Abstract:
In a display panel drive circuit, discharge is caused by applying a positive display pulse to a common electrode. The discharge is inhibited by applying a positive control voltage to the individual electrodes. Applying an opposite polarity pulse to the common electrode erases any charge remaining after an unstable discharge and effectively prevents unstable discharges.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a drive circuit for a display panel, disposed with a common electrode and an individual electrode in each of a plurality of display cells arranged in a matrix configuration, for controlling gas discharges in each display cell by applying display pulses to a common electrode to perform display operations and by individually applying control voltages to individual electrodes to control the discharge in each display cell. 
     2. Description of the Related Art 
     Heretofore, display panels, such as plasma displays, are known for performing display operations by controlling the gas discharge of each display cell. In such display panels, it is necessary to continually maintain a favorable state for discharge of the stored charge in order to perform normal discharge. Accordingly, initialization is performed periodically in all display cells, such as by erasing the stored charge that causes discharge. 
     Japanese Patent Laid-Open Publication Nos. Hei 10-143106(1998/05/29), 8-278766(1996/10/22), 7-40927(1995/06/02), 9-325736(1997/12/16), and 8-212930(1996/08/20) disclose this sort of initialization. 
     Although various types of initialization methods have been proposed in this manner, if the discharge conditions change, for example, a different method must be adopted. 
     RELATED APPLICATION 
     The present applicant has proposed an invention concerning a display panel with a novel drive method in an international application (application number PCT/JP98/01444) of the Patent Cooperation Treaty. This display panel comprises individual display cell electrodes and a common electrode. The individual electrodes are driven individually for every display cell and the common electrodes are driven together for a plurality of display cells. The discharge is then controlled for each display cell so as to control the overall display by applying a positive display pulse to the common electrode and by individually controlling the application of a positive control voltage to the individual electrodes. 
     The drive for the common electrode in this display panel employs display pulses having voltage that changes in two levels. One two-level display pulse performs discharge for storing charge and discharge for erasing charge. Theoretically, therefore, erasure of the charge is performed automatically, even though display discharge is repeated. However, storage of the charge due to insufficient voltage application when the power is turned on or storage of charge due to repetition of discharges does occur. In order to eliminate this, a positive pulse (initialization pulse) is supplied, once per frame, to all individual electrodes so as to invert the display cell charges to perform initialization. 
     This sort of initialization can solve the problem of inappropriate charge storage and maintain normal discharge. However, this method requires that a-sufficiently large positive voltage be applied to the individual electrodes. The voltage application onto the individual electrodes drives a control element (ex. transistor) corresponding to each display cell. Thus, the entire drive circuitry for the individual electrodes must be adapted for high voltages. Furthermore, the insertion of initialization pulses raises the frequency for driving the individual electrodes, resulting in the problem that the power consumption of the drive circuitry is increased. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to solve the above-mentioned problems by providing display panel drive circuit capable of driving the individual electrodes at low voltages and low frequencies. 
     The display panel drive circuit of this invention applies a reset pulse having a polarity opposite to that of the display pulse in gaps between applied display pulses to the common electrode. Thus, the control of the individual electrode is unaffected even if reset pulses are inserted. Accordingly, it is sufficient to have one on-off operation of the individual electrode in one frame necessary to determine when to halt the discharge. Therefore, the individual electrode can be driven at a low frequency so as to achieve a reduction in power consumption of this drive circuit. Furthermore, a high voltage initialization pulse (reset pulse) for the individual electrode is unnecessary, making it possible to eliminate the need for handling high voltages in the drive circuit for the individual electrode. 
     The above-mentioned display pulse is formed from voltages of two levels and the voltage rises and lowers in a stepwise fashion. It is preferable for the absolute voltage value of the above-mentioned reset pulse to be greater than or equal to the first level voltage value of the display pulse. With this type of display pulse, one display pulse can cause two discharges, one for storing charge and another for erasing the stored charge. When a stable discharge is performed, the insertion of the reset pulse becomes unnecessary. 
     It may also be preferable to apply the above-mentioned reset pulse either once for one frame or once for a plurality of frames. This enables frames to be created without inserting the reset pulse and results in reduced processing requirements. 
     It may also be preferable to provide a sequence memory for storing a plurality of sequences for driving the above-mentioned common electrode and individual electrode, and to control the drive of the common electrode on the basis of the sequence data read out from the sequence memory. This enables repetitive output of the same display pulse to be easily performed. 
     Furthermore, it may be preferable to provide a loop memory for storing the sequence readout order from the above-mentioned sequence memory, and to read out the sequence data from the sequence memory on the basis of the data read out from the loop memory. This raises the degree of freedom of sequence use and permits various types of drive with a small storage capacity. In particular, the operation of the sequence of insertion of the reset pulse through the loop memory can be performed as desired. 
