (also referred to as an OLED, an organic EL element, or the like), an inorganic EL element, or an EL element containing an organic material and an inorganic material), an electron emitting element, a liquid crystal element, electronic ink, or the like. In addition, a carbon nanotube can be used as the electron emitting element. Note that as an example of a display device using the electron emitting element, a field emission display (FED), an SED (Surface-conduction Electron-emitter Display) that is a kind of FED, or the like can be given. Furthermore, any display element that is used for a liquid crystal display (LCD), a plasma display (PDP), an electronic paper display, a digital micromirror device (DMD), a piezoelectric ceramic display, or the like may be used. 
     Note that the storage capacitor  405  in  FIG. 15  serves to hold the gate potential of the driver transistor  406 . Therefore, it is connected between a gate of the driver transistor  406  and the power supply line  403 , but the invention is not limited thereto. The storage capacitor  405  may be positioned so as to be able to store the gate potential of the driver transistor  406 . In the case where the gate potential of the driver transistor  406  can be held using the gate capacitance of the driver transistor  406  or the like, the storage capacitor  405  may be omitted. 
     The display mode-specific video signal generation circuit  106  in  FIG. 1  may be located over the same substrate as the pixel array  101 , the same substrate as the source driver  102 , or may be on an FPC (flexible printed circuit), or a PCB (printed circuit board). 
     In addition, the display mode-specific video signal generation circuit  106  may be formed with a transistor similar to that included in the pixel array  101 . Alternatively, it may be formed with another transistor. For example, the pixel array  101  may include a thin film transistor, and the display mode-specific video signal generation circuit  106  may be formed with a MOS transistor or a bipolar transistor formed over a bulk substrate or an SOI substrate. 
     Next, details of the display mode-specific video signal generation circuit  106  are shown in  FIG. 3 . Based on a signal input from the controller  107 , a display mode control circuit  301  performs control so that display according to a display mode can be performed. For example, in the case of a digital mode, switches  303  and  304  are turned on. Then, the input video signal is processed by a binarization circuit  302  and is output to the source driver  102 . In that case, a switch  305  is in an off state. On the other hand, in the case of an analog mode, the switch  305  is turned on, and the input video signal is output to the source driver  102  without any change. In the case where the video signal input to the display mode-specific video signal generation circuit  106  is an analog value, the video signal is output without any change; therefore, the video signal is output as an analog value also to the source driver  102 . 
     In  FIG. 3 , the case where the display mode is in an analog mode and in a digital mode is shown; however, the invention is not limited thereto. Note that a display mode where a video signal is a discrete value but not a binary value is referred to as a multi-valued mode. Examples of the relationship between a video signal and luminance are shown in  FIGS. 4A to 4C . 
       FIG. 4A  shows the case of an analog mode. A video signal varies in an analog manner, and accordingly, luminance also varies in an analog manner. 
       FIG. 4B  shows the case of a digital mode. A video signal is a binary value, and light is emitted at one value and not emitted at the other. 
       FIG. 4C  shows the case of a multi-valued mode. A video signal takes a discrete value, but not a binary value. Note that in the multi-valued mode, display is performed using a multi-valued digital signal output from the display mode-specific video signal generation circuit  106 . 
     Then, details of the display mode-specific video signal generation circuit  106  corresponding also to the case of the multi-valued mode are shown in  FIG. 5 . Based on a signal input from the controller  107 , a display mode control circuit  501  performs control so that display according to a display mode can be performed. For example, in the case of a digital mode, switches  303  and  304  are turned on. Then, the input video signal is processed by a binarization circuit  302  and is output to the source driver  102 . In that case, switches  313 ,  404 , and  305  are in an off state. On the other hand, in the case of an analog mode, the switch  305  is turned on, and the input video signal is output to the source driver  102  without any change. In the case where the video signal input to the display mode-specific video signal generation circuit  106  is an analog value, the video signal is output without any change; therefore, the video signal is output as an analog value also to the source driver  102 . In the case of a multi-valued mode, the switches  313  and  404  are turned on. Then, the input video signal is processed by a value multiplexing circuit  312  and is output to the source driver  102 . In that case, the switches  303 ,  304 , and  305  are in an off state. 
     Next, details of the binarization circuit  302  are shown in  FIGS. 6A and 6B . As shown in a circuit diagram of  FIG. 6A , a comparator circuit is formed using an operational amplifier  601 . The comparator circuit outputs the signal of H or L depending on whether an input voltage is higher or lower than a reference potential Vref to perform binarization. Note that the comparator circuit is formed using the operational amplifier, but the invention is not limited thereto. A chopper inverter comparator circuit may be used, or the comparator circuit may be formed using another circuit. 
       FIG. 6B  shows a circuit for generating the reference potential Vref. The value of the reference potential Vref corresponds to the voltage between voltages V 1  and V 2  and becomes a value divided by resistors R 1  and R 2 . Switches  602  and  603  may be turned on only when the binarization circuit is operated. Accordingly, a period during which current flows to the resistors R 1  and R 2  can be shortened, so that power consumption can be reduced. 
     Note that when the reference potential Vref is desired to be changed depending on the situation, it is preferable that many resistors are connected as shown in  FIG. 7  and contacts for outputting are switched. 
