Patent Publication Number: US-11037518-B2

Title: Display driver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The Present application claims priority from Japanese application JP 2015-011953 filed on Jan. 26, 2015, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     The invention relates to a display driver operable to drive a display panel, and especially relates to a technique useful in application to e.g., a liquid crystal display driver. 
     Examples of an interface method which allows a display driver to receive display data from a host device include: a synchronous interface method by which a display driver receives display data in synchronization with a display timing; and an asynchronous interface method by which a display driver receives display data in asynchronization with a display timing. In the former method, display data are input as pixel data streams in synchronization with a display timing, whereas in the latter method, data to write in a frame buffer memory are input in asynchronization with a display timing, and data written in the frame buffer memory are read out in synchronization with the display timing. The former synchronous interface method is used for input of moving-image data and the like, and the latter asynchronous interface method is used for input of still image data, menu operation data and the like. Whether to handle input of display data by synchronous interface or asynchronous interface will be appropriately changed depending on the display control state of a host device. Especially, synchronous interface does not require that display data be accumulated in a frame buffer memory, but in case that the synchronous interface has been switched to the asynchronous interface, asynchronously input display data must be written once in a frame buffer and then, be read out from there in synchronization with a display timing. Therefore, it takes a time until the display of asynchronously input display data are enabled after switching of the input of display data from the synchronous interface to the asynchronous interface, during which image display is disturbed. To even out such disturbance, the action of displaying in all white or black during only a period of one to several frames is performed. For instance, in the case of making an attempt to display an operation menu in display of a moving picture, a situation as described above develops. 
     A technique for avoiding the disturbance of display as described above and further, the disturbance of display attributed to white or black display insertion has been already disclosed. According to such a technique, display data input through a synchronous interface are sequentially stored in a frame buffer memory in parallel with the display thereof; and such disturbance of displays can be suppressed by switching the display action to the one which uses display data stored in the frame buffer memory immediately on the switching of the synchronous interface to asynchronous one. 
     Examples of the above technique include the one disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2014-89314. 
     SUMMARY 
     Display drivers are described herein. In one example, a display driver is provided that includes an external interface circuit and a control circuit. The external interface circuit has input modes as interface modes to input display data, through which a display panel is driven based on display data input. The control circuit is configured to keep scan driving of the display panel stopped during a predetermined period until the driving of the display panel by display data input in the interface mode after switching is enabled in case that the interface mode of the external interface circuit is switched in the middle of driving the display panel based on the input display data. 
     In another example, a display driver is provided that includes an external interface circuit, a frame buffer memory, a control circuit and a drive circuit. The external interface circuit has a first interface mode to input display data as streams of pixel data in synchronization with a display timing, and a second interface mode to input display data for write on the frame buffer memory in asynchronization with the display timing. The drive circuit is configured to output signals for scanning pixels of a display panel in synchronization with the display timing, and to output pixel drive signals for display driving the scanned pixels according to display data. The control circuit is configured to stop the pixel scan on the display panel during a predetermined period until the output of the pixel drive signals is enabled based on display data input in the second interface mode in case that display data input is switched from the first interface mode to the second interface mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing, by example, the schematic structure of a liquid crystal display driver according to one embodiment of the invention; 
         FIG. 2  is a circuit diagram showing, by example, the schematic structure of a liquid crystal display panel; 
         FIG. 3  is a timing chart showing, by example, the action timing when switching the interface mode of display data during display; and 
         FIG. 4  is a block diagram showing an example of a portable communication terminal device to which the liquid crystal display driver of  FIG. 1  is applied. 
     
    
    
     DETAILED DESCRIPTION 
     The inventor has made a consideration about the problems of the action of accumulating display data input through the synchronous interface in a frame buffer memory in parallel with the display thereof. 
     The first problem is that even if display data input through the synchronous interface are accumulated in the frame buffer memory in parallel with the display thereof, most of the data end up being unused and overwritten, resulting in a large wasteful power consumption. 
     The second problem is the one accompanying the scale-up of display data. Specifically, as the rise in the resolution of a display panel provided on a personal digital assistance such as a smart phone advances, display data are decreased in quantity by performing a scale-up process of data by the calculation of data interpolation or the like instead of providing RAM for holding image data, such as a frame buffer memory, which accommodates data of one display frame, which also enables the shortening of the time for writing display data into a frame buffer memory. However, the scale-up process is arranged to target display data input through the asynchronous interface and therefore, even if an attempt is made to sequentially store display data input through the synchronous interface in a frame buffer memory in parallel with display thereof as described above, the process of previously accumulating display data of a necessary size in a frame buffer cannot be performed adequately because of the frame buffer memory size smaller than a display frame size, and the disturbance of display cannot be prevented. 