     This invention is composed as described above and achieves the effects given below. 
     (i) Control of the individual electrode remains unaffected, even if a reset pulse is inserted, because the reset pulse having a polarity opposite to that of the display pulse is applied in the interval between applied display pulses to the common electrode. Accordingly, one on-off operation of the individual electrode in one frame is sufficient to determine when to halt the discharge. Therefore, it is sufficient to drive the individual electrode at an extremely low frequency so as to achieve a reduction in power consumption of this drive circuit. Furthermore, a high voltage initialization pulse for the individual electrode is unnecessary, making it possible to lower the load in the drive circuit for the individual electrode and sufficiently lower the voltages to be handled. 
     (ii) The above-mentioned display pulse is formed from voltages of two levels and the voltage rises and lowers in a stepwise fashion. It may be preferable for the absolute voltage value of the above-mentioned reset pulse to be greater than or equal to the first level voltage value of the display pulse. With this sort of display pulse, one display pulse can cause two discharges, one for storing charge and another for erasing the stored charge. When a stable discharge is performed, the insertion of the reset pulse becomes unnecessary. 
     (iii) It may be preferable to apply the above-mentioned reset pulse once for one frame or once for a plurality of frames. This enables frames to be created without inserting the reset pulse and results in lighter processing requirements. 
     (iv) It may be preferable to provide a sequence memory for storing a plurality of sequences for driving the above-mentioned common electrode and individual electrode, and to control the drive of the common electrode on the basis of the sequence data read out from the sequence memory. This enables the drive to repetitively output the same display pulse to be easily performed. 
     (v) It may be preferable to provide a loop memory for storing the sequence readout order from the above-mentioned sequence memory, and to read out the sequence data from the sequence memory on the basis of the data read out from the loop memory. This raises the degree of freedom of sequence use and permits various types of drive with a small storage capacity. In particular, the operation of the sequence of insertion of the reset pulse can be easily performed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the structure of a display cell to be driven by a drive circuit for display panel of the present invention. 
     FIG. 2 illustrates the structure of the drive circuit for display panel concerning an embodiment of the present invention. 
     FIG. 3 shows a relationship between drive and discharge waveform in a stable state. 
     FIG. 4 shows a state of discharge in the stable state. 
     FIG. 5 shows a relationship between drive and discharge waveform in an unstable state. 
     FIG. 6 shows a state of discharge in the un stable state. 
     FIG. 7 shows the structure of the display control circuit. 
     FIG. 8 show s the structure of a sequencer. 
     FIG. 9 shows the operation of the sequencer. 
     FIG. 10 shows the operation of an insertion sequence by the sequencer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 1 shows one display cell (one color) in a display panel of embodiment 1. On a rear of the display panel is provided a back glass plate  10 . On the inner surface of a recess  12  formed in the back glass plate  10  is formed a fluorescent layer  14 . On a back side (side facing the back glass plate  10 ) of a front glass plate  20  are disposed a pair of transparent electrodes  24   a  and  24   b . So as to cover them, a dielectric layer  26  is formed, and a protective layer  28  is further formed. Therefore, the protective layer  28 , which is usually formed from MgO, faces the recess  12 . A positive display pulse is applied to a common-electrode and an individual electrode is maintained at a sufficiently low voltage (for example 0 V) so that a discharge occurs at a part close to the protective layer within the recess  12 . A positive voltage is applied to the individual electrode so that the voltage value between the individual electrode and common electrode drops and the discharge ceases to occur. 
     FIG. 2 shows a drive circuit for the common electrode. For example, a 160 V power supply Vs is grounded via transistors Q 1  and Q 2 . The gates of transistors Q 1  and Q 2  are connected to a first controller  30 . The on-off operations of transistors Q 1  and Q 2  are controlled by control signals from the first controller  30 . By turning on transistor Q 1  and turning off transistor Q 2 , a voltage Vs is output to a subsequent stage from a point (Vs output point) between transistors Q 1  and Q 2 . The circuit of transistors Q 1  and Q 2 , a circuit at the power supply side, is formed on a circuit board that is separate from the subsequent circuit denoted by the broken line in the figure and has a separate ground. 
     To the point between transistors Q 1  and Q 2  is connected a capacitor C 1 , which has its other end connected to ground. To the Vs output point are connected a transistor Q 3  and a transistor Q 4 , which has its other end connected to ground. To the gates of transistors Q 3  and Q 4  is connected a second controller  32 , which controls the on-off operations of the transistors Q 3  and Q 4 . Furthermore, to the Vs output point are connected, via a diode D 1 , a transistor Q 5  and a transistor Q 6 , which has its other end connected to ground. To the gates of transistors Q 5  and Q 6  is connected a third controller  34 , which controls the on-off operations of the transistors Q 5  and Q 6 . 
     With transistor Q 1  on and transistor Q 2  off, transistors Q 3 , Q 4 , Q 5 , and Q 6  turn on and off in the following manner so that a two-level display pulse shown in FIG. 3 is supplied to the common electrode. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Q3 
                 Q4 
                 Q5 
                 Q6 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 (1) At 0 V 
                 OFF 
                 ON 
                 OFF 
                 ON 
               