     Details of the value multiplexing circuit  312  are shown in  FIG. 8 . An input signal is input to a determination circuits  811 . In addition, two voltages corresponding to reference potentials are input to each of the determination circuit  811 . The determination circuit  811  outputs an H signal when the potential of the input signal is between the two reference potentials. Accordingly, one of switches  821  to  824  is turned on, and a multi-valued voltage is output. Note that switches  801  to  804  may be turned on only when the value multiplexing circuit  312  is operated. Accordingly, a period during which current flows between Va and Vb can be shortened, so that power consumption can be reduced. 
       FIG. 9  shows details of the determination circuit  811 . A comparator circuit is formed using operational amplifiers  901  and  902 . Each of the operational amplifiers  901  and  902  outputs an H signal when the potential Vin of an input signal is in the range of a reference potential Vx to a reference potential Vy. Then, the signals are input to an AND circuit  903 . When both of the input signals to the AND circuit  903  are H signals, an H signal is output. 
     Note that the determination circuit is formed using the AND circuit in  FIG. 9 , but the invention is not limited thereto. A similar function can also be performed when an OR circuit, a NAND circuit, or a NOR circuit is used. 
     When display is performed in a digital mode or a multi-valued mode, thresholding is performed and sampling of image information is performed. Accordingly, even an image data including noise can be displayed with the noise removed when the image is actually displayed. In addition, the image can be seen clearly because a luminance change in each gray scale level is significant, so that contrast is enhanced. 
     Note that the switches shown in  FIGS. 2 ,  3 , and  5 , for example the analog switch  201 , may each be an electrical switch, a mechanical switch, or any other switch. Any switch that can control current flow can be used. The switch may be a transistor, a diode, or a logic circuit that is a combination thereof. In the case of using a transistor as the switch, the transistor operates as a mere switch. Therefore, the polarity (conductivity type) of the transistor is not particularly limited. However, in the case where lower off-current is desired, it is desirable to use a transistor having a polarity with lower off-current. As the transistor with low off-current, a transistor provided with an LDD region, a transistor having a multi-gate structure, or the like can be used. In addition, it is desirable to use an n-channel transistor when a transistor to be operated as a switch operates in a state where a potential of a source terminal thereof is close to a lower potential side power source (such as Vss, Vgnd, or 0 V), whereas it is desirable to use a p-channel transistor when a transistor operates in a state where a potential of a source terminal thereof is close to a higher potential side power source (such as Vdd). This is because the absolute value of a gate-source voltage can be increased, so that the transistor easily operates as a switch. Note that the switch may be of a CMOS type using both an n-channel transistor and a p-channel transistor. 
     Examples of the switch are shown in  FIGS. 14A to 14D .  FIG. 14A  schematically shows a switch.  FIG. 14B  shows a switch using an AND circuit. Whether or not a signal of an input  1501  is transmitted to an output  1503  is controlled using a control line  1502 . In the case of  FIG. 14B , control can be performed in such a way that the output  1503  is an L signal regardless of the input signal. However, the output  1503  is never in a floating state. Thus, the switch in  FIG. 14B  is suitably used such as when the output  1503  is connected to an input of a digital circuit. In the case of the digital circuit, an output is not put in a floating state even when an input is put in a floating state. When the input is put in a floating state, the output becomes unstable, which is not desirable. Therefore, it is preferable to use the switch in  FIG. 14B  such as when the output is connected to the input of a digital circuit. 
     Note that the switch in  FIG. 14B  is formed using the AND circuit, but the invention is not limited thereto. A similar function can also be performed when an OR circuit, a NAND circuit, or a NOR circuit is used. 
     On the other hand, a switch in  FIG. 14C  or  14 D may be used when the input is desired to be put in a floating state. The switch in  FIG. 14C  is a circuit referred to as a transmission gate, an analog switch, or the like. The switch in  FIG. 14C  transmits the potential of an input  1511  to an output  1513  with almost no change. Therefore, the switch is suitable for analog signal transmission. The switch in  FIG. 14D  is a circuit referred to as a clocked inverter or the like. The switch in  FIG. 14D  inverts and transmits a signal of an input  1521  to an output  1523 . Therefore, the switch is suitable for digital signal transmission. Note that whether or not the signals of the inputs  1511  and  1521  are transmitted to the outputs  1513  and  1523  respectively is controlled using control lines  1512  and  1522 . 
     According to the above, the switch in  FIG. 14C  is preferably used as the analog switch  201 , the switch  305 , the switch  602 , the switch  801 , or the like. The switch in FIG.  14 C or  14 D is suitable for the switch  304  or the like since the input thereof needs to be put in a floating state. Note that the output from the switch  304  is a digital signal; therefore, the switch in  FIG. 14D  is more suitable. 
     (Embodiment Mode 2) 
     The case where the video signal input to the display mode-specific video signal generation circuit  106  is an analog value is described in Embodiment Mode 1. Next, the case where a digital value is input is described. 
       FIG. 24  shows an overall configuration diagram. A video signal input to a source driver  102  is generated in a display mode-specific video signal generation circuit  2306  in accordance with each display mode. The display mode-specific video signal generation circuit  2306  is controlled using a controller  2307 . An original digital video signal is input to the display mode-specific video signal generation circuit  2306 . Then, using the original video signal, the video signal in accordance with each display mode is generated in the display mode-specific video signal generation circuit  2306  and is output to the source driver  102 . 
     The display modes can be roughly divided into an analog mode and a digital mode. In the analog mode, a video signal input to a pixel is an analog value. On the other hand, in the digital mode, a video signal input to a pixel is a digital value. 
     Next, details of the display mode-specific video signal generation circuit  2306  are shown in  FIG. 25 . Based on a signal input from the controller  2307 , a display mode control circuit  2401  performs control so that display according to a display mode can be performed. For example, in the case of a digital mode, switches  2513  and  2514  are turned on, and only the most significant bit (MSB) of the video signal is output to the source driver  102 . However, there is a case where a potential level does not meet. In that case, the potential level needs to be converted into a necessary level. Thus, when that is necessary, a level converter circuit  2504  is provided. On the other hand, in the case of an analog mode, the video signal is transmitted to a DA converter circuit (digital analog converter circuit)  2502  and an appropriate analog value is output to the source driver  102  through a switch  2511 . 
     In  FIG. 25 , the case where the display mode is in an analog mode and a digital mode is shown; however, the invention is not limited thereto. 
     Then, details of the display mode-specific video signal generation circuit  2306  corresponding also to the case of a multi-valued mode is shown in  FIG. 26 . Based on a signal input from the controller  2307 , a display mode control circuit  2501  performs control so that display according to a display mode can be performed. The cases of an analog mode and a digital mode are similar to  FIG. 25 . In the case of a multi-valued mode, only a higher bit of a video signal is input to a DA converter circuit  2503  when a switch  2512  is on state. A lower bit is not input. Thus, not smooth display but sampled display is performed. 
     Note that in the multi-valued mode, sampling may be performed without using a lower bit; therefore, the invention is not limited to the configuration shown in  FIG. 26 . For example, a lower bit data removal circuit  2702  may be provided in an input portion of a DA converter circuit  2502  as shown in  FIG. 27 . As a result, the value of the lower bit is forced to 0 (or an L signal) in accordance with the signal of the display mode control circuit. Thus, not smooth display but sampled display is performed. 
     Then, an example of the lower bit data removal circuit  2702  is shown in  FIG. 28 . Data of lower 3 bits can be forced to 0 (or an L signal) using an AND circuit. 
     Note that the AND circuit is used in  FIG. 28 , but the invention is not limited thereto. A similar function can also be performed when an OR circuit, a NAND circuit, or a NOR circuit is used. In addition, in  FIG. 28 , a video signal of 6 bits is input and data of lower 3 bits thereof are forced to 0 (or an L signal), but the invention is not limited thereto. Modification may be made appropriately. 
     Thus, the number of bits of which data are to be forced to 0 (or an L signal) may be changed. A diagram of a circuit  2902  in that case is shown in  FIG. 29 . Since signals input to AND circuits are separated, the signals can be controlled individually. 
     Next, details of the DA converter circuits shown in  FIGS. 25 to 27  are shown in  FIG. 30 . A decoder circuit  3021  decodes the input digital signal, and accordingly, any of switches  3011  to  3016  is turned on to output an analog voltage. Then, switches  3001  and  3002  may be turned on only when the DA converter circuit is operated. As a result, a period during which a current flows to a resistor can be shortened, so that power consumption can be reduced. 
     When display is performed in a digital mode or a multi-valued mode as described above, thresholding is performed and sampling of image information is performed. Accordingly, even an image data including noise can be displayed with the noise removed when the image is actually displayed. In addition, the image can be seen clearly because a luminance change in one gray scale level is significant, so that contrast is enhanced. 
     The content described in this embodiment mode can be freely combined with the content described in Embodiment Mode 1. 
     (Embodiment Mode 3) 
     In this embodiment mode, the case of performing display using each display mode is described. 
     First, the case of performing display on an entire screen in the same display mode can be given. In other words, the case of performing display in an analog mode on the entire screen can be given. In this case, normal display can be performed. Since a smooth gray scale can be expressed, the case is suitable for displaying a picture and the like. 
     Next, the case of performing display in a digital mode on the entire screen can be given. In this case, contrast is enhanced and excellent visibility is obtained; therefore, it is suitable when characters are mainly displayed, such as for reading e-mail, reading an electronic book, and the like. 
     Subsequently, the case of performing display in a multi-valued mode on the entire screen can be given. In this case, contrast is enhanced and excellent visibility is obtained; therefore, it is suitable when displaying an illustration, an animation, a cartoon, and the like for which a gray scale is desired to be expressed, but not as precise as a picture and the like. 
     Next, the case where the entire screen is divided into a plurality of regions and each region performs display in a corresponding display mode, can be given. This is achieved due to the fact that a video signal in accordance with a display mode can be generated for each pixel in the display mode-specific video generation circuit  106  as  FIG. 1  shows. 
     For example, a screen is divided into three regions as shown in  FIG. 10 . Then, an upper region  1001  performs display in a digital mode. For example, the upper region  1001  displays time, battery information, electric wave information, and the like to increase visibility. A middle region  1002  performs display in an analog mode as normal. The middle region  1002  can display an image such as a picture with a smooth gray scale. A lower region  1003  performs display in a multi-valued mode and displays a simple animation and the like. 
     In  FIG. 11 , an upper region  1101  performs display in a multi-valued mode, and displays a simple animation and the like. A middle region  1102  performs display in a digital mode, and is suitable to read e-mail, an electronic book, and the like. In addition, a lower region  1103  also performs display in a multi-valued mode, and displays a simple animation and the like. This makes it possible to display a pretty, highly-attractive image rather than an unattractive image as in the case of performing display in a digital mode on the entire screen, since a simple colorful icon and the like can be displayed while being suitable for read e-mail, an electronic book, and the like by mainly performing display in a digital mode. 