     The modes for input of display data by a display driver are roughly classified into a synchronous interface and an asynchronous interface. Ina display driver arranged to be able to select interface specifications of MIPI (Mobile Industry Processor Interface), MDDI (Mobile Display Digital Interface), Bus Interface, etc., the problem concerning the disturbance of display as described above can arise not only when switching from the synchronous interface to the asynchronous interface, but also when switching between other input modes. For instance, in case that the switching of input mode changes the number of input lanes in MIPI, display data are still in danger of suffering the disturbance of display in a new input mode until the internal state transition caused by the change is stabilized. 
     A benefit of the invention provides a display driver arranged so that the disturbance of display owing to the switching of the input mode of display data can be substantially eliminated. 
     The above and other benefits of the invention and the novel features thereof will become apparent from the description hereof and the accompanying diagrams. 
     Of the embodiments herein disclosed, the representative embodiment will be briefly outlined below. Now, it is noted that what is described in paired round brackets here is just an example for easier understanding. 
     Stopping the Scan Driving of a Display Panel Following the Interface Mode Switching 
     The display driver ( 1 ) according to one example includes an external interface circuit ( 20 ) having, as interface modes to input display data, input modes (i.e., a video mode and a command mode). The display driver drives a display panel ( 2 ) based on display data input through the external interface circuit. The display driver further includes a control circuit ( 9 ) which keeps the scan driving of the display panel stopped during a predetermined period until the driving of the display panel by display data input in the interface mode after switching is enabled in case that the interface mode of the external interface circuit is switched in the middle of driving the display panel based on the input display data. 
     According to the above arrangement, with the scan driving of the display panel temporarily stopped, all of the pixels of the display panel can hold the information of signals driven based on the immediately preceding display data without losing it. Therefore, in the event of the switching of the interface mode in the middle of driving the display panel, the scan driving of the display panel is stopped during a predetermined period until the driving of the display panel by display data input in the interface mode after switching is enabled, whereby the occurrence of display disturbance can be suppressed until the internal state transition owing to the change in the interface mode is stabilized. 
     Stop During a Period of one or more Display Frames 
     In the display driver, the predetermined period may be made e.g., a period of one or more display frames, of which the unit is a one-display frame period. 
     Taking into account that the display control and the write of display data into the frame buffer memory are performed in units of the display frame, the above arrangement significantly simplifies the control for temporarily stopping the scan driving of the display panel. 
     Register to set Predetermined Periods on 
     In the display driver, the control circuit has a register ( 33 ) on which the stop-period-setting data (MP 1 , MP 0 ) specifying a period of one or more display frames as e.g., the predetermined period are set overwritably. 
     According to the above arrangement, the scan-driving-stop period can be optimized according to the change in the period until the internal state transition owing to the interface mode change is stabilized, which depends on a display data interface speed and an internal action speed. 
     Stop-Period-Setting Data Supplied from Outside 
     In the display driver, the stop-period-setting data may be supplied e.g., from outside the display driver. 
     According to the above arrangement, it becomes easier to optimize the scan-driving-stop period according to control from the outside. 
     Interface Mode Arranged to be in Synchronization/Asynchronization with a Display Timing 
     In the display driver, the interface modes include: e.g., a first interface mode (video mode) to input display data in synchronization with a display timing; and a second interface mode (command mode) to input display data in asynchronization with the display timing. In this case, the control circuit performs control for stopping the scan driving of the display panel during a predetermined period until the driving of the display panel by display data input in the second interface mode is enabled in case that display data input is switched from the first interface mode to the second interface mode. 
     According to the above arrangement, in case that the interface mode is switched from the first interface mode to the second interface mode, the scan driving of the display panel is stopped during the predetermined period. Therefore, even in the case of performing the process of storing display data input in the second interface mode in the frame buffer memory for display in synchronization with a display timing during the period, no disturbance of display is caused because a display state just before the switching is maintained. Further, it is unnecessary to perform the operation of previously accumulating, in the frame buffer, display data input in the first interface mode and in parallel with display thereof. 
     Frame Buffer Memory and Scale-Up Circuit 
     The display driver has e.g., a frame buffer memory ( 22 ) for storing display data input in the second interface mode, and a scale-up circuit ( 23 ) for scaling up image data so as to enlarge the number of display pixels according to image data stored in the frame buffer memory. 
     According to the above arrangement, even if the memory capacity of the frame buffer memory is insufficient and smaller than the data size of one-frame display data supplied in the first interface mode, and image data stored in the frame buffer memory needs to be scaled up, nothing interferes with the switching of the interface mode, and the disturbance of display can be prevented as described above. 
