Abstract:
A display device has pixels, signal lines coupled to the pixels, and a video signal drive circuit for supplying signals to the signal lines in parallel, based upon data transferred serially from an external system. The video signal drive circuit includes plural stages each formed by a column of switching elements. The switching elements of each stage double successively in number as a final stage is approached. Each of the switching elements of each stage excluding the final stage is connected to a pair of switching elements in a succeeding stage. Each of the pair of switching elements is repeatedly and alternately switched ON with the other of the pair of switching elements being switched OFF, and a frequency of the ON-OFF switching of the pair of switching elements of each stage is successively halved as the final stage is approached.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a display device, and in particular, to a display device having an improved video signal drive circuit.  
           [0002]    For example, in an active matrix type liquid crystal display device, a liquid crystal layer is sandwiched between a pair of opposing substrates, formed on a liquid-crystal layer-side surface of one of the pair of substrates are a plurality of gate signal lines extending in an x direction and arranged in a y direction, and a plurality of drain signal lines extending in the y direction and arranged in the x direction, and each of a plurality of pixel areas is surrounded by two adjacent ones of the gate signal lines and two adjacent ones of the drain signal lines.  
           [0003]    Each pixel area is provided with a switching element driven by a scanning signal via a gate signal line, and is provided with a pixel electrode supplied with a video signal via the switching element from a drain signal line. The pixel electrode generates an electric field between the pixel electrode and a counter electrode formed on one of the two substrates and thereby controls light transmission through the liquid crystal layer.  
           [0004]    One end of each of the gate signal lines is connected to a vertical scanning circuit which sequentially selects one of the gate signal lines based upon the scanning signal. One end of each of the drain signal lines is connected to a video signal drive circuit which supplies a video signal to each of the drain signal lines in synchronism with selection of a corresponding one of the gate signal lines.  
           [0005]    Data from an external system such as a microcomputer, for example, are transferred to the video signal drive circuit serially, and then they are supplied to respective ones of the drain signal lines in parallel by a shift register provided within the video signal drive circuit.  
         SUMMARY OF THE INVENTION  
         [0006]    In the liquid crystal display device having the above-described configuration, these day it is pointed out that the speed of transmission of data within the video signal drive circuit has become insufficient as the degree of high definition and the size of the viewing screen are increased.  
           [0007]    As the liquid crystal display device is made large-sized and thereby the length of wiring within the display device is increased, parasitic capacitances are increased which are formed by wiring and pixels into which signals are not written, that is, which are not selected, and consequently, time constant is increased. Therefore, time available for writing data into one pixel is decreased as the number of pixels is increased, since rise times of clock and drain signal pulses are approximately represented by τ=CR in the shift register incorporated within the video signal drive circuit.  
           [0008]    Such problems have been pronounced especially in the liquid crystal display devices of the type in which the video signal drive circuit (also the vertical scanning drive circuit) is fabricated directly on one of the two substrates, and transistors constituting the shift register within the drive circuits are fabricated by using polysilicon (p-Si) semiconductor layers simultaneously with switching element (a thin film transistor) disposed within the pixel area, because ON-resistance of those transistors is high. Consequently, in a case where the high-speed transmission of data is required, the shift register itself becomes inoperative, and there has been a demand for elimination of the problem.  
           [0009]    The present invention has been made in view of the above-explained situation, and it is one of the present invention to provide a display device capable of high-speed transmission of data.  
           [0010]    The following explains the representative ones of the present inventions disclosed in this specification briefly.  
           [0011]    In accordance with an embodiment of the present invention, there is provided a display device comprising: a plurality of pixels; a plurality of signal lines for supplying signals to the plurality of pixels; and a video signal drive circuit for receiving data transferred serially from an external system and supplying the signals based upon the data to the plurality of signal lines in parallel, wherein the video signal drive circuit includes a plurality of stages each comprising a column of switching elements, the switching elements constituting the column of each of the plurality of stages double successively in number as a final one of the plurality of stages is approached, each of the switching elements of each of the plurality of stages excluding the final one is connected to a pair of switching elements in a next succeeding one of the plurality stages, each of the pair of switching elements is repeatedly and alternately switched ON with the other of the pair of switching elements being switched OFF, and a frequency of the ON-OFF switching of the pair of switching elements of each of the plurality of stages is successively halved as the final one of the plurality of stages is approached.  
           [0012]    In accordance with another embodiment of the present invention, there is provided a display device comprising: a plurality of pixels; a plurality of signal lines for supplying signals to the plurality of pixels; a data conversion circuit for converting an arrangement of data transferred serially from an external system; and a video signal drive circuit for receiving the data transferred serially from the data conversion circuit and supplying the signals based upon the data to the plurality of signal lines in parallel, wherein the video signal drive circuit includes a plurality of stages each comprising a column of switching elements, the switching elements constituting the column of each of the plurality of stages double successively in number as a final one of the plurality of stages is approached, each of the switching elements of each of the plurality of stages excluding the final one is connected to a pair of switching elements in a next succeeding one of the plurality stages, each of the pair of switching elements is repeatedly and alternately switched ON with the other of the pair of switching elements being switched OFF, a frequency of the ON-OFF switching of the pair of switching elements of each of the plurality of stages is successively halved as the final one of the plurality of stages is approached, and the data conversion circuit has a configuration that is a mirror image of the video signal drive circuit.  
           [0013]    In accordance with another embodiment of the present invention, there is provided a display device comprising: a plurality of pixels; a plurality of signal lines for supplying signals to the plurality of pixels; and a video signal drive circuit for receiving data transferred serially from an external system and supplying the signals based upon the data to the plurality of signal lines in parallel, wherein the video signal drive circuit includes a plurality of stages each comprising a column of switching elements, the switching elements constituting the column of each of the plurality of stages double successively in number as a final one of the plurality of stages is approached, each of the switching elements of each of the plurality of stages excluding the final one is connected to a pair of switching elements in a next succeeding one of the plurality stages, each of the pair of switching elements is repeatedly and alternately switched ON with the other of the pair of switching elements being switched OFF, a frequency of the ON-OFF switching of the pair of switching elements of each of the plurality of stages is successively halved as the final one of the plurality of stages is approached, and the video signal drive circuit further includes a store memory section for grouping and storing therein the data transferred from the final one of the plurality of stages.  
           [0014]    The display devices having the above configurations are capable of reducing substantial input load-capacitances and thereby time constant τ=CR greatly in their video signal drive circuit, and thereby are capable of increasing the speed of transmission of data (digital data) and also reducing their power consumption even when the size of their viewing screen and the degree of high definition are increased. Although a clock having the highest speed is required for a column of switching elements in the first stage, the present invention makes it possible to provide the highest speed clock to the column of switching elements externally, and thereby is capable of relaxing restrictions imposed on high-speed read-in operation by insufficient driving capability of the switching elements provided in the pixel areas. These advantages become more pronounced when the switching elements are thin film transistors fabricated by using polysilicon semiconductor layers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the accompanying drawings, in which like reference numerals designate similar components throughout the figures, and in which:  
         [0016]    [0016]FIG. 1 is a circuit diagram illustrating an embodiment of a display device in accordance with the present invention, showing a major circuit portion of a video signal drive circuit employed in the display device;  
         [0017]    [0017]FIG. 2 illustrates a liquid crystal display as an embodiment of a display device in accordance with the present invention, showing its liquid crystal display panel and its peripheral circuits;  
         [0018]    [0018]FIGS. 3A and 3B are circuit diagrams of examples of two types of switching elements used in the video signal drive circuit of FIG. 1, respectively;  
         [0019]    [0019]FIG. 4 is a timing chart of clock signals supplied to the switching elements shown in FIG. 1;  
         [0020]    [0020]FIG. 5A is a circuit diagram of an example of a frequency divider for generating clock signals supplied to the video signal drive circuit of FIG. 1, and FIG. 5B is a timing chart of the generated clock signals;  
         [0021]    [0021]FIG. 6 is a circuit diagram of an example of an inverse conversion circuit employed in the display device in accordance with the present invention;  
         [0022]    [0022]FIG. 7 is a major circuit portion of another embodiment of a video signal drive circuit employed in a display device in accordance with the present invention; and  
         [0023]    [0023]FIG. 8 is a timing chart of clock signals supplied to a store memory section employed in the video signal drive circuit shown in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    An embodiment of the liquid crystal display device in accordance with the present invention will be explained by reference to the drawings.  
         [0025]    Embodiment 1  
         [0026]    Overall Configuration  
         [0027]    [0027]FIG. 2 illustrates a liquid crystal display panel PNL and its peripheral circuits as an embodiment of a display device in accordance with the present invention.  
         [0028]    The liquid crystal display panel PNL shown in FIG. 2 comprises a pair of opposing substrates SUB 1  and SUB 2 , a liquid crystal layer sandwiched between the substrates SUB 1  and SUB 2 . Formed on a liquid-crystal-layer side surface of the substrate SUB 1  are a plurality of gate signal lines GL extending in an x direction and arranged in a y direction and a plurality of drain signal lines DL extending in the y direction and arranged in the x direction.  
         [0029]    Each of rectangular areas surrounded by two adjacent ones of the gate signal lines GL and two adjacent ones of the drain signal lines DL forms one pixel area, and a matrix array of the pixel areas form a liquid crystal display section AR.  
         [0030]    Each of the pixel areas is provided with a thin film transistor TFT driven by a scanning signal from a corresponding one of the gate signal lines GL, and a pixel electrode PX supplied with a video signal via the thin film transistor TFT from a corresponding one of the drain signal lines DL. The pixel electrode PX generates an electric field between the pixel electrode PX and a counter electrode (not shown) formed on a liquid-crystal-layer side surface of one of the two substrates SUB 1 , SUB 2  and thereby controls light transmission through the liquid crystal layer.  
         [0031]    The substrate SUB 1  fabricated as explained above is superposed on the other substrate SUB 2  with the liquid crystal layer interposed therebetween in the liquid crystal display section AR, and the two substrates SUB 1  and SUB 2  are fixed together by a sealing member which also serves to seal up the liquid crystal layer therebetween. Each of the gate signal lines GL disposed in the liquid crystal display section AR extends beyond the sealing member such that both its ends are connected to two vertical scanning circuits V fabricated on the substrate SUB 1 , respectively. Each of the drain signal lines DL disposed in the liquid crystal display section AR extends beyond the sealing member such that one of its two ends are connected to a video signal drive circuit He fabricated on the substrate SUB 1 .  
         [0032]    Each of the gate signal lines GL is selected by a scanning signal from the vertical scanning circuit V, turns ON all the thin film transistors TFT of a group of the pixels coupled to the selected one of the gate signal lines GL, and in synchronism with this, video signals are output to respective ones of the drain signal lines DL from the video signal drive circuit He. The video signals are supplied to respective ones of the pixel electrodes PX the pixels of the group via the turned-ON thin film transistors TFT. The video signal drive circuit He will be explained in further detail subsequently.  
         [0033]    On the other hand, there is provided an external system such as a microcomputer system or the like, and this external system supplies data, sync pulses and supply voltages to external circuits disposed around the liquid crystal display panel PNL. The external circuit includes data conversion circuits and a timing controller for taking in the data and sync pulses from the external system, respectively. The data conversion circuit is configured so as to change the arrangement of the data supplied from the external system such that the converted data from the conversion circuit suit with the configuration of distribution ports (a first distribution port, a second distribution port, a third distribution port, and an eighteenth distribution port) serving as the first-stage circuit of the video signal drive circuit He for the liquid crystal display panel PNL, and further details of the data conversion circuit will be explained subsequently.  
         [0034]    Each of the distribution ports is configured such that the arrangement of data supplied from the data conversion circuit are changed, and therefore the data conversion circuit changes the arrangement of the data in advance, taking into account the subsequent conversion of the arrangement of the data by the distribution ports. In other words, initially the data conversion circuit changes data supplied in the regular arrangement from the external system, and thereafter the distribution ports convert the data from the data conversion circuit into the data in the regular arrangement.  
         [0035]    Voltages corresponding to gray scale levels are selected by a decoder included in the video signal drive circuit in accordance with the data from the distribution ports, and they are supplied to the respective drain signal lines DL.  
         [0036]    Configuration of the Distribution Ports  
         [0037]    [0037]FIG. 1 is a circuit diagram of an example of the distribution port used as the above-mentioned first, second, . . . , and eighteenth distribution ports.  
         [0038]    As is apparent from FIG. 1, one distribution port comprises a column SL 1  of switching elements constituting the first stage serving as an input stage, a column SL 2  of switching elements constituting the second stage, a column SL 3  of switching elements constituting the third stage, a column SL 4  of switching elements constituting the fourth stage, a column SL 5  of switching elements constituting the fifth stage, and a store memory section SM. The column SL 1  of switching elements of the first stage is composed of two (2 1 ) switching elements, the column SL 2  of switching elements of the second stage is composed of four (2 2 ) switching elements, the column SL 3  of switching elements of the third stage is composed of eight (2 3 ) switching elements, the column SL 4  of switching elements of the fourth stage is composed of sixteen (2 4 ) switching elements, and the column SL 5  of switching elements of the fifth stage is composed of thirty-two (2 5 ) switching elements.  
         [0039]    Each of the switching elements SW constituting the columns of the switching elements of the respective stages has one of configurations enclosed by broken lines in FIGS. 3A and 3B. When switching elements SW of one type (for example, switching elements denoted by “+” in FIG. 1) is turned ON, depending upon a clock signal supplied thereto, switching elements SW of the other type (for example, switching elements denoted by “−” in FIG. 1) is turned OFF, and vice versa. The clock signal alternately turns ON each type of the two types of the switching elements with turning OFF the other type of the two types of the switching elements. This cycle of the ON and OFF operation is repeated.  
         [0040]    In each of the columns SL of the switching elements SW of the respective stages, the switching elements SW of two different types are arranged alternately. Each of the switching elements SW arranged in a switching-element column SL in one stage is connected to a pair of adjacent ones of the switching elements SL arranged in a switching-element column SL in the next succeeding stage.  
         [0041]    For example, in FIG. 1, a switching element SWl disposed in an upper half of the column SL 1  of the first stage is connected to a pair of switching elements SW 21  and SW 22  disposed in an upper half of the column SL 2  of the second stage, a switching element SW  12  disposed in a lower half of the column SL 1  of the first stage is connected to a pair of switching elements SW 23  and SW 24  disposed in a lower half of the column SL 2  of the second stage.  
         [0042]    The switching elements SW in the columns SL of the respective stages are supplied with clock pulses φi (i=1, 2, 3, 4, 5) or clock pulses/φi (i=1, 2, 3, 4, 5) having opposite phase from the clock pulses φi as illustrated in FIG. 4 (in this specification, a slant “/” is used to indicate that a clock pulse/φi has opposite phase from a clock pulse φi, but the bar “−” is used instead of the slant “/” in the drawings).  
         [0043]    A frequency of a clock pulse φ 2  supplied to the switching elements in the column SL 2  of the second stage is half that of a clock pulse φ 1  supplied to the switching elements in the column SL 1  of the first stage, a frequency of a clock pulse φ 3  supplied to the switching elements in the column SL 3  of the third stage is one fourth of that of the clock pulse φ 1 , a frequency of a clock pulse φ 4  supplied to the switching elements in the column SL 4  of the fourth stage is one eighth of that of the clock pulse φ 1 , and a frequency of a clock pulse φ 5  supplied to the switching elements in the column SL 5  of the fifth stage is one sixteenth of that of the clock pulse φ 1 . These clock pulses φi (i=1-5) are supplied from an internal frequency divider shown in FIG. 2, and an example of the internal frequency divider is illustrated in detail in FIG. 5A. In FIG. 5A, the frequency divider is composed of five flip-flops connected in series, and in response to inputs of data clock pulses CK and a clear pulse CGL shown in FIG. 5B, the clock pulses φ 1 , φ 2 , φ 3 , φ 4  and φ 5  are output from the first, second, third, fourth and fifth flip-flops, respectively.  
         [0044]    In the distribution port of this configuration, first the data from the above-explained data conversion circuit is input to the switching elements SW 11  and SW 12  in the column SL 1  of the first stage, and the switching elements SW 11  and SW 12  are supplied with the clock pulses φ 1  and /φ 1 . The relationship between the clock pulses φ 1  and /φ 1  is illustrated in FIG. 4, where the clock pulses φ 1  and /φ 1  are represented by solid lines and broken lines, respectively.  
         [0045]    With this configuration, when one of the switching elements SW 11  and SW 12  is turned ON, and the other one of the switching elements SW 11  and SW 12  is turned OFF, and vice versa. The clock signals alternately turn ON each of the switching elements SW 11  and SW  12  with turning OFF the other of the switching elements SW 11  and SW  12 . This operation is repeated. Consequently, among data {circle over ( 1 )}, {circle over ( 2 )}, {circle over ( 3 )}, {circle over ( 4 )}, {circle over ( 5 )}, {circle over ( 6 )}, {circle over ( 7 )}, {circle over ( 8 )}, {circle over ( 9 )}, {circle over ( 10 )} . . . supplied serially from the data conversion circuit, data {circle over ( 1 )} and data {circle over ( 2 )} are transferred to the switching element column SL 2  of the second stage via the switching element SW 11  and the switching element SW 12 , respectively, and data {circle over ( 3 )} and data {circle over ( 4 )} are transferred to the switching element column SL 2  of the second stage via the switching element SW 11  and the switching element SW 12 , respectively.  
         [0046]    The switching element column SL 2  of the second stage is composed of four switching elements SW 21 , SW 22 , SW 23  and SW 24 , which are supplied with the clock pulses φ 2  and /φ 2 . The relationship between the clock pulses φ 2  and /φ 2  is illustrated in FIG. 4, where the clock pulses φ 2  and /φ 2  are represented by solid lines and broken lines, respectively. The frequency of the clock pulses φ 2  and /φ 2  is half that of the clock pulses φ 1  and /φ 1 .  
         [0047]    With this configuration, when the switching elements SW 21  and SW 23  among the switching elements SW 21 , SW 22 , SW 23  and SW 24  are turned ON, and the switching elements SW 22  and SW 24  are turned OFF, and vice versa. The clock signals alternately turns ON each of one pair of the switching elements SW 21  and SW  23  and another pair of the switching elements SW 22  and SW  24  with turning OFF the other of the two pairs. This operation is repeated. As a result, the data {circle over ( 1 )} transferred via the switching element SW 11  is transferred to the switching element column SL 3  of the third stage via the switching element SW 21 , the data {circle over (2)} transferred via the switching element SW 12  is transferred to the switching element column SL 3  of the third stage via the switching element SW 23 , data {circle over ( 3 )} transferred via the switching element SW 11  is transferred to the switching element column SL 3  of the third stage via the switching element SW 22 .  
         [0048]    Such operations are successively repeated, and finally the data transferred via respective ones of the elements SW 501 -SW 532  in the column SL 5  of the fifth stage are stored in the store memory section SM for subsequent processing.  
         [0049]    Data Conversion Circuit  
         [0050]    Apparent from operation of the circuit of the distribution port, the arrangement of the data stored in the store memory section SM is different from the arrangement of the data input into the distribution port as indicated in FIG. 1 at the extreme right of which the arrangement of some of the data stored in the store section SM is indicated.  
         [0051]    In view of the above, in this embodiment, the data conversion circuit is configured so as to change the arrangement of the data transferred from the external system in advance before inputting the data into the distribution ports, taking into account the subsequent conversion of the arrangement of the data by the distribution ports. In other words, a so-called inverse conversion is performed by the data conversion circuit.  
         [0052]    As shown in FIG. 6, data supplied via a data bus from the external system are input into a latch memory section, and then are input into the store memory section SM. The data from respective memory elements of the store memory SM are input into the data conversion circuit, which has the same configuration as a configuration obtained by reversing the arrangement of the input and output sides of the distribution port shown in FIG. 1. That is to say, the data conversion circuit is comprised of the switching element column SL 1  of the first stage, the switching element column SL 2  of the second stage, the switching element column SL 3  of the third stage, the switching element column SL 4  of the fourth stage, and the switching element column SL 5  of the fifth stage.  
         [0053]    The switching element column SL 1  of the first stage is composed of thirty-two (2 5 ) switching elements, and corresponds to the switching element column SL 5  of the fifth stage of the distribution port. The switching element column SL 2  of the second stage is composed of sixteen (2 4 ) switching elements, and corresponds to the switching element column SL 4  of the fourth stage of the distribution port. The switching element column SL 3  of the third stage is composed of eight (2 3 ) switching elements, and corresponds to the switching element column SL 3  of the third stage of the distribution port. The switching element column SL 4  of the fourth stage is composed of four (2 2 ) switching elements, and corresponds to the switching element column SL 2  of the second stage of the distribution port. The switching element column SL 5  of the fifth stage is composed of two (2 1 ) switching elements, and corresponds to the switching element column SL 1  of the first stage of the distribution port. A pair of switching elements SW in each of the switching element columns SLi (i=1-5) are connected to one of switching elements of the switching element column of the next succeeding stage.  
         [0054]    Since the above-explained data conversion circuit has a configuration that is the mirror image of the distribution port, irrespective of how data are converted in the distribution port, the data supplied serially from the external system can be arranged in parallel with retaining the original arrangement of the data. Further, such a data conversion circuit has the same configuration as that of the distribution port, and is supplied with the same clock signals as those supplied to the distribution port, and consequently, problems such as increasing of time constants are not caused.  
         [0055]    Embodiment 2  
         [0056]    [0056]FIG. 7 illustrates another embodiment of a distribution port employed in the liquid crystal display device in accordance with the present invention,. and is a circuit diagram similar to that of FIG. 1. The configuration of FIG. 7 differs from that of FIG. 1 in that data each comprising six bits representing color information for one pixel are input to the distribution port, and the data are grouped and stored in the store memory section. The store memory section is formed by memory blocks into which six-bit data supplied via the respective switching elements of the switching element column of the fifth stage are successively stored. Pulses φA-φF and /φA-/φF driving the store memory section are illustrated in FIG. 8, and they are synchronized with ON-Off operation of the switching element column SL 5  of the fifth stage.  
         [0057]    Initially, first-bit data are input successively into the input stage of the distribution port, are transferred to the switching element column SL 5  of the fifth stage as explained in connection with Embodiment 1, and then are stored in the store memory section. Then second-bit data are input successively into the input stage of the distribution port, are transferred to the switching element column SL 5  of the fifth stage, and then are stored in the store memory section as in the case of the first-bit data.  
         [0058]    In this case, all the bit data for representing color information of one pixel take the same route to the switching element column SL 5  in FIG. 7, and therefore they are transferred to the corresponding memory positions of the store memory section via the corresponding switching elements SW. Consequently, six bits representing color information for each pixel are grouped together and then are stored in the store memory section, and this provides an advantage of facilitating subsequent processing of the data.  
         [0059]    Further, even in a case where the store memory section is formed of shift registers, for example, since the frequency of pulses for driving the shift registers is relatively lower, no problems associated with employment of the shift registers occur, such as a problem caused by high-speed operation of the shift registers.  
         [0060]    As explained above, the display device in accordance with the present invention makes possible high-speed data transmission within the video signal drive circuit.  
         [0061]    While the above-explained embodiments are applicable to all types of liquid crystal display devices, it is very effective for the present invention to be applied to a liquid crystal display device of the type in which the video signal drive circuit He is fabricated directly on the transparent substrate SUB 1 , for example, (in this case, usually the vertical scanning drive circuit V is also fabricated), and transistors constituting the shift register within the drive circuits are fabricated by using polysilicon (p-Si) semiconductor layers simultaneously with thin film transistors TFT disposed within the pixel areas. Although driving capability of those transistors are not very great at the present time, the present invention is capable of a large-sized and high-definition display device.  
         [0062]    The above embodiments have been explained in connection with the liquid crystal display devices, but the present invention is not limited to those, and it is needless to say that the present invention is applicable to other display devices such as an electroluminescent display device, because the basic configuration of the video signal drive circuit for such display devices is the same as that for the liquid crystal display devices.  
         [0063]    As is apparent from the above explanation, the display device in accordance with the present invention makes possible the high-speed data transmission within its video signal drive circuit.