     In  FIG. 12 , a central region  1201  performs display in an analog mode, and can clearly display an image such as a picture with a smooth gray scale. A peripheral region  1202  performs display in a digital mode, and can display an icon of time, battery information, electric wave information, or the like. 
     In  FIG. 13 , a peripheral region  1302  performs display in an analog mode, and can clearly display an image such as a picture with a smooth gray scale. A central region  1301  performs display in a multi-valued mode. In a portion performing display in a multi-valued mode, a smooth gray scale is converted into a stepwise gray scale. Therefore, in the case of displaying a human face in a multi-valued mode, the human face looks like a facial illustration or a cartoon. This function can be utilized to perform display like a simple photo sticker. 
     Note that the number, position, and shape of portions of a screen to be divided are not limited to those described above. In addition, which region and which display mode a display is performed in is not limited to those described above. 
     Note that this embodiment mode describes Embodiment Modes 1 and 2 in detail. Therefore, the content described in this embodiment mode can be freely combined with the content described in Embodiment Modes 1 and 2. 
     (Embodiment Mode 4) 
     In this embodiment mode, a method for driving a pixel in an analog mode is described. 
       FIGS. 16A and 16B  show the relationship between a voltage and a current applied to a driver transistor and a light emitting element.  FIG. 16A  shows a circuit of a driver transistor  631  and a light emitting element  632 . The driver transistor  631  and the light emitting element  632  are connected serially between a wire  633  and a wire  634 . Since the wire  633  has a higher potential than that of the wire  634 , a current flows to the light emitting element  632  from the driver transistor  631 . 
     The driver transistor  406  in  FIG. 15  corresponds to the driver transistor  631  in  FIG. 16A , and the light emitting element  407  in  FIG. 15  corresponds to the light emitting element  632  in  FIG. 16A . 
       FIG. 16B  shows the relationship between a gate-source voltage (or the absolute value thereof) of the driver transistor  631  and a current flowing to the driver transistor  631  and the light emitting element  632 . As the gate-source voltage (or the absolute value thereof) is increased, a current value is also accordingly increased. This is because the driver transistor  631  operates in a saturation region. In a saturation region, a current value increases in proportion to the square of a gate-source voltage of a transistor. As the gate-source voltage (or the absolute value thereof) is further increased, a voltage applied to the light emitting element  632  is increased. Accordingly, a drain-source voltage is decreased, and the driver transistor  631  operates in a linear region. As the drain-source voltage is decreased, the rate of increase in the current value is also decreased. Then, a current equal to or more than a certain current value does not flow. 
     In an analog mode, a gray scale is expressed using an analog gray scale method. Thus, by changing the gate-source voltage (or the absolute value thereof) of the driver transistor  631  in an analog manner, the driver transistor  631  is preferably operated in such a state that the current flowing to the driver transistor  631  and the light emitting element  632  also changes in an analog manner. Therefore, the gate-source voltage (or the absolute value thereof) of the driver transistor  631  is preferably changed from a threshold voltage to a gate-source voltage at which the driver transistor  631  operates in a saturation region. Note that the upper limit of change is not limited to within a saturation region, and the gate-source voltage may be changed to a linear region. In other words, the gate-source voltage (or the absolute value thereof) of the driver transistor  631  may be in a region where a current value I EL  changes with respect to the gate-source voltage (or the absolute value thereof). In addition, the lower limit of change may be a gate-source voltage (or the absolute value thereof) at which the driver transistor  631  is turned off. 
     For example, as in a voltage range  620 , the gate-source voltage (or the absolute value thereof) of the driver transistor  631  may be controlled in such a state that the transistor operates in a saturation region from a state that almost no current flows. The state in which almost no current flows corresponds to the case where the gate-source voltage of the driver transistor  631  is approximately equal to the threshold voltage of the driver transistor  631 . 
     Alternatively, as in a voltage region  621 , the gate-source voltage (or the absolute value thereof) is increased and controlled from a state in which the gate-source voltage (or the absolute value thereof) of the driver transistor  631  is certainly lower than the threshold voltage of the driver transistor  631 , and the gate-source voltage (or the absolute value thereof) of the driver transistor  631  may be controlled in such a state that the driver transistor  631  operates in a saturation region. By making the gate-source voltage of the driver transistor  631  in a black state certainly lower than the threshold voltage of the driver transistor  631 , a black state can be assured. For example, if the current characteristic of the driver transistor  631  varies, the threshold voltage also varies. Thus, even when one pixel is in a black state, another pixel may slightly emit light. As a result, a decrease in contrast is caused. Thus, in order to prevent that, it is preferable to operate the driver transistor  631  in such a voltage range as  621 . 
     Note that the driver transistor  631  is operated in a saturation region in the voltage range  620  and the voltage range  62 l even if the gate-source voltage (or the absolute value thereof) is increased in the above-mentioned example. However, the invention is not limited thereto. As in a voltage range  622  and a voltage range  623 , the driver transistor  631  may be operated not only in a saturation region but also in a linear region. By changing the gate-source voltage (or the absolute value thereof) of the driver transistor  631  in an analog manner, the driver transistor  631  may be operated also in a linear region as long as it is within such a range that a current flowing to the driver transistor  631  and the light emitting element  632  also changes in an analog manner. 
     Note that in the case where the driver transistor  631  is operated in a saturation region, a certain amount of current can be supplied to the light emitting element  632  even when the light emitting element is deteriorated. In addition, in the case of a linear region, the driver transistor can be driven without being affected by characteristic variation of the transistor. 
     Subsequently, the case of performing optimization depending on the color light emitted from of the light emitting element  632  is described. The luminance and necessary current value of the light emitting element  632  vary depending on color. Thus, a color balance needs to be adjusted. In order for that to be done, the gate-source voltage (or the absolute value thereof) of the driver transistor  631  is desirably changed depending on color. Alternatively, the current supply capacity of the driver transistor  631  (for example, a wide of a channel region of a transistor or the like) is desirably changed depending on color. Alternatively, the light emitting area of the light emitting element  632  is desirably changed depending on color. Further alternatively, some of these are desirably combined. This enables to adjust the color balance. 
     Note that the potential of the wire  633  can be changed depending on color. However, there is the disadvantage that the voltage at which the driver transistor  631  is turned off is also changed depending on color. Therefore, the potential of the wire  633  may be equivalent in all colors. 
     Note that the case where the driver transistor  631  is a p-channel transistor is described, but the invention is not limited thereto. It is easy for one skilled in the art to reverse the direction of current flow by using an n-channel transistor. In addition, it is also easy for one skilled in the art to reverse the direction of current flow either in the case of a p-channel transistor or in the case of an n-channel transistor. In that case, the amount of gate-source voltage is affected by the voltage-current characteristic of the light emitting element  632 . 
     Note that the case of an analog mode is described in this embodiment mode, but this embodiment mode can be similarly applied to the case of a multi-valued mode. 
     Note that this embodiment mode describes the pixel of Embodiment Modes 1 to 3 in detail. Therefore, the content described in this embodiment mode can be freely combined with the content described in Embodiment Modes 1 to 3. 
     (Embodiment Mode 5) 
     In this embodiment mode, a method for driving a pixel in a digital mode is described. 
     The relationship between the gate-source voltage (or the absolute value thereof) of the driver transistor  631  and a current flowing to the driver transistor  631  and the light emitting element  632 , shown in  FIG. 16B , is referred to. In a digital mode, control is performed in binary like on and off, or H and L. In other words, Whether or not a current flows to the light emitting element  632  is controlled. First, the case where current does not flow is considered. In that case, the gate-source voltage (or the absolute value thereof) of the driver transistor  631  may be 0 V or more as indicated by a voltage  624 , a voltage  625 , and a voltage  626  and equal to or lower than the threshold voltage of the driver transistor  631 . 
     Next, the case where current flows is considered. In that case, the driver transistor  631  may be operated in a saturation region, a linear region, or a region in a linear region where voltage is further increased and a current value is not increased, or the like with the gate-source voltage (or the absolute value thereof) of a voltage  627 , a voltage  628 , and a voltage  629 . Note that the voltage  627  is located at the boundary of a linear region and a saturation region in the diagram, but it is acceptable as long as it is within the saturation region. Thus, there is no particular limitation on the gate-source voltage as long as it can cause a current to flow to the light emitting element  632  from the driver transistor  631 . 
     For example, operation in a saturation region has the advantage that the value of a current flowing through the light emitting element  632  does not vary even when the voltage-current characteristic thereof is deteriorated. Therefore, the current value is hardly affected by burn-in. However, when the current characteristics of the driver transistors  631  vary, the current flowing therethrough also varies. Thus, uneven display may be caused. 
     On the other hand, if the driver transistor  631  is operated in a linear region, the value of a current flowing through the driver transistor  631  is hardly affected even when the current characteristic of the driver transistor  631  varies. Therefore, uneven display is hardly caused. In addition, power consumption can be reduced because the gate-source voltage (or the absolute value thereof) of the driver transistor  631  does not become too high and the voltage between the wire  633  and the wire  634  does not need to be high. 
     Furthermore, when the gate-source voltage (or the absolute value thereof) of the driver transistor  631  is high, the value of a current flowing through the driver transistor  631  is hardly affected even if the current characteristic thereof varies. However, when the voltage-current characteristic of the light emitting element  632  is deteriorated, the value of a current flowing therethrough may vary. Therefore, the current value is easily affected by burn-in. 
     As described above, when the driver transistor  631  is operated in a saturation region, the value of a current flowing therethrough does not vary even if the characteristics of the light emitting element  632  vary. Therefore, in that case, it can be assumed that the driver transistor  631  operates as a current source. Thus, such drive is referred to as constant current drive. 
     When the driver transistor  631  is operated in a linear region, the current value does not vary even if the current characteristic of the driver transistor  631  varies. Therefore, in that case, it can be assumed that the driver transistor  631  operates as a switch. Accordingly, the voltage of the wire  633  can be considered to be applied to the light emitting element  632  without any change. Thus, such drive is referred to be constant voltage drive. 
     In a digital mode, either constant voltage drive or constant current drive may be employed. However, constant voltage drive is preferable since the constant voltage drive is not affected by variations in transistors and can reduce power consumption. 
     Next, the case of performing optimization depending on the color of emission form the light emitting element  632  is described. The case of constant current drive is similar to an analog mode. 