     MIPI 
     In the display driver, the interface circuit is e.g., MIPI circuit ( 20 ); the first interface mode is an action mode (video mode) compliant with MIPI video mode; and the second interface mode is an action mode (command mode) compliant with MIPI command mode. In MIPI video mode, display data are input as streams of pixel data in synchronization with a display timing. In MIPI command mode, display data to write into the frame buffer memory are input in asynchronization with the display timing. An instruction for the action of writing display data input in MIPI command mode into the frame buffer memory is given by an appropriate command. 
     LCD and OELD 
     In regard to the display driver, the above-described display panel is e.g., a liquid crystal display panel or an organic electroluminescence display panel. A display panel of this type is a representative example having a pixel structure arranged so that each time the pixels are selected by scan driving, their display signal information is overwritten; and it is ensured for such a display panel that even in case that its scan driving is stopped, the pixels continue holding information of signals driven based on immediately preceding display data thereof without losing such signal information. 
     LSI 
     The display driver is formed on a semiconductor substrate, for example. A display driver arranged like this is superior from the viewpoint of the downsizing, and also has a lower power consumption. 
     Stopping the Scan Driving of the Display Panel Following the Interface Mode Switching 
     The display driver ( 1 ) according to the invention from another aspect has: an external interface circuit ( 2 ); a frame buffer memory ( 22 ); a control circuit ( 9 ); and drive circuits ( 27 ,  29 ). The external interface circuit has: a first interface mode (video mode) to input display data as streams of pixel data in synchronization with a display timing; and a second interface mode (command mode) to input display data for write on the frame buffer memory in asynchronization with the display timing. The drive circuit outputs signals (Scn_ 1  to Scn_m) for scanning pixels of a display panel in synchronization with the display timing, and pixel drive signals (Sig_ 1  to Sig_n) for display-driving the scanned pixels according to display data. The control circuit performs control for stopping the pixel scan on the display panel during a predetermined period until the output of the pixel drive signals is enabled based on display data input in the second interface mode in case that display data input is switched from the first interface mode to the second interface mode. 
     According to the above arrangement, with the scan driving of the display panel temporarily stopped, all of the pixels of the display panel can hold the information of signals driven based on the immediately preceding display data without losing it. Therefore, in the event of the switching of the interface mode in the middle of driving the display panel, the scan driving of the display panel is stopped during a predetermined period until the driving of the display panel by display data input in the second interface mode after switching is enabled, whereby the occurrence of display disturbance of display data in the additional second interface mode can be suppressed until the internal state transition owing to the change in the interface mode is stabilized. Further, in case that the interface mode is switched from the first interface mode to the second interface mode, the scan driving of the display panel is stopped during the predetermined period. Therefore, even in the case of performing the process of storing display data input in the second interface mode in the frame buffer memory in synchronization with a display timing during the period, no disturbance of display is caused because a display state just before the switching is maintained. Further, it is unnecessary to perform the operation of previously accumulating, in the frame buffer, display data input in the first interface mode and in parallel with display thereof. 
     Stop During Periods of one or more Display Frames 
     In the display driver, the predetermined period is e.g., periods of one or more display frames, of which the unit is a one-display frame period. 
     Taking into account that the display control and the write of display data into the frame buffer memory are performed in units of the display frame, the above arrangement significantly simplifies the control for temporarily stopping the scan driving of the display panel. 
     Register to Set Predetermined Periods on 
     In the display driver, the control circuit has a register ( 33 ) on which the stop-period-setting data (MP 1 , MP 0 ) specifying a period of one or more display frames as e.g., the predetermined period are set overwritably. 
     According to the above arrangement, the scan-driving-stop period can be optimized according to the change in the period until the internal state transition owing to the interface mode change is stabilized, which depends on a display data interface speed and an internal action speed. 
     Stop-Period-Setting Data Supplied from Outside 
     In the display driver, the stop-period-setting data are supplied from outside the display driver, for example. 
     According to the above arrangement, it becomes easier to optimize the scan-driving-stop period according to control from the outside. 
     Scale-Up Circuit, which is Arranged on the Premise that no Data is Accumulated in FBM in Parallel with the Video Mode 
     The display driver further includes a scale-up circuit ( 23 ) for scaling up image data so as to enlarge the number of display pixels according to image data stored in the frame buffer memory. 
     According to the above arrangement, even if the memory capacity of the frame buffer memory is insufficient and smaller than the data size of one-frame display data supplied in the first interface mode, and image data stored in the frame buffer memory need to be scaled up, nothing interferes with the switching of the interface mode and the disturbance of display can be prevented as described above. 
     The effect achieved by the representative embodiment of the embodiments disclosed herein will be briefly described below. 
     The disturbance of display owing to the switching of the input mode of display data can be substantially eliminated. 
     Turning now to  FIG. 1 ,  FIG. 1  shows, by example, the schematic structure of a liquid crystal display driver according to one embodiment of the invention. As an example of the display driver, a liquid crystal display driver (LCDDRV)  1  is described here. Although no special restriction is intended, the liquid crystal display driver  1  is formed on a semiconductor substrate such as a bulk of monocrystalline silicon by a known CMOS integrated circuit manufacturing technique and the like. 
     The liquid crystal display driver  1  receives a command and display data from a host device  3 , and drives a liquid crystal display panel (PNL)  2  in synchronization with a display timing based thereon, thereby having a moving picture or a still image displayed. 
     For instance, the liquid crystal display panel  2  has pixels  40  formed on a glass substrate and arranged like a matrix as shown in  FIG. 2  by example; and each pixel  40  has a thin-film transistor  41  and a liquid crystal element  42  which are connected in series. A common potential Vcom is supplied to the liquid crystal elements  42  of the pixels. The select terminals of the thin-film transistors  41  are connected with scan electrodes Scn_ 1  to Scn_m corresponding to them in X direction, respectively; and the signal terminals of the thin-film transistors  41  are connected with signal electrodes Sig_ 1  to Sig_n corresponding to them in Y direction, respectively. The pixel lines of the respective scan electrodes Scn_ 1  to Scn_m are made display lines. The thin-film transistors  41  of the pixels  40  are turned ON in display lines, whereby the display lines are selected (scan of the display lines); and gradation voltages are applied to the liquid crystal elements  42  through the signal electrodes Sig_ 1  to Sig_n in each select period (horizontal display period) of the display lines. Turning OFF the thin-film transistors  41 , the gradation voltages thus applied are held by capacitor components of the liquid crystal elements and serve to maintain the shutter condition of the liquid crystal element until the next time the pixels are selected. 
     The liquid crystal display driver  1  has: MIPI circuit  20  as an example of an external interface circuit having a plurality of input modes as the interface modes to input display data; and a system interface circuit  21 . In this embodiment, the system interface circuit  21  is shown as a circuit operable to exchange commands and data with the host device bi-directionally. As a matter of course, the circuit configuration of the system interface circuit is compliant with MIPI, and it can be naturally arranged to be compliant with MDDI or bus interface specifications. 
     The MIPI circuit  20  has: a first interface mode to input display data in synchronization with a display timing, which is an action mode (also simply referred to as “video mode”) compliant with MIPI video mode; and a second interface mode to input display data in asynchronization with the display timing, which is an action mode (also simply referred to as “command mode”) compliant with MIPI command mode. The MIPI interface specifications of MIPI video mode, MIPI command mode and the like are described in MIPI Alliance Standard for Display Serial Interface V1.0 and others. According to the description thereof, the operations in MIPI command mode are a data write to the frame buffer memory and a data read therefrom, and a command write to a register and a command read therefrom. The operations in MIPI video mode are the action of inputting display data as streams of pixel data in synchronization with a display timing. In this embodiment, the video mode in MIPI circuit  20  is an action mode to input display data as streams of pixel data in synchronization with a display timing, and the command mode in MIPI circuit  20  is an action mode to input display data in asynchronization with a display timing because display data are written into a frame buffer memory according to an instruction of a command. In the command mode, the command input is performed on a register circuit  33  of a logic control part  30  from the system interface circuit  21 . As described above, it is sufficient for the system interface circuit  21  compliant with MDDI to write a command and control data into the register circuit  33  of the logic control part  30  by the operation in MDDI command mode. 
     The logic control part  30  has a register circuit  33  into which the host device  3  writes control data, a command, etc. The logic control part decrypts the written command, refers to written control data, and produces various control signals for controlling the actions in the liquid crystal display driver. In the diagram, input-mode data DM of display data, and stop-period-setting data MP 1 , MP 0  of two-bit scan, which are to be described later, are shown as control data set on the register circuit  33 . Although no special restriction is intended, the video mode is specified when DM=1, whereas the command mode is specified when DM=0. The stop-period-setting data MP 1 , MP 0  are to be described later in detail. 
     The display timing necessary for the liquid crystal display driver  1  to work is generated by the timing generation circuit  31 . In case that display timing signals such as a horizontal synchronizing signal HSYNC and a vertical synchronizing signal VSYNC are supplied from the host device  3  together with display data as in input in the video mode, the timing generation circuit  31  produces, based on the horizontal synchronizing signal HSYNC and the vertical synchronizing signal VSYNC, a horizontal synchronizing signal HSYNC_int and a vertical synchronizing signal VSYNC_int for display control. On the other hand, in the case of inputting display data in asynchronization with a display timing as in input in the command mode, neither the horizontal synchronizing signal HSYNC nor the vertical synchronizing signal VSYNC is supplied from the outside, the timing generation circuit  31  produces a horizontal synchronizing signal HSYNC_int and a vertical synchronizing signal VSYNC_int for display control according to an instruction from the logic control part  30 . In addition, the timing generation circuit  31  is arranged to be able to receive necessary operation clock signals and synchronizing clock signals from an oscillation circuit  28 . 
     In response to the video mode being set on the register circuit  33 , display data Ddat_vd input in a packet format according to the video mode are passed through a selector  24  and then, latched by a data latch circuit  25  in synchronization with a display timing sequentially. The display timing in this time is included in the packet format of the video mode, and created based on the horizontal synchronizing signal HSYNC and the vertical synchronizing signal VSYNC which are supplied to the timing generation circuit  3   l . The data latch circuit  25  latches, in each horizontal display period defined by the horizontal synchronizing signal HSYNC, display data (display-line data) corresponding to display lines and provides the latched display data to a gradation voltage select circuit  26  of the subsequent stage. If the number of pixels of one display line is 1024, the data latch circuit  25  outputs data of 1024 pixels to the gradation voltage select circuit  26  in parallel. The gradation voltage select circuit  26  selects gradation voltages for driving the pixels according to display-line data thus transmitted, and provides the selected gradation voltages to a source drive circuit  27  in parallel. The source drive circuit  27  uses the supplied gradation voltages, namely supplies the gradation voltages to the corresponding signal electrodes Sig_ 1  to Sig_n to drive the signal electrodes. The action of driving the signal electrodes Sig_ 1  to Sig_n is repeated in every horizontal display period defined by the horizontal synchronizing signal HSYNC, during which a gate drive circuit  29  drives the scan electrodes Scn_ 1  to Scn_m to a select level, thereby switches the display lines to be selected sequentially. The time of switching between the display lines is set at intervals of the horizontal display period based on the horizontal synchronizing signal HSYNC_int, and the period for making a round of the select of the scan electrodes Scn_ 1  to Scn_m is a frame display period based on the vertical synchronizing signal VSYNC_int. The horizontal synchronizing signal HSYNC_int is defined by the horizontal synchronizing signal HSYNC included in the packet format of the video mode, whereas the vertical synchronizing signal VSYNC_int is defined by the vertical synchronizing signal VSYNC included in the packet format of the video mode. 
     In response to the command mode being set on the register circuit  33 , display data Ddat_cm input in a packet format according to the command mode are written into the frame buffer memory  22  under the control of the logic control part  30 . The control form of write into the frame buffer memory  22  is defined by e.g., a command and control data provided to the register circuit  33  through the system interface circuit  21  from the host device  3  in advance. For instance, the logic control part  30  is provided with vertical and horizontal pixel sizes of display data, input format of input display data according to the command mode, etc. in the form of control data, and it controls an address for write on the frame buffer memory  22  based on input synchronizing clocks of input data according to the command mode while counting the number of input words. The input of display data according to the command mode is asynchronous to the display timing and therefore, the write of display data to the frame buffer memory  22  is also asynchronous to the display timing. Although no special restriction is intended, it is presupposed here that display data input in the command mode are data which need the scale-up process, of which the number of pixels is small for the display frame in scale. Therefore, display data stored in the frame buffer memory  22  are subjected to a process such as interpolation in the scale-up circuit  23  and thus, scaled up. The display data thus scaled up are passed through the selector  24 , and then, sequentially latched by the data latch circuit  25  in synchronization with a display timing. The timing generation circuit  31  creates the display timing at the time based on control data such as a dot clock frequency and the number of pixels of the display frame which are previously set on the logic control part  30 . The display timing thus created is reflected on the horizontal synchronizing signal HSYNC_int and the vertical synchronizing signal VSYNC_int, and provided to the gate drive circuit  29 . Further, the display timing is reflected on control signals to the data latch circuit  25 , the gradation voltage select circuit  26 , and the source drive circuit  27 . The action frequency when accumulating display data in the frame buffer memory  22  according to the command mode is faster than the dot clock frequency in synchronization with the display timing and therefore, the data latch circuit  25  is capable of sequentially latching display data in units of display lines so as to be in time for the display timing. The data latch circuit  25  latches display data (display-line data) corresponding to display lines and provides the display data to the gradation voltage select circuit  26  of the subsequent stage in each required horizontal display period. The gradation voltage select circuit  26  selects gradation voltages for driving the pixels according to the display-line data transmitted thereto, and provides the selected gradation voltages to the source drive circuit  27  in parallel. The source drive circuit  27  uses the gradation voltages thus provided, namely supplies the gradation voltages to the corresponding signal electrodes Sig_ 1  to Sig_n to drive the signal electrodes. The action of driving the signal electrodes Sig_ 1  to Sig_n is repeated in every required horizontal display period, during which the gate drive circuit  29  drives the scan electrodes Scn_ 1  to Scn_m to a select level, thereby switches the display lines to be selected sequentially. The time of switching between the display lines is set at intervals of the horizontal display period based on the horizontal synchronizing signal HSYNC_int, and the period for making a round of the select of the scan electrodes Scn_ 1  to Scn_m is a frame display period based on the vertical synchronizing signal VSYNC_int. 
     The liquid crystal display driver  1  has a control circuit  9  which includes a scan-mask-signal generation circuit  32  in addition to the logic control part  30  and the timing generation circuit  31 . While the display panel is driven for display drive, the scan-mask-signal generation circuit  32  outputs a scan-mask signal SCNMSK for stopping the scan driving on the liquid crystal display panel  2  for a predetermined period during which the driving of the display panel is enabled by display data input according to the command mode and supplies the scan-mask signal to the gate drive circuit  29  in case that the input mode of display data is switched from the video mode to the command mode in the MIPI circuit  20 . As in  FIG. 1 , the scan-mask-signal generation circuit  32  is supplied with a mode signal ϕdm, a suspension-period signal ϕmp, and a vertical synchronizing signal VSYNC_int from the timing generation circuit  31 . As the mode signal ϕdm, a logical value of mode data DM is supplied. The suspension-period signal ϕmp provides values of stop-period-setting data MP 1 , MP 0  of two bits. The stop-period-setting data MP 1 , MP 0  are control data which the host device  3  sets on the register circuit  33  programmably. For instance, stop-period-setting data MP 1 , MP 0  take values as follows: (0, 0)=0; (0, 1)=1; (1, 0)=2; or (1, 1)=3. Depending on the values, the stop-period-setting data mean one to four fold the vertical display period (frame display period). 
     The scan-mask-signal generation circuit  32  produces a scan-mask signal SCNMSK=0 under the condition of ϕdm=1 (being in the video mode). The gate drive circuit  29  receiving the scan-mask signal SCNMSK performs the scan driving in each vertical display period defined by the vertical synchronizing signal VSYNC_int; and in the scan driving, the scan electrodes Scn_ 1  to Scn_m are sequentially selected in synchronization with the horizontal synchronizing signal HSYNC_int. On switching of from ϕdm=1 (video mode) to ϕdm=0 (command mode), the scan-mask-signal generation circuit  32  validates the value of the suspension-period signal ϕmp, and keeps the value of the scan-mask signal SCNMSK one(1) from the subsequent vertical display period to a vertical display period specified by the validated value. The gate drive circuit  29  stops selecting the scan electrodes Scn_ 1  to Scn_m during each vertical display period during which the scan-mask signal SCNMSK=1. An instruction by the host device  3  for switching the video mode to the command mode in display action is enabled by the logic control part  30  in a vertical line-return period, and is reflected on the subsequent display data input and display control. Therefore, it is not required to presuppose that the instruction by the host device  3  for switching the video mode to the command mode be executed in a vertical line-return period. 
       FIG. 3  shows, by example, the action timing when the video mode is switched to the command mode during display. 
     The host device  3  overwrites the value of input-mode data DM of the register circuit  33  into one(1)(t 0 ), and display data (video data A) are supplied to the MIPI circuit  20  in synchronization with display synchronizing signals VSYNC, HSYNC (t 2 ). The liquid crystal display driver  1  recognizes DM=1 in synchronization with display timing signals VSYNC_int, HSYNC_int in the vertical line-return period thereof (t 1 ). The liquid crystal display driver  1  sends display data (video data A), which are input in synchronization with display synchronizing signals VSYNC, HSYNC in the video mode, to the source drive circuit  27  through the data latch circuit  25  (t 3 ) and then, sequentially selects the display lines by use of the scan electrodes Scn_ 1  to Scn_m, and drives, by video data A, the signal electrodes Sig_ 1  to Sig_n while synchronizing with display timing signals VSYNC_int, HSYNC_int. Thereafter the host device  3  outputs display data of the subsequent display frame starting from t 4  and then, the liquid crystal display driver  1  performs, with the subsequent display data (video data A), the scan driving of the liquid crystal display panel  2  and the signal electrode driving thereof while synchronizing with display timing signals VSYNC_int, HSYNC_int from t 5  in the same way as described above. Incidentally, the scan-mask signal SCNMSK remains negated into a value of zero(0) because of DM=1. 
     Subsequently, the host device  3  overwrites the input-mode data DM of the register circuit  33  into the value of zero(0) to stop the supply of display data in the video mode (t 6 ). Then, the liquid crystal display driver  1  recognizes DM=0 in a vertical line-return period of the display frame at the time of the stop, and turns the display data input mode to the command mode (t 8 ), whereby the liquid crystal display driver is allowed to work for display in synchronization with display timing signals VSYNC_int, HSYNC_int. The host device  3  issues a command (2Ch) for writing display data into the frame buffer memory  22  (t 7 ) and in parallel, starts supplying display data (data B) to write into the frame buffer memory  22  to the MIPI circuit  20 . The liquid crystal display driver  1  starts the action of accumulating display data (data B) supplied in the command mode in the frame buffer memory  22  (t 9 ). Since the frame buffer memory  22  is not used in the video mode, data (FBM data) remaining stored in the frame buffer memory  22  before that are overwritten by display data (FBM data B). It is presupposed here that it takes the time of a one-display frame period to finish the write. During the write action, the value of the scan-mask signal SCNMSK remains asserted into one(1). Only during the period, the scan driving on the scan electrodes Scn_ 1  to Scn_m is stopped (panel scan is stopped). Thus, the respective pixels of the liquid crystal display panel  2  are allowed to hold signal charges of the preceding display frame during the period. Therefore, even if during the period, the frame buffer memory  22  is overwritten, undesired data are put in the data latch circuit  25 , or the source drive circuit  27  is supplied with an undesired gradation voltage, an immediately preceding image based on the preceding video mode can be displayed without stopping these circuit actions. On condition that display data (FBM data B) enough for display according to the command mode have been stored in the frame buffer memory  22  during a one-display frame period, the value of the scan-mask signal SCNMSK is negated into zero(0) (t 10 ), thereby starting to display the display data (FBM data B) in synchronization with the display timing signals VSYNC_int, HSYNC_int. The display of display data (FBM data B) from the time t 10  is continued from the display of display data (video data A) of a display frame from the time t 9 ; and neither disturbance of display owing to an undesired image nor disturbance of display like dummy display in all white or black is interposed between the display from t 9  and the display from t 10 . After that, the host device  3  issues a command (2Ch) to write display data for the subsequent display frame into the frame buffer memory  22  at the time t 11  and concurrently, supplies display data (data C) to the liquid crystal display driver  1  at the time t 12 . In a display frame from the time t 13 , an already displayed data region is overwritten by the subsequent display data (FBM data C) in parallel with the display of display data (FBM data B) on the frame buffer memory  22 . 
       FIG. 4  shows an example of a portable communication terminal device to which the liquid crystal display driver  1  shown in  FIG. 1  is applied. The personal digital assistance shown in the diagram is e.g., a portable telephone or smart phone, which is an embodiment of a data processing system. 
     The personal digital assistance PDA includes: a liquid crystal display module  4  as a display part; an antenna  7  for transmission and reception; a speaker  6  for audio output; a microphone  5  for audio input; and a host device  3 . The liquid crystal display module  4  includes: a liquid crystal display panel  2  formed on a glass substrate; and a liquid crystal display driver  1  mounted on the glass substrate. Although no special restriction is intended, the host device  3  has: an audio interface  16  for performing signal input and output of the speaker  6  and the microphone  5 ; a high-frequency interface  15  for performing signal input and output to the antenna  7 ; a memory  14 ; and a base-band/application processor part (BB/APP)  10  for controlling a communication protocol process and other application processes. Although no special restriction is intended, BB/APP  10  has: DSP (Digital Signal Processor)  11  which performs signal processes in connection with audio signals and transmission and reception signals; ASIC (Application Specific Integrated Circuits)  12  which offers a custom function (user logic); and a microprocessor or microcomputer (hereinafter abbreviated as “MICON”)  13  as a data processing unit which performs the control of the whole device including display control. 
     Although no special restriction is intended, the liquid crystal display panel  2  is a dot matrix panel of FHD (Full High Definition) which has a number of display pixels of e.g., 1920×1080 arranged like a matrix. In the case of a liquid crystal panel of color display type, one pixel is configured of three kinds of dots, namely red, blue and green ones. In the case of adopting a touch sensor panel as an input device, a touch sensor panel according to an electrostatic capacitance method or the like is arranged over a surface of the liquid crystal display panel  2 , which is not shown particularly. In addition, a touch sensor panel controller operable to perform the drive control of the touch sensor panel and a sensing action, which is not shown, is arranged. The memory  14  is composed of e.g., a flash memory collectively erasable in units of a predetermined block, in which a control program to be executed by MICON  13  in communication control and display control, and control data used for communication control and display control are stored. 
     In the liquid crystal display panel  2 , the scan electrodes and signal electrodes are arranged like a matrix; TFT (Thin Film Transistor) switches are formed at intersections thereof. Each TFT switch has a gate connected with the corresponding scan electrode, and a drain connected with the corresponding signal electrode. TFT switches are each connected with a liquid crystal pixel electrode of a liquid crystal capacitance forming a sub-pixel on the source side thereof; and electrodes of the liquid crystal capacitances on the opposite side make a common electrode. The signal electrodes are supplied with signal voltages output by the liquid crystal display driver  1  A scanning pulse from the liquid crystal display driver  1  is applied to the gate electrodes e.g., in the order of the array thereof, whereby the gate electrodes are driven. 
     A combination of the audio interface  16 , BB/APP  10 , and the memory  14  can be arranged as a SoC (System on Chip) semiconductor device of one chip. Further, the high-frequency interface  15  may be added to them to form a multi-chip or one-chip semiconductor device. 
     The effects and advantages achieved according to the above embodiments are as follows. 
     Switching the display data input from the video mode to the command mode, the display driver performs control for stopping the scan driving of the pixels on the scan electrodes Scn_ 1  to Scn_m of the liquid crystal display panel  2  during a predetermined period until the output of pixel drive signals to the signal electrodes Sig_ 1  to Sig_n is enabled based on display data input in the command mode. Therefore, in a period during which the scan driving of the liquid crystal display panel  2  is stopped temporarily, all the pixels of the liquid crystal display panel  2  can hold information of signals activated based on their immediately preceding display data without losing it. Hence, if the input mode of image information is switched with the liquid crystal display panel activated, the scan driving of the liquid crystal display panel  2  is kept stopped during a predetermined period in which the activation of the liquid crystal display panel  2  by display data input in the command mode after switching is enabled, whereby the occurrence of disturbance of display can be suppressed until the internal state transition owing to the change in image information input mode is stabilized. Even if the frame buffer memory  22  has a sufficient memory capacity, it is unnecessary to perform the operation of previously accumulating display data input in the video mode in the frame buffer memory  22  in parallel with display thereof. So, the power consumption attributed to access to the frame buffer memory can be reduced. 
     The scale-up circuit  23  is adopted in the above embodiment, which scales up image data so as to enlarge the number of the display pixels according to image data stored in the frame buffer memory  22 . In this case, even if the memory capacity of the frame buffer memory  22  is insufficient and smaller than the data size of one-frame display data supplied in the video mode, and image data stored in the frame buffer memory  22  need to be scaled up, nothing interferes with the switching of the display data input mode and the disturbance of display can be prevented as described above. 
     To stop the scan driving for selecting the display lines for a predetermined period is all that is needed. It poses no problem to leave the circuit on the drive side of the signal electrodes working and therefore, the disturbance of display owing to the switching of the input mode of display data can be substantially eliminated. 
     The predetermined period during which the scan driving remains stopped is a period of at least one frame, of which the unit is e.g., a one-display frame period. Taking into account that the display control and the write of display data into the frame buffer memory  22  are performed for each display frame, the control arranged to temporarily stop the scan driving for the liquid crystal display panel  2  is remarkably simple. 
     The display driver has the register circuit  33  on which stop-period-setting data MP 1 , MP 0  each specifying a period of one or more display frames as the predetermined period to keep the scan driving stopped are set overwritably. Taking into account that a period until the internal state transition owing to the change in display data input mode is stabilized changes depending on a display data interface speed and an internal action speed, the scan-driving-stop period can be optimized according to the change. 
     The stop-period-setting data can be supplied e.g., from outside the display driver and therefore, it becomes easier to optimize the scan-driving-stop period according to control from the outside. 
     While the invention made by the inventor has been concretely described above based on the embodiments, the invention is not limited to the embodiments. It is contemplated that various changes and modifications thereof may be made without departing from the subject matter thereof. 
     For instance, the interface modes to input display data are not limited to the video mode and the command mode compliant with MIPI. The interface modes of MDDI, RGB interface and the like may be included therein. In addition, the interface modes to input display data are not limited to the interface modes arranged to be in synchronization/asynchronization with a display timing as the video mode and the command mode are. For instance, in the case of MIPI, they may be interface modes different in the number of data lanes. 
     The period during which the scan driving remains stopped is not limited to a period of which the unit is the period of a display frame. Even in the event of the switching of the interface mode in the middle of a display frame period, the time of the end of the stop period thereof may be arranged to be coincident with an end of a display frame period (or a vertical line-return period). 
     The invention is not limited to the structure having the scale-up circuit. Also, in the case of the display driver having a frame buffer of a sufficient memory capacity, the action of accumulating display data in the frame buffer in parallel with display in the video mode can be omitted. It is obvious that the invention is applicable to such a structure that display data stored in the frame buffer memory are selectively passed through the scale-up circuit. 
     The display panel is not limited to a liquid crystal one. It may be another display panel, such as an organic electroluminescence display panel. In short, the display panel may be any one as long as it has a display form in which the preceding display drive signal information can be held by the pixels at the stop of the scan driving. 
     The display driver is not limited to one formed on a semiconductor substrate independently. The display driver may be mounted in a semiconductor substrate together with another circuit, e.g., a touch panel controller, a microcomputer or the like, or it may be installed on a single module substrate.