               
                   
                 (2) At first-level pulse rise 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 (3) 
                 OFF 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 (4) At second-level pulse rise 
                 OFF 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 (5) 
                 ON 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 (6) At second-level pulse fall 
                 OFF 
                 OFF 
                 ON 
                 ON 
               
               
                   
                 (7) 
                 OFF 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 (8) At first-level pulse fall 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 (9) 
                 OFF 
                 ON 
                 OFF 
                 ON 
               
               
                   
                   
               
             
          
         
       
     
     Namely, by turning off transistor Q 5  and turning on transistor Q 6 , the potential of the common electrode is at ground (0 V), and by turning on transistor Q 5  and turning off transistor Q 6 , the potential of the common electrode reaches Vs. When Q 4  is then turned on, a charge equivalent to Vs is stored into a capacitor C 2 . Then, by turning off transistor Q 4  and turning on Q 3 , the transistor Q 3  side of the capacitor C 2  is Vs. Since the capacitor C 2  is charged with Vs, the voltage at the common electrode becomes 2Vs. In this manner, voltages of two levels, Vs and 2Vs, can be generated. The voltage of the common electrode returns to Vs by turning off transistor Q 3  off and turning on Q 4 , and the voltage of the supply electrode returns to 0 with transistor Q 5  off and Q 6  on so that a two-level display pulse can be formed. 
     Next, with Q 5  off and Q 6  on, transistor Q 1  is turned off and Q 2  is turned on. This causes the potential of the upper side of capacitor C 1  to be fixed at the ground potential of 0 V on the power supply side. The ground on the lower side of capacitor C 1  serves as the ground of the drive circuit and is not necessarily 0 V. This ground becomes −Vs, and the potential at the common electrode, which is connected to ground via transistor Q 6 , becomes −Vs. This causes the reset pulse in FIG. 3 to be applied to the common electrode. 
     This reset pulse has a polarity opposite to that of the display pulse and a magnitude of Vs that is identical to the first level pulse. This Vs is, for example, 160 V (approximately 150 V to 200 V) and is a voltage at which discharge occurs when a wall charge remains. Therefore, the application of this reset pulse causes discharge to occur when the wall charge remains so that the wall charge erases. 
     FIGS. 3 to  6  illustrate the relationship of the applied voltages to the common electrode and individual electrode and the discharge. FIGS. 3 and 4 show the states of normal discharges and FIGS. 5 and 6 show the states during unstable discharges with wall charges remaining. When the unstable discharge occurs and wall charges remain as shown, the reset pulse causes discharge to occur, and thereby erase the wall charges. 
     As described above, it is preferable for the erase pulse to have a voltage near the first level of the display pulse so that a reliable erasing discharge is performed when wall charges remain. Furthermore, if the voltages are identical, the drive circuit can be simplified. 
     In the case where wall charges remain after discharge, it is necessary for the reset pulse to have a duration during which discharge can be reliably performed. In order to reliably perform discharge, a duration of about 5 μsec is necessary in this embodiment. This duration is affected by the size of the display cell and so forth. The time for this discharge is identical to that for the discharge by the display pulse and it is preferable to insert a reset pulse with a time of about 5 μsec after the elapse of about 15 μsec from the fall of the display pulse to 0 V (GND). If the display cell size changes, the discharge time changes so that both the above-mentioned values of 15 μsec and 5 μsec change. It is preferable for the time from the end of the display pulse to the start of the reset pulse and the duration of the reset pulse to have a relationship of around 3:1. It should be noted that this relationship applies to the case where minimum times are used for both values, and setting sufficient times for both values poses no problem. 
     Embodiment 2 
     FIG. 7 shows the structure of a display control circuit for controlling the drive of the individual electrode and common electrode. Image data, which is RGB digital data for every pixel, is input by a multiplier  40 . In the display panel, one pixel comprises three RGB display cells. One RGB data item at a time causes the discharge of the corresponding display cell to be controlled. The description below is based on the case where a single luminance data value is input. 
     Correction data is supplied to the multiplier  40  from a correction memory  42 , and correction is performed by multiplication of the image data and correction data. The correction memory  42  stores correction data for every display cell. The correction data corresponding to the image data is read from the correction memory  42  and multiplied on the basis of the image position data that is input to yield error-corrected image data for every cell. This allows variations in luminance of the display cells to be corrected. It should be noted that the corrections need not necessarily be performed by multiplication but may be performed by the addition of differential data. In this embodiment, the image data has 9 bits and the correction data has 8 bits. With a “ 1 ” added to the most significant bit of the correction data for a total of 9 bits, 9×9 multiplication is performed and the most significant 9 bits are output from the multiplier  40  as the calculation result. 
     The corrected image data, which is the output of the multiplier  40 , is stored in an image memory  44 . The image data for at least one frame is stored in the image memory  44 . Usually, the image data for one frame at a time is stored for R, G, and B, respectively. 
     In the meantime, a sequencer  50  generates and outputs a drive signal for common electrode drive after detecting the start of one frame with a vertical synchronizing signal. The display pulse is repeated in one frame period and supplied to the common electrode. The sequencer  50  then supplies a pulse signal, which is synchronized to the display pulse, to a sequence counter  52 . Thus, a count value in the sequence counter  52  is determined by the number of display pulse outputs. The luminance of the display cell corresponds to the number of discharges in one frame. Since the number of discharges corresponds to the number of display pulses, the count value becomes the assumed luminance (assumed luminance data) when light is emitted due to the display pulses. 
     The output of the sequence counter  52  is supplied to a lookup table (LUT)  54 . A predetermined conversion is performed according to this lookup table  54  and the converted assumed luminance data is input by a comparator  56 . To another input terminal of this comparator  56  is input the image data from the image memory  44 . A one-bit signal is then obtained from the comparator  56  in order to control the supply of the control voltage to the individual electrode of the display cell. 
     One data item is output from the lookup table  54  for each display cell in the display of one frame display. For a color display, there are three types of RGB data for one display unit (pixel: three types (RGB) of data for one pixel) so the image data for one frame (three types of RGB data for three frame memories) is output in parallel from the image memory  44 . The comparator  56  is provided for each color, and at each comparator  56 , the image data for each display cell and the assumed luminance data from the lookup table  54  are compared. The comparison results are individually output from the comparators  56  one by one as display data of each display cell. Controlling the voltage applied to each individual electrode of each display cell by one frame of pixels×3 (RGB) items of display data controls the light emission in each display cell so that an image is displayed on the display panel. 
     For example, if the image data has  256  gradations and the number of pulses to be output from the sequencer  50  is  256  pulses, it is sufficient to cause the display cell to emit light by performing the discharge according to the display pulses until the output value of the sequence counter  52  is the same as the gradations of the image data. When the values that are input are identical at the comparator  56 , it is sufficient to change the value of the display data and at this time to control the control voltage to be applied to the individual electrode so that the light emission ceases. In this embodiment, an arbitrary conversion can be performed for the assumed luminance data by means of the contents of the lookup table  54 . Therefore, the light emission time can be set as desired in accordance with the gradations of the image data. 
     In this embodiment, the number of display pulse outputs in one frame is 765 pulses. If the lookup table  54  is set so that 0, 3, 6, 9 , . . . , 765 are output with respect to inputs 0, 1, 2, 3 , . . . , 255, one gradation corresponds to three discharges and both the input and output have a linear relationship. 
     In the meantime, if the amount of the increment or decrement is varied, such as if the value of the lookup table  54  is initially incremented by 1 and subsequently incremented by 5, the amount of light emission can be arbitrarily set according to the change in gradation. Thus, gamma correction can be achieved by settings of the content of the lookup table  54 . Furthermore, through each of the RGB colors, the tint and so forth can be set by rewriting the contents of the lookup table  54 . 
     The operation of the sequencer  50  will next be described. The sequencer  50  internally contains a sequence bit register  50   a , which is a sequence memory for storing a drive sequence internally, and a loop count register  50   b , which is a loop memory for controlling a readout of sequences. Their structures are shown in FIG.  8 . 
     The sequence bit register  50   a  stores the sequence (or pattern) for the drive signal and its period. Sequence bits B 0  to B 23  of each address A 0  to A 63  indicate values for output, and these values are, for example, commands for the drive voltage for the common electrode. Counter bits B 0  to B 7  indicate the output periods of the sequence bits. The counter bits can, for example be, the number of system clock pulse. 
     The loop count register  50   b  stores the address of the sequence bit register and the number of sequence outputs. Sequence address bits B 0  to B 4  of each address A 0  to A 63  indicate the address of the sequence bit register  50   a , and the sequence output is performed according to this address setting. Furthermore, the counter bits B 0  to B 7  indicate the number of loops of the sequence to be performed at the specified address. 
     The operation at the sequencer  50  will be described here with reference to FIG.  9 . The sequencer  50  first reads (S 1 ) the top address A 0  of the loop count register  50   b . Next, the sequence bit of the sequence bit register  50   a  at the address specified by the sequence address of the loop count register is output for the period specified by the counter bit (S 2 ). When the output of S 2  terminates, the address of the sequence bit register  50   a  is incremented by 1 (A 1  follows A 0 ) (S 3 ). It is then judged whether the count value of the sequence bit register  50   a  has been set to 0 (S 4 ). 
     If the count value of the sequence register  50   a  is a specific value (in this case 0), the setting is made to signify the termination of the successive output of the sequence in the sequence register  50   a.    
     If the result of the judgment in S 4  is NO, the sequence bit of the next address (address in the previous process incremented by 1) of the sequence bit register  50   a  is output for the count period (S 5 ). When this is terminated, the operation returns to S 3 , which increments the sequence bit register  50   a  by one. The output of the sequence stored in the sequence bit register  50   a  is repeated, and the output of the sequence in the sequence bit register  50   a  is repeated until the count value of the sequence bit register  50   a  reaches 0. A count value other than 0 signifies some type of output is to be performed while a count value of 0 signifies the output is not to be performed or that the sequence is to be terminated. 
     Then, when the count value of the sequence bit register  50   a  becomes 0 and the result of S 4  is YES, the operation returns to the loop count register  50   b  where it is judged whether the specified number of loops of the count has been performed (S 6 ). If the specified number of loops has not been performed, the operation returns to S 2  where the sequence of the sequence bit register of the address specified by the loop count register  50   b  at the time is output. 
     In this manner, if the process specified by one address of the loop count register  50   b  terminates (termination of the specified number of loops of the count of the loop count register  50   b ) and the result of S 6  is YES, the address of the loop count register  50   b  is incremented by 1 (S 7 ). It is then judged whether the count value of the loop count register  50   b  is 0 (S 8 ). 
     If the count value is 0, this signifies that the corresponding sequence is not to be performed. Therefore, not performing the output signifies the termination of the sequence so in this case the sequence is terminated. On the other hand, if the count value of the loop count register  50   b  is not 0, the operation returns to S 2  and the sequence bit of the sequence bit register of the address specified by the loop count register  50   b  is output for the count period. 
     In this manner, the signal for controlling the output of the common pulse to the common electrode is output from the sequencer  50  so as to operate the drive circuit shown in FIG.  1 . Controlling the voltage of the individual electrode on the basis of the display data in the period in which the output of this common pulse is performed enables the light emission of each display cell to be controlled. 
     In addition to the synchronization sequence for synchronizing to the vertical synchronizing signal to be executed every time in each frame so that the display pulse is applied onto the common electrode as a sequence, the sequencer  50  of this embodiment also contains an insertion sequence for inserting the reset pulse only into a predetermined frame. The execution of this insertion sequence is identical to the execution of the above-mentioned sequence, except that the output differs. 
     This insertion sequence is inserted before the actual display (discharge due to display pulses) begins. This is described with reference to FIG.  10 . It is first judged whether the vertical synchronizing signal has arrived (S 11 ). Although this vertical synchronizing signal indicates the termination of the vertical retrace period, it may also be judged as the start or middle of the vertical retrace period. 
     The vertical synchronizing signal is counted (S 12 ) when it arrives. This is then compared with the value stored in the register (S 13 ). For example, if this sequence is to be performed every three frames, a “3” is stored in the register. Then, if the count is greater than or equal to the stored value of the register, the insertion sequence is performed (S 14 ). 
     If the operation of the insertion sequence terminates or if the count value in S 13  has not reached the value stored in the register, the synchronization sequence is performed (Sl 5 ). As a result, according to the value stored in the register, the sequence for the output of the reset pulse stored in the sequence bit register is read out at every predetermined frame, and the reset pulse is inserted. It is preferable to execute this insertion sequence prior to the start of the synchronization sequence that is to be performed each time. 
     Changing the stored value in the register enables the timing for the execution of the insertion sequence to be arbitrarily set, and enables the insertion sequence to be executed as desired in the sequencer  50 . 
     While there has been described what is at present considered to be a preferred embodiment of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.