     In the case of constant voltage drive, even if the gate-source voltage (or the absolute value thereof) of the driver transistor  631  and the current supply capacity of the driver transistor  631  (for example, a transistor width or the like) are changed depending on color, the value of a current flowing therethrough does not vary so much. This is because the driver transistor operates  631  as a switch. 
     Therefore, the light emitting areas of the light emitting elements  632  are desirably differentiated depending on color. Alternatively, the potentials of the wire  633  can be differentiated depending on color. Alternatively, these are desirably combined. This enables to adjust the color balance. 
     Note that in the case of performing color display in a digital mode, display is performed in binary in each of RGB; therefore, display can be performed in eight colors in total. 
     Note that this embodiment mode describes the pixel and the like of Embodiment Modes 1 to 4 in detail. Therefore, the content described in this embodiment mode can be freely combined with the content described in Embodiment Modes 1 to 4. 
     (Embodiment Mode 6) 
     Next, description is made on the layout of a pixel in the display device of the present invention. As an example,  FIG. 17  is a layout diagram of the circuit diagram shown in  FIG. 15 . Note that the circuit diagram and the layout diagram are not limited to  FIGS. 15 and 17 . 
     Electrodes of the selection transistor  404 , the driver transistor  406 , and the light emitting element  407  are arranged. A source and a drain of the selection transistor  404  are connected to the source signal line  402  and a gate of the driver transistor  406 , respectively. A gate of the selection transistor  404  is connected to the gate signal line  401 . A source and a drain of the driver transistor  406  are connected to the power supply line  403  and an electrode  417  of the light emitting element  407 , respectively. The storage capacitor  405  is connected between the gate of the driver transistor  406  and the power supply line  403 . 
     The source signal line  402  and the power supply line  403  are formed with a second wire, and the gate signal line  401  is formed with a first wire. 
     In the case of a top gate structure, a substrate, a semiconductor layer, a gate insulating film, the first wire, an interlayer insulating film, and the second wire are formed in this order. In the case of a bottom gate structure, a substrate, the first wire, a gate insulating film, a semiconductor layer, an interlayer insulating film, and the second wire are formed in this order. 
     Note that the content described in this embodiment mode can be freely combined with the content described in Embodiment Modes 1 to 5. 
     (Embodiment Mode 7) 
     In this embodiment mode, description is made on hardware for controlling the display devices described in Embodiment Modes 1 to 6. 
       FIG. 18  is a rough constitution diagram. A pixel array  2704  is arranged over a substrate  2701 . In addition, a source driver  2706  and a gate driver  2705  are arranged in many cases. Besides, a power supply circuit, a precharge circuit, a timing generation circuit, and the like may be arranged. There is also a case where the source driver  2706  and the gate driver  2705  are not arranged. In that case, a circuit which is not provided over the substrate  2701  is formed on an IC in many cases. The IC is mounted on the substrate  2701  by using a COG (Chip On Glass) method in many cases. Alternatively, the IC may be mounted on a connecting substrate  2707  which connects a peripheral circuit substrate  2712  and the substrate  2701 . 
     A signal  2703  is input to the peripheral circuit substrate  2712 , and a controller  2708  performs control to store the signal in a memory  2709 , a memory  2710 , or the like. In the case where the signal  2703  is an analog signal, the signal is stored in the memory  2709 , the memory  2710 , or the like in many cases after analog-digital conversion is performed. Then, the controller  2708  outputs a signal to the substrate  2701  by using the signal stored in the memory  2709 , the memory  2710 , or the like. 
     In order to realize the driving methods described in Embodiment Modes 1 to 5, the controller  2708  controls various pulse signals or the like and outputs a signal to the substrate  2701 . 
     Note that the content described in this embodiment mode can be complemented in free combination with the content described in Embodiment Modes 1 to 6. 
     (Embodiment Mode 8) 
     An example of the structure of a mobile phone which has the display device of the invention, which is a display device using the driving method of the invention in a display portion is explained with reference to  FIG. 19 . 
     A display panel  5410  is incorporated in a housing  5400  so as to be detachable. The shape and size of the housing  5400  can be appropriately changed in accordance with the size of the display panel  5410 . The housing  5400  to which the display panel  5410  is fixed is fitted in a printed circuit board  5401  and assembled as a module. 
     The display panel  5410  is connected to the printed circuit board  5401  through an FPC  5411 . The printed circuit board  5401  is provided with a speaker  5402 , a microphone  5403 , a transmitting and receiving circuit  5404 , and a signal processing circuit  5405  including a CPU, a controller, and the like. Such a module, an input means  5406 , and a buttery  5407  are combined and stored using a chassis  5409  and a chassis  5412 . Note that a pixel portion of the display panel  5410  is arranged so as to be seen from a window formed in the chassis  5412 . 
     In the display panel  5410 , the pixel portion and part of peripheral driver circuits (a driver circuit having a low operation frequency among a plurality of driver circuits) may be formed using TFTs in an integrated manner over a substrate, and another part of the peripheral driver circuits (a driver circuit having a high operation frequency among the plurality of driver circuits) may be formed on an IC chip. The IC chip may be mounted on the display panel  5410  by using a COG (Chip On Glass) method. The IC chip may alternatively be connected to a glass substrate using a TAB (Tape Automated Bonding) method or a printed circuit board. Note that  FIG. 20A  shows an example of constitution of a display panel where part of peripheral driver circuits is integrated with a pixel portion over a substrate and an IC chip on which the another part of peripheral driver circuits is formed is mounted by a COG method or the like. 
     The display panel in  FIG. 20A  may have a structure in which a pixel portion  5302  and its peripheral driver circuits (a first scan line driver circuit  5303  and a second scan line driver circuit  5304 ) are integrated over a substrate  5300 , and a signal line driver circuit  5301  is formed on an IC chip and mounted on the display panel by using a COG method or the like. Note that the pixel portion  5302  and the peripheral driver circuits integrated over the substrate are sealed by attaching a sealing substrate  5308  to the substrate  5300  with a sealant  5309 . In addition, IC chips (semiconductor chips provided with a memory circuit, a buffer circuit, and the like)  5306  and  5307  may be mounted on a connection portion of an FPC  5305  and the display panel by using a COG (Chip On Glass) method or the like. Note that only the FPC is shown here; however, a printed wiring board (PWB) may be attached to this FPC. 
     As described above, only part of a signal line driver circuit which requires high speed operation is formed on an IC chip using a CMOS or the like to reduce power consumption. In addition, higher-speed operation and lower power consumption can be achieved by using a semiconductor chip of a silicon wafer or the like as the IC chip. Furthermore, cost reduction can be achieved by integrating the first scan line driver circuit  5303  and the second scan line driver circuit  5304  with the pixel portion  5302 . In addition, a substrate area can be used efficiently by mounting an IC chip provided with a functional circuit (a memory or a buffer) on a connection portion of the FPC  5305  and the substrate  5300 . 
     In order to further reduce power consumption, all of the peripheral driver circuits may be formed on an IC chip, and the IC chip may be mounted on the display panel by using a COG method or the like. For example, a pixel portion  5312  may be formed over a substrate  5310 , and a signal line driver circuit, a first scan line driver circuit, and a second scan line driver circuit may be formed on IC chips, which may be mounted on a display panel by using a COG method or the like as shown in  FIG. 20B . Note that an FPC  5315 , an IC chip  5316 , an IC chip  5317 , a sealing substrate  5318 , and a sealant  5319  in  FIG. 20B  correspond to the FPC  5305 , the IC chip  5306 , the IC chip  5307 , the sealing substrate  5308 , and the sealant  5309  in  FIG. 20A , respectively. 
     By employing the above-described structure, power consumption of the display device can be reduced and operating time per charge of the mobile phone can be made longer. In addition, cost reduction of the mobile phone can be achieved. 
     In addition, by converting the impedance of a signal set to a scan line or a signal line by using a buffer, a write period for pixels of each row can be shortened. Accordingly, a high-definition display device can be provided. 
     By using the display device of the invention, it becomes possible to see a high-contrast clear image. 
     The structure described in this embodiment mode is an example of a mobile phone, and the display device of the invention can be applied not only to the mobile phone having the above-described structure but also to mobile phones having various kinds of structures. 
     (Embodiment Mode 9) 
       FIG. 21  shows an EL module in which a display panel  5701  and a circuit board  5702  are combined. The display panel  5701  includes a pixel portion  5703 , a scan line driver circuit  5704 , and a signal line driver circuit  5705 . Over the circuit board  5702 , for example, a control circuit  5706 , a signal dividing circuit  5707 , and the like are formed. The display panel  5701  and the circuit board  5702  are connected to each other by a connection wiring  5708 . As the connection wiring, an FPC or the like can be used. 
     The control circuit  5706  corresponds to the controller  2708 , the memory  2709 , the memory  2710 , or the like in Embodiment Mode 7. Mainly in the control circuit  5706 , the appearance order of subframes or the like is controlled. 
     In the display panel  5701 , the pixel portion and part of peripheral driver circuits (a driver circuit having a low operation frequency among a plurality of driver circuits) may be formed using TFTs in an integrated manner over a substrate, and another part of the peripheral driver circuits (a driver circuit having a high operation frequency among the plurality of driver circuits) may be formed on an IC chip. The IC chip may be mounted on the display panel  5701  by using a COG (Chip On Glass) method or the like. The IC chip may alternatively be mounted on the display panel  5701  by using a TAB (Tape Automated Bonding) method or a printed circuit board. Note that an example of constitution of a display panel where part of peripheral driver circuits is integrated with a pixel portion over a substrate and an IC chip on which another part of the peripheral driver circuits is formed is mounted by using a COG method or the like, is shown in  FIG. 20A . By using the above-described structure, power consumption of the display device can be reduced, and operating time per charge of a mobile phone can be made longer. In addition, cost reduction of a mobile phone can be achieved. 
     In addition, by converting the impedance of a signal set to a scan line or a signal line by using a buffer, a write period for pixels of each row can be shortened. Accordingly, a high-definition display device can be provided. 
     In order to further reduce power consumption, a pixel portion may be formed using TFTs over a glass substrate, and all signal line driver circuits may be formed on an IC chip, which may be mounted on a display panel by using a COG (Chip On Glass) method or the like. 
     Note that a pixel portion may be formed using TFTs over a substrate, and all peripheral driver circuits may be formed on an IC chip, which may be mounted on a display panel by using a COG (Chip On Glass) method. Note that an example of constitution where a pixel portion is formed over a substrate and an IC chip provided with a signal line driver circuit is mounted on the substrate by using a COG method or the like, is shown in  FIG. 20B . 
     An EL TV receiver can be completed with the above-described EL module.  FIG. 22  is a block diagram showing main constitution of an EL TV receiver. A tuner  5801  receives a video signal and an audio signal. The video signal is processed by a video signal amplifier circuit  5802 , a video signal processing circuit  5803  for converting a signal output from the video signal amplifier circuit  5802  into a color signal corresponding to each color of red, green and blue, and a control circuit  5706  for converting the video signal into the input signal according to specification of a driver circuit. The control circuit  5706  outputs respective signals to the scan line side and the signal line side. In the case of driving in a digital manner, constitution in which the signal dividing circuit  5707  is provided on the signal line side to supply an input digital signal divided into m pieces may be adopted. 
     The audio signal among the signals received by the tuner  5801  is transmitted to an audio signal amplifier circuit  5804 , an output of which is supplied to a speaker  5806  through an audio signal processing circuit  5805 . A control circuit  5807  receives control information of a receiving station (reception frequency) or sound volume from an input portion  5808  and transmits signals to the tuner  5801  and the audio signal processing circuit  5805 . 
     By incorporating the EL module into a chassis, a TV receiver can be completed. A display portion is formed with the EL module. In addition, a speaker, a video input terminal, and the like are provided appropriately. 
     Naturally, the present invention is not limited to the TV receiver, and can be applied to various uses as a large-sized display medium such as an information display board at a train station, an airport, or the like, or an advertisement display board on the street, as well as a monitor of a personal computer. 
     By using the display device of the invention as described above, it becomes possible to see a high-contrast clear image. 
     (Embodiment Mode 10) 
     The present invention can be applied to various electronic devices. Specifically, it can be applied to a display portion of an electronic device. Examples of such an electronic device are as follows: a camera such as a video camera or a digital camera, a goggle type display (a head-mounted display), a navigation system, a sound reproducing device (such as a car audio or an audio component), a computer, a game machine, a portable information terminal (such as a mobile computer, a mobile phone, a portable game machine, or an electronic book), an image reproducing device provided with a recording medium reading portion (specifically, a device which can reproduce a recording medium such as a digital versatile disc (DVD) and includes a light emitting device capable of displaying images thereof), and the like. 
       FIG. 23A  shows a light emitting device, which includes a chassis  35001 , a support  35002 , a display portion  35003 , a speaker portion  35004 , a video input terminal  35005 , and the like. The display device of the present invention can be used for the display portion  35003 . Note that the light emitting device includes in its category all light emitting devices used for displaying information, for example, for a personal computer, for TV broadcast reception, or for advertisement display. The light emitting device using the present invention for the display portion  35003  makes it possible to see a high-contrast clear image. 
       FIG. 23B  shows a camera, which includes a main body  35101 , a display portion  35102 , an image receiving portion  35013 , an operation key  35104 , an external connection port  35105 , a shutter  35106 , and the like. 
     The camera using the present invention for the display portion  35102  makes it possible to see a high-contrast clear image. 
       FIG. 23C  shows a computer, which includes a main body  35201 , a chassis  35202 , a display portion  35203 , a keyboard  35204 , an external connection port  35205 , a pointing mouse  35206 , and the like. The computer using the present invention for the display portion  35203  makes it possible to see a high-contrast clear image. 
       FIG. 23D  shows a mobile computer, which includes a main body  35301 , a display portion  35302 , a switch  35303 , an operation key  35304 , an infrared port  35305 , and the like. The mobile computer using the present invention for the display portion  35302  makes it possible to see a high-contrast clear image. 
       FIG. 23E  shows a portable image reproducing device provided with a recording medium reading portion (specifically, a DVD reproducing device), which includes a main body  35401 , a chassis  35402 , a display portion A  35403 , a display portion B  35404 , a recording medium (DVD or the like) reading portion  35405 , an operation key  35406 , a speaker portion  35407 , and the like. The display portion A  35403  mainly displays image information, and the display portion B  35404  mainly displays character information. The image reproducing device using the present invention for the display portion A 35403  and the display portion B  35404  makes it possible to see a high-contrast clear image. 
       FIG. 23F  shows a goggle type display, which includes a main body  35501 , a display portion  35502 , an arm portion  35503 , and the like. The goggle type display using the present invention for the display portion  35502  makes it possible to see a high-contrast clear image. 
       FIG. 23G  shows a video camera, which includes a main body  35601 , a display portion  35602 , a chassis  35603 , an external connection port  35604 , a remote control receiving portion  35605 , an image receiving portion  35606 , a battery  35607 , an audio input portion  35608 , an operation key  35609 , and the like. The video camera using the present invention for the display portion  35602  makes it possible to see a high-contrast clear image. 
       FIG. 23H  shows a mobile phone, which includes a main body  35701 , a chassis  35702 , a display portion  35703 , an audio input portion  35704 , an audio output portion  35705 , an operation key  35706 , an external connection port  35707 , an antenna  35708 , and the like. The mobile phone using the present invention for the display portion  35703  makes it possible to see a high-contrast clear image. 
     As described above, the applicable range of the present invention is so wide that the invention can be applied to electronic devices of various fields. In addition, the electronic device of this embodiment mode may use a display device having any of the structures described in Embodiment Modes 1 to 9. 
     This application is based on Japanese Patent Application serial no. 2005-133825 filed in Japan Patent Office on May 2, 2005, the contents of which are hereby incorporated by reference.