Liquid crystal display apparatus

Abstract: In the operation of liquid crystal display apparatus, the last display line of one frame is followed by a vertical blanking term during which data for the first display line of the next frame is used to pre-charge the signal electrodes at least one horizontal duration before the first display line of the next frame is displayed by the arrival of a frame pulse. This removes the effect of data for the last display line of one frame on the first display line of the next frame.

Description: The present invention relates to liquid crystal display apparatus. Figure 4 shows display control signals and display data in a conventional liquid crystal display (LCD) apparatus. A frame pulse FP and a latch pulse LP correspond to a vertical synchronizing signal and a horizontal synchronizing signal for a display controller, respectively. A shift clock pulse SCP is a clock signal for data transfer and data stored in a shift register for signal electrodes shifts each time the shift clock pulse SCP is provided to the shift register. From the shift register for the signal electrodes, data is transferred in parallel to a driver output stage for the signal electrodes each time the latch pulse LP is provided to the shift register. The signal driver output stage outputs a potential which corresponds to the data to a signal electrode. The latch pulse LP is provided at the same time not only to the shift register for the signal electrodes, but also to a shift register for the scanning electrodes. Each time the latch pulse LP is provided to the shift register for the scanning electrodes, the frame pulse FP is shifted as data within the shift register for the scanning electrodes and thus the scanning electrodes are sequentially turned on each time the latch pulse LP is provided to the shift register for the scanning electrodes. As a result, the scanning electrodes are turned on one after another simultaneously with the output of a data signal to the signal electrodes, that is, the display operations of display lines change one after another. A vertical blanking term is established between frames. Immediately after the frame pulse FP is shifted to a scanning electrode for the last display line and the scanning electrode for the last display line has been turned on, the following frame pulse FP does not generate, but does so after a predetermined interval and thus the vertical blanking term is established. That is, the scanning electrode for the last display line is turned on and then, after the predetermined interval, a scanning electrode for the first display line of the following frame is turned on to establish the vertical blanking term. However, in such a conventional LCD apparatus, it became clear that the effect of data for the last display line appeared in the first display line in some cases. The cause of such phenomenon has been investigated. In the conventional LCD apparatus, even during a vertical blanking term after the last display line has been displayed, the application of a potential according to data for the last display line to a signal electrode continues. However, no scanning electrode being turned on during the vertical blanking term, no display line is in a display state, regardless of the potential of the signal electrode. If a potential according to data for the last display line is applied to the signal electrode for a long period of time and then a potential according to data for the first display line of the following frame is applied to the signal electrode, the voltage applied to the signal electrode does not immediately change. In a LCD apparatus using a two-split driving method, constructed so that the upper half and the lower half areas on one LCD panel are driven by different columns of signal electrodes, the first display line of the lower area being in the middle of the LCD panel, the phenomenon described above remarkably appears. In particular, if a pattern in which only one horizontal line is displayed at the bottom of the lower area and none of lines is displayed in other areas, is to be displayed, the horizontal line appears in the middle of the screen, that is, in the first display line on the lower half screen. In a LCD apparatus in multiple display modes in which the number of display lines is different from one another, if the display area includes only the number of partial display lines smaller than that of display lines which can be displayed, an LCD controller blanks out margins at the top and the bottom of the screen (fixes at either on or off level) due to the centring of the display area. In this case, if reversed display is present in the middle of the screen, as expected, the horizontal line appears in the middle of the screen (in the first line on the lower screen). A driver for the signal electrodes of the lower screen is arranged under columns of the signal electrodes and the display lines are usually driven so that they move from the top to the bottom of the screen. Therefore, the first display line of the lower screen is arranged at the longest distance from a driver output stage for the signal electrodes. This causes the impedance from the driver output stage to the first display line for the signal electrodes to increase and it becomes difficult to change a potential corresponding to the first display line rapidly. Also in this respect, the phenomenon shown above becomes apt to occur. A typical liquid crystal display controller is disclosed in EP-A-0206 178. Another typical liquid crystal display, in which signal lines are pre-charged to an intermediate potential prior to application of an input video signal, is disclosed in JP 2204718. An object of the present invention is to remove the effect of data for the last display line on the first display line. According to the present invention, prior to a horizontal duration immediately before scanning electrodes for the first display line of the following frame are turned on after the completion of the last display of the scanning electrodes, signal electrodes are pre-charged with an electric potential according to data for the first line of the following frame to accomplish this object. This is, in the present invention, an electric potential according to data for the first display line is applied to the signal electrodes, earlier than in the prior art, to give the signal electrodes sufficient time to change to the potential according to data for the first display line and thus the object is accomplished. Other aspects of the invention appear from the appended claims; and how the invention can be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 shows waveform charts of control signals in an embodiment of LCD controller according to the present invention; Figure 2 is a block diagram showing an embodiment of LCD apparatus constructed in accordance with the present invention; Figure 3 is a block diagram showing an embodiment of an information processing apparatus constructed in accordance with the present invention; and Figure 4 shows waveform charts of control signals in a conventional LCD controller. In an embodiment of an information processing apparatus (Figure 3) including an LCD display controller according to the present invention, a bus 3 is connected to a CPU 1, a screen memory 5 and a display controller 7, and an LCD module 9 is connected to the display controller 7. The screen memory 5 stores picture image data to be displayed by a matrix-type LCD cell 10 (Figure 2) in the LCD module 9 and the picture image data is rewritten by the CPU 1. The display controller 7 transfers the picture image data stored in the screen memory 5 and each signal for display control (as shown in Figure 1) together to the LCD module 9. The LCD module 9 (Figure 2) comprises the LCD cell 10 and a liquid crystal driver 15. The LCD cell 10 driven by a two-split driving method, includes columns of signal (data or segment) electrodes Y1, Y2, Y3, ... Yn and rows of scanning (common) electrodes X1, X2, X3, ... Xm for an upper half screen 10A and a lower half screen 10B. The liquid crystal driver 15 includes data side driver blocks 17A and 17B and a scanning side driver block 19. The data side driver blocks 17A and 17B include driver output stages 20A and 20B and shift registers 30A and 30B for the signal electrodes, respectively. The scanning side driver block 19 includes a shift register 40 for the scanning electrodes and a driver output stage 45. Each of the columns of signal electrodes Y1, Y2, Y3, ... Yn for the upper half screen and the lower half screen is connected to the driver output stages 20A and 20B, respectively. The driver output stages 20A and 20B are connected to the shift registers 30A and 30B for the signal electrodes, respectively. The respective scanning electrodes X1, X2, X3, ... Xm for the upper and the lower screens 10A and 10B are connected to the common shift register 40 for the scanning electrodes. The operