Abstract:
Regarding the time gradation method as the digital gradation expression method, it is an object of the invention to provide a display device which prevents a reduction of frame frequency and implements a low-power-consumption SRAM. The invention overcomes the aforementioned disadvantages of conventional technique by synchronizing a writing and reading by utilizing read and write signals of which states read at a certain timing to select which of two memories is efficient to be written.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a display device, and more particularly such a display device having a light emitting element and a memory control circuit. The memory control circuit controls a writing and reading to memories such as SRAM. 
         [0003]    2. Description of the Related Art 
         [0004]    Hereinafter explained is a display device which disposes a light emitting element at each pixel and displays an image by controlling the emission of the light emitting elements. 
         [0005]    The explanation throughout this specification uses elements (OLED elements) having a structure in which an organic compound layer for emitting light when an electric field is generated is sandwiched between an anode and a cathode, for the light emitting elements, but the present invention is not limited to this structure. 
         [0006]    Further, the explanation within this specification uses elements that utilize light emitted when making a transition from singlet excitons to a base state (fluorescence), and those that utilize light emitted when making a transition from triplet excitons to a base state (phosphorescence). 
         [0007]    An organic compound layer includes a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, and the like. The basic structure of a light emitting element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, an electron injection layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, and a cathode layered in this order. 
         [0008]    A display device is constituted by a display and peripheral circuits for inputting signals to the display. 
         [0009]    The structure of the display is shown in a block diagram of  FIG. 8 . 
         [0010]    In  FIG. 8 , the display  2000  is constituted by a source signal line driver circuit  2107 , a gate signal line driver circuit  2108 , and a pixel portion  2109 . The pixel portion has pixels disposed in a matrix shape. 
         [0011]    Thin film transistors (hereafter referred to as TFTs) are arranged in each pixel. A method of placing two TFTs in each pixel and controlling light emitted from the light emitting element of each pixel is explained. 
         [0012]      FIG. 9  shows a structure of a pixel portion of a display device. 
         [0013]    Source signal lines S 1  to Sx, gate signal lines G 1  to Gy, and electric power source supply lines V 1  to Vx are arranged in a pixel portion  2700 , and x columns and y rows (where x and y are natural numbers) of pixels are also placed in the pixel portion. Each pixel  2705  has a switching TFT  2701 , a driver TFT  2702 , a storage capacitor  2703 , and a light emitting element  2704 . 
         [0014]    The pixel is constituted by one source signal line S of the source signal lines S 1  to Sx, one gate signal line G of the gate signal lines G 1  to Gy, one electric power source supply line V of the electric power source supply lines V 1  to Vx, the switching TFT  2701 , the driver TFT  2702 , the storage capacitor  2703 , and the light emitting element  2704 . 
         [0015]    A gate electrode of the switching TFT  2701  is connected to the gate signal line G, and either a source region or a drain region of the switching TFT  2701  is connected to the source signal line S, while the other is connected to a gate electrode of the driver TFT  2702  and to one electrode of the storage capacitor  2703 . Either a source region or a drain region of the driver TFT  2702  is connected to the electric power source supply line V, while the other is connected to an anode or a cathode of the light emitting element  2704 . The electric power source supply line V is connected to one of the two electrodes of the storage capacitor  2703 , namely the electrode on a side to which the driver TFT  2702  and the switching TFT  2701  are not connected. 
         [0016]    The anode of the light emitting element  2704  is referred to as a pixel electrode, and the cathode of the light emitting element  2704  is referred to as an opposing electrode, within this specification for cases in which the source region or the drain region of the driver TFT  2702  is connected to the anode of the light emitting element  2704 . On the other hand, if the source region or the drain region of the driver TFT  2702  is connected to the cathode of the light emitting element  2704 , then the cathode of the light emitting element  2704  is referred to as the pixel electrode, and the anode of the light emitting element  2704  is referred to as the opposing electrode. 
         [0017]    Further, an electric potential imparted to the electric power source supply line V is referred to as an electric power source electric potential, and an electric potential imparted to the opposing electrode is referred to as an opposing electric potential. 
         [0018]    The switching TFT  2701  and the driver TFT  2702  may be either p-channel TFTs or n-channel TFTs. However, it is preferable that the driver TFT  2702  is a p-channel TFT, and that the switching TFT  2701  is an n-channel TFT for cases in which the pixel electrode of the light emitting element  2704  is the anode. Conversely, it is preferable that the driver TFT  2702  is an n-channel TFT, and that the switching TFT  2701  is a p-channel TFT if the pixel electrode is the cathode. 
         [0019]    Operations during display of an image with the aforementioned pixel structure are explained below. 
         [0020]    A signal is inputted to the gate signal line G, and the electric potential of the gate electrode of the switching TFT  2701  changes, then a gate voltage is changed. The signal is inputted to the gate electrode of the driver TFT  2702  by the source signal line S, via source and drain of the switching TFT  2701  which thus has been placed in a conductive state. Further, the signal is stored in the storage capacitor  2703 . The gate voltage of the driver TFT  2702  changes in accordance with the signal inputted to the gate electrode of the driver TFT  2702 , then the source and drain are placed in a conductive state. The electric potential of the electric power source supply line V is imparted to the pixel electrode of the light emitting element  2704  through the driver TFT  2702 . The light emitting element  2704  thus emits light. 
         [0021]    A method of expressing gradations with pixels having such a structure is explained. Gradation expression methods can be roughly divided into an analog method and a digital method. The digital method has advantages of being good at variation of TFTs compared with the analog method. A digital gradation expression method is focused upon here. A time gradation method can be given as the digital gradation expression method. A time gradation driving method is explained in detail now. 
         [0022]    The time gradation driving method is a method of expressing gradations by controlling the period that each pixel of a display device emits light. If a period for displaying one image is taken as one frame period, then one frame period is divided into a plurality of subframe periods. 
         [0023]    Turn on and turn off, namely whether or not the light emitting element of each pixel is made to emit light or to not emit light, is performed for each subframe period. The period during which the light emitting element emits light in one frame period is controlled, and a gradation for each pixel is expressed. 
         [0024]    The time gradation driving method is explained in detail using timing charts of  FIGS. 10A and 10B . Note that an example of expressing gradation using a 4-bit digital image signal is shown in  FIGS. 10A and 10B . Note also that  FIG. 9  may be referred to regarding the structure of the pixel portion and the structure of the pixels, respectively. In accordance with an external electric power source (not shown in the figure), the opposing electric potential can be switched over between an electric potential on the same order as the electric potential of the electric power source supply lines V 1  to Vx (electric power source electric potential), and an electric potential difference of the electric power source supply lines V 1  to Vx on an order sufficient to make the light emitting element  2704  emit light. 
         [0025]    One frame period F is divided into a plurality of subframe periods SF 1  to SF 4 . The gate signal line G 1  is selected first in the first subframe period SF 1 , and a digital image signal is inputted from the source signal lines S 1  to Sx to each of the pixels having the switching TFTs  2701  with gate electrodes connected to the gate signal line G 1 . The driver TFT  2702  of each pixel is placed in an ON state or an OFF state by the inputted digital image signal. 
         [0026]    The term “ON state” for a TFT in this specification indicates that the TFT is in a state in which there is a state of conduction between the source and the drain in accordance with a gate voltage. Further, the term “OFF state” for a TFT indicates that there is a non-conductive state between the source and the drain in accordance with a gate voltage. 
         [0027]    The opposing electric potential of the light emitting elements  2704  is set nearly equal to the electric potential of the electric power source supply lines V 1  to Vx (electric power source electric potential) at this point, and therefore the light emitting elements  2704  do not emit light even in pixels having their driver TFT  2702  in an ON state. The aforementioned operations are repeated for all of the gate signal lines G 1  to Gy, and a write-in period Ta 1  is completed. Note that a period for write-in during the first subframe period SF 1  is called Ta 1 . In general, a write-in period of a j-th sub-frame period (where j is a natural number) is called Taj. 
         [0028]    The opposing electric potential changes when the write-in period Ta 1  is complete, so as to have an electric potential difference from the electric power source electric potential on an order so that the light emitting element  2704  will emit light. A display period Ts 1  thus begins. Note that the display period of the first subframe period SF 1  is called Ts 1 . In general, a display period of the j-th sub-frame period (where j is a natural number) is denoted by using a reference symbol Tsj. The light emitting elements  2704  of each pixel are placed in a light emitting state or a non-light emitting state, corresponding to the inputted signal, in the display period Ts 1 . 
         [0029]    The above operations are repeated for all of the subframe periods SF 1  to SF 4 , one frame period F 1  is completed. The length of the display periods Ts 1  to Ts 4  of the subframe periods SF 1  to SF 4  are set appropriately here, and gradations are expressed by an accumulation of the display periods of the subframe period during which the light emitting elements  2704  emit light. In other words, the total amount of the turn on time within one frame period is used to express the gradations. 
         [0030]    A method of generally expressing 2 n  gradations by inputting an n-bit digital video signal, is explained. One frame period is divided into n sub-frame periods SF 1  to SFn at this point, for example, and the ratios of the lengths of the display periods Ts 1  to Tsn of the sub-frame periods SF 1  to SFn are set so as to be Ts 1 :Ts 2 : . . . :Tsn=2 0 :2 −1 : . . . :2 −n+2 :2 −n+1 . Note that the lengths of the write-in periods Ta 1  to Tan are all the same. 
         [0031]    Within one frame period, the gradation of the pixels in the frame period is determined by finding the total of the display period Ts during which a light emitting state is selected in the light emitting element  2704 . For example, if the brightness for a case in which a pixel emits light during all of the display periods is taken to be 100% when n=8, then a brightness of 1% can be expressed if the pixel emits light in the display period Ts 8  and in the display period Ts 7 . A 60% brightness can be expressed for cases in which the pixel emits light in the display periods Ts 6 , Ts 4 , and Ts 1 . 
         [0032]    A circuit to convert signals is needed in order to display in such time gradation method as shown above. Schematic of the conventional control circuit is shown in  FIG. 2 . A control circuit  200  is constituted by memories A 201  and B 202  for storing data, a logic circuit for reading data and writing into the memory (W-LOGIC  203 ), and a logic circuit for reading the memory and outputting data (R-LOGIC  204 ). 
         [0033]    A timing chart of the conventional control circuit is shown in  FIG. 3 . Data is written and read alternately using memories A 201  and B 202 , in order to make the digital data inputted to W-LOGIC  203  synchronize with a time gradation method. 
         [0034]    When R-LOGIC 204  reads a signal in the memory A 201 , a digital video signal for the next frame period is inputted to the memory B 202  through W-LOGIC  203  and starts being stored. 
         [0035]    In this way, the control circuit  200  includes the memories A 201  and B 202  which can store digital video signal of  1  frame period each, to sample a digital video signal by using them alternately. 
         [0036]    Conventionally, however, there was a state of Wait until the next read signal occurred after writing into the memories A 201  and B 202 . A switching function between writing and reading of the memories A 201  and B 202  was operated in timing with reading which takes more time. ( FIG. 3 ) 
       SUMMARY OF THE INVENTION 
       [0037]    In the conventional method, a time for reading was set much longer than a time for writing. Therefore there was no problem with a method in which a writing occurs as needed and operating functions are switched after reading. 
         [0038]    However, there was a problem. In a driving method which has little difference between a time for reading and a time for writing of memory, a conventional method that there is a state of Wait until reading is done after writing pulled back the timing of writing to memory. As a result of this, a frame frequency decreases. 
         [0039]    To solve the above-mentioned problem of related art, the present invention took the following method. Namely, by reading states of reading signal and writing signal at a certain timing, synchronization is taken and which one of two memories will be written to is decided through the signals. 
         [0040]    Namely, by using a display device having: 
         [0041]    a first memory and a second memory which store data; 
         [0042]    a writing device which reads data and writes to the first memory and the second memory; 
         [0043]    a reading device which reads data from the first memory or the second memory, and outputs data; 
         [0044]    a means to decide the roles for writing and reading to the first memory and the second memory in accordance with the states of a writing device and a reading device; and 
         [0045]    a first memory selector and a second memory selector which select writing and reading to the first memory and the second memory; 
         [0046]    the writing device and the reading device can be synchronized to solve the problem. 
         [0047]    As a means to decide the roles for writing and reading to a first memory and a second memory from the states of a writing device and a reading device, a display device provides a circuit, wherein: 
         [0048]    state of a writing device is denoted by a first signal and state of a reading device is denoted by a second signal; 
         [0049]    a third signal decides the roles for writing and reading to a first memory and a second memory, and inverts to switch the roles of the first memory and the second memory when the first and second signals come into a second state; 
         [0050]    a fourth signal holds the third signal; 
         [0051]    said first and second memories are given the roles of writing and reading respectively; 
         [0052]    the first signal is inputted to the reading device and the second signal is inputted to the writing device; 
         [0053]    when the writing device is in a state of writing, the first signal and the second signal come into a first state, therefore, the third signal is not inverted and the fourth signal overwrites a state of the third signal; 
         [0054]    when the writing device is in a waiting state, the first signal comes into the second state and the second signal also comes into the second state to invert the third signal, therefore, the roles of writing and reading of two memories are switched. Then the second signal returns to the first state again. The fourth signal is compared with the third signal, and a state of the first signal is returned to the first state at the time the state of the third signal changes and the writing device starts writing. 
         [0055]    Then, the reading device and the writing device may be not only FPGAs but also LSIs. Furthermore, they may be constituted on the same substrate with the display device. 
         [0056]    Thereby, even when there is little difference between the time for reading and writing to memories, the operating functions can be switched in the optimum period. The problem that the frame frequency decreases can thus be solved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0057]      FIG. 1  is a block diagram of the present invention. 
           [0058]      FIG. 2  is a block diagram of the conventional example. 
           [0059]      FIG. 3  is a timing chart of the operation of conventional example. 
           [0060]      FIG. 4  is a timing chart of the operation of the present invention. 
           [0061]      FIG. 5  is a timing chart of the operation of the present invention. 
           [0062]      FIG. 6  is a diagram showing an embodiment using the present invention. 
           [0063]      FIG. 7  is a diagram showing an example of a display device using the present invention. 
           [0064]      FIG. 8  is a block diagram of the conventional example. 
           [0065]      FIG. 9  is a circuit diagram of the pixels disposed in a matrix shape. 
           [0066]      FIGS. 10A and 10B  are timing charts of the operation of the conventional example. 
           [0067]      FIG. 11  is a diagram showing an example of a display device using the present invention. 
           [0068]      FIGS. 12A to 12G  are diagrams showing electric devices using the present invention. 
           [0069]      FIG. 13  is a diagram showing an example of a display device using the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0070]      FIG. 1  shows a block diagram of a major structure of the present invention. 
         [0071]    A control circuit  100  has memories A 101  and B 102 , a Selector  103  for writing a memory, a Selector  104  for output, a logic circuit for writing into a memory (W-LOGIC  105 ), and a logic circuit for reading the memory and outputting the data (R-LOGIC  106 ). When video data is inputted to W-LOGIC  105 , it writes data in either of memories A 101  or B 102  selected by the Selector  103  for writing memory. Then the Selector  104  selects the other memory which was not selected by the Selector  103  as a memory for R-LOGIC  106  to read. 
         [0072]    Signals of SYNC, WFLAG, RFLAG and RAM_SELECT are newly adopted to achieve a synchronization. W-LOGIC  105  inputs the writing state WFLAG to R-LOGIC  106 , and the reading state RFLAG from memory is inputted to W-LOGIC  105  as needed. RAM_SELECT selects a memory to write in accordance with each state of WFLAG and RFLAG. R-LOGIC  106  holds RAM_SELECT and makes a comparison with the RAM_SELECT of the moment when SYNC is inputted. 
         [0073]    In the structure in  FIG. 1 , R-LOGIC  106  holds RAM_SELECT particularly, however, W-LOGIC  105  may hold RAM_SELECT as well. 
         [0074]    A timing chart of the operations of W-LOGIC  105  and R-LOGIC  106  is shown in  FIG. 4 . 
         [0075]    WFLAG is Low when W-LOGIC  105  is in Write state and RFLAG also becomes Low when Low of WFLAG is inputted to R-LOGIC  106 . 
         [0076]    WFLAG is High when W-LOGIC  105  is in the state of Wait, and when High of WFLAG is inputted to R-LOGIC  106 , RFLAG becomes High as well. RFLAG becomes Low when both WFLAG and RFLAG are High and R-LOGIC  106  finishes reading data from the memory selected by the Selector  104  for output. In timing with RFLAG becoming Low, RAM_SELECT is inverted and the memory selected by the Selectors  103  and  104  switches. 
         [0077]    When SYNC is inputted, RAM_SELECT at that point is compared with the RAM_SELECT stored in R-LOGIC  106 . During a Wait period, the RAM_SELECT is inverted and WFLAG becomes Low when the state of the inverted RAM_SELECT is different from the RAM_SELECT stored in R-LOGIC  106 , and again W-LOGIC  105  becomes Write state. 
         [0078]    In  FIG. 5 , a timing chart regarding the synchronization and the timing of writing and reading is shown. When SYNC is inputted, R-LOGIC  106  writes the state of the RAM_SELECT. During a Write (WFLAG is Low) period, a new state of RAM_SELECT is overwritten, and the state is kept during a Wait (WFLAG is High) period. 
         [0079]    Furthermore, when inverted RAM_SELECT during Wait period is different in state from RAM_SELECT stored in R-LOGIC  106 , WFLAG becomes Low and again W-LOGIC  105  becomes Write state. 
         [0080]    As RFLAG is Low when RAM_SELECT is inverted, writing and reading can be synchronized at this point. 
         [0081]    Embodiments of the invention will be described. 
       Embodiment 1 
       [0082]    In this embodiment, an example of the constitution of a control circuit which outputs to a display for displaying using OLED elements referring to  FIG. 6 . 
         [0083]    18 bits (6 bits×RGB) of Video_Data and control signals are inputted to a control circuit  601 . The operation from the input of Video_Data to the output to a display  608  is described. 
         [0084]    Reading of each line is controlled by VCLK (a cycle is 148.8 μs). First, the input of Video_Data starts by being inputted a SYNC signal. After being inputted a SYNC signal and a certain period of off time passes, the input of Video_Data to W-LOGIC  602  starts. One line of Video_Data is read per half cycle of VCLK. After inputting  220  lines and a certain period of off time passes, a SYNC signal is inputted again, and Video_Data is inputted. An input cycle for full page is 18.1536 ms (122 cycles of VCLK). 
         [0085]    Reading to each block in one line is controlled by HCLK (a cycle is 400 ns). HCLK reads Video_Data during Video_Enable is high. After reading one line, more specifically, 176 blocks of data, and a certain period of off time (Video_Enable is Low) passes, then reading the next line of Video_Data. By repeating this for 220 lines, data for one screen is completed. 
         [0086]    On the other hand, a memory A 606  and a memory B 607  are connected to the control circuit  601 , and a signal RAM_SELECT from the control circuit  601  decides which memory is written and read. Each memory is constituted of 24 (8×3) flip flops. Each flip-flop can store data (6 bits) for one color at a certain point. Data is moved to next flip flop sequentially by HCLK. When the memory has eight blocks of data, one memory is selected for writing and the other memory is selected for reading data in accordance with a value of RAM_SELECT. After finishing a cycle of reading data, and receive data, RAM_SELECT is switched. 
         [0087]    Because the display on a display  608  is done by time gradation, data written to the memory A 606  or the memory B 607  are changed their orders for the output to the display and sequentially outputted to the display  608 . R-LOGIC  603  takes data for 8 blocks into the memory A 606  and the memory B 607 , followed by reading the first period of 1 to 4 blocks, the first period of 5 to 8 blocks, the second period of 1 to 4 blocks, the second period of 5 to 8 blocks . . . up to the sixth period in this order, and outputs them to the display  608 . 
         [0088]    In displaying on the display  608 , Video_Data is processed in 12 bits (4×RGB). G 1 _CK, G 2 _CK, G 1 _CKB, G 2 _CKB are clocks whose cycles are 12 μs each. In timing with G 1 _CK and G 1 _CKB rising or dropping, the row where Video_Data is inputted moves. 
         [0089]    After 2 cycles after G 1 _SP drops, writing is done from the top row in sequence. Writing 220 lines makes a display for one screen, however, 4 dummy cycles (48 μs) come into to delay writing before displaying the next image. G 2 _SP is risen in cleaning the writing, as needed. 
         [0090]    S_CK and S_CKB are clocks whose cycles are 200 ns each. In timing with S_CK and S_CKB rising or dropping, the block where Video_Data is inputted moves. After 4 cycles (800 ns) after rising or dropping of G 1 _CLK, S_LAT becomes High to hold an electric charge, and then when S_SP changes from High to Low, the input of Video_Data starts. As input is done every 4 blocks, repeating it 44 times completes writing for one line. 
         [0091]    Inputting clocks from an oscillation element  609  through PLL  610  take synchronization between W-LOGIC  602  and R-LOGIC  603 . The timing of writing and reading to the memory A 606  and the memory B 607  is controlled by the rise and drop of the clocks through PLL  610 . 
         [0092]    Known LSI as well as FPGA may be used for W-LOGIC  602  and R-LOGIC  603 . 
         [0093]    The invention is used for W-LOGIC  602 , R-LOGIC  603 , the memory A 606 , the memory B 607 , and Selectors  604  and  605  which select memory. 
       Embodiment 2 
       [0094]    In  FIG. 7 , an example of a display device using OLED elements with a control circuit of the embodiment 1 is shown. 
         [0095]    A display device is constituted by a panel  700 , a control circuit  701 , a source signal line driving circuit  702 , a gate signal line driving circuits  703  and  704 , a display portion  705 , an SRAM  706 , an FPC 707 , and a connector  708 . Each circuit of the display device is formed over the panel  700 , otherwise attached externally. 
         [0096]    Operation of the display device is now described. Data and a control signal sent from the FPC  707  through the connector  708  are inputted to the control circuit  701  and the data are rearranged for output in SRAM  706 , and then sent to the control circuit  701  again. The control circuit  701  sends signals for data and display to the source signal line driving circuit  702  and the gate signal line driving circuits  703  and  704 , and then image is displayed at the display portion  705  using OLED elements. 
         [0097]    The source signal line driving circuit  702  and the gate signal line driving circuits  703  and  704  can be substituted for the known circuits. Furthermore, the gate signal line driving circuit can be reduced to one depending on the structure of the circuit. 
         [0098]    The invention is applied to the control circuit  701 . 
       Embodiment 3 
       [0099]    In this embodiment, an example of the display device using OLED elements with a control circuit of the embodiment 1 which is different from the embodiment 2 is described in  FIG. 13 . 
         [0100]    A panel  900  is constituted by a control circuit  901 , a source signal line driving circuit  902 , a gate signal line driving circuits  903  and  904 , a display portion  905 , an SRAM  906 , an FPC  907 , and a connector  908 . Each circuit of the display device is formed over the panel  900 , otherwise attached externally. 
         [0101]    Operation of the display device is now described. Data and a control signal sent from the FPC  907  through the connector  908  are inputted to the control circuit  901  and their data are returned to the SRAM  906  in the FPC  907 , and then rearranged for output and sent to the control circuit  901  again. The control circuit  901  sends signals used for data and display to the source signal line driving circuit  902  and the gate signal line driving circuits  903  and  904 , and then display of the picture image is performed at the display portion  905  using OLED elements. 
         [0102]    The difference with the embodiment 2 is that the SRAM  906  is incorporated in the FPC  907 . Display device can be made smaller thereby. 
         [0103]    As with the embodiment 2, the source signal line driving circuit  902  and the gate signal line circuits  903  and  904  can be substituted for the known circuits. Furthermore, the gate signal line driving circuit can be reduced to one depending on the structure of the circuit. 
         [0104]    The invention is applied to the control circuit  901 . 
       Embodiment 4 
       [0105]    In this embodiment, an example of the control circuit for output to the display using OLED elements having the different structure from the embodiments 1 to 3 is described referring to  FIG. 11 . 
         [0106]    Time gradation method display naturally takes more operating frequencies compared with an analog display. In order to achieve a high image quality, pseudocontour needs to be avoided and subframe needs to be increased to 10 or more. Therefore, operating frequency also needs to be decupled or more. 
         [0107]    To drive the device with such an operating frequency, SRAM needs a high speed operation using an SRAM-IC for high speed operation. 
         [0108]    SRAM for high speed operation, however, consumes rather big power when storing, so that it is not appropriate for mobile devices. In order to use an SRAM of low-power-consumption, frequency needs to be more decreased. 
         [0109]    As shown in  FIG. 11 , a serial-parallel conversion circuit  1702  is constituted which changes data from serial to parallel before writing digital image signals to SRAMs  1703  and  1704 . Writing is made through a Switch  1706  thereafter. 
         [0110]    By taking such a measure, parallel calling can be made with low frequency. Hence, a low-power-consumption SRAM can be used with low frequency to achieve the low power consumption of mobile devices. 
       Embodiment 5 
       [0111]    The invention may be applied to electric devices such as a video camera, a digital camera, a goggle display (head mount display), a navigation system, a sound reproduction device (car audio, audio component and the like), a laptop personal computer, a game device, a Personal Digital Assistant (mobile computer, mobile phone, portable game device or a digital book and the like), picture reproducer with recording medium (specifically a device with a display which plays the recording medium such as Digital Versatile Disc (DVD) and display the images) and the like. Examples of those electric devices are shown in  FIG. 12 . 
         [0112]      FIG. 12(A)  illustrates a liquid crystal display or an OLED display constituted by a case  1001 , a stand  1002 , a display portion  1003  and the like. The present invention can be applied to a driving circuit of the display device having the display portion  1003 . 
         [0113]      FIG. 12(B)  illustrates a video camera constituted by a main body  1011 , a display portion  1012 , an audio input portion  1013 , operating switches  1014 , a battery  1015 , an image receiving portion and the like. The present invention can be applied to a driving circuit of the display device having the display portion  1012 . 
         [0114]      FIG. 12(C)  illustrates a laptop personal computer constituted by a main body  1021 , a case  1022 , a display portion  1023 , a keyboard  1024  and the like. The present invention can be applied to a driving circuit of the display device having the display portion  1023 . 
         [0115]      FIG. 12(D)  illustrates a Personal Digital Assistant constituted by a main body  1031 , a stylus  1032 , a display portion  1033 , operating buttons  1034 , an external interface  1035  and the like. The present invention can be applied to a driving circuit of the display device having the display portion  1033 . 
         [0116]      FIG. 12(E)  illustrates a sound reproduction device, especially an audio device mounted in a motor vehicle constituted by a main body  1041 , a display portion  1042 , operating switches  1043  and  1044  and the like. The invention can be applied to a driving circuit of the display device including the display portion  1042 . Furthermore, the invention can be applied to any of portable or home audio devices other than the above-described audio device mounted in a motor vehicle. 
         [0117]      FIG. 12(F)  illustrates a digital camera constituted by a main body  1051 , a display portion (A) 1052 , an ocular portion  1053 , operating switches  1054 , a display portion (B)  1055 , a battery  1056  and the like. The present invention can be applied to a driving circuit of the display device having the display portions (A)  1052  and (B)  1055 . 
         [0118]      FIG. 12(G)  illustrates a mobile phone constituted by a main body  1061 , an audio output portion  1062 , an audio input portion  1063 , a display portion  1064 , operating switches  1065 , an antenna  1066  and the like. The present invention can be applied to a driving circuit of the display device having the display portion  1064 . 
         [0119]    A plastic substrate with high heat resistance other than a glass substrate can also be applied to the display device of these electronic devices. Further weight saving can be achieved thereby. 
         [0120]    It is to be noted that the above-described devices of this embodiment are only examples and that the invention is not exclusively applied to them. 
         [0121]    This embodiment can be freely combined with the embodiment mode as well as any of embodiments 1 to 4. 
         [0122]    In the case of the display device with light emitting elements, the reduction of frame frequency can be prevented by switching writing and reading efficiently by utilizing the control circuit of the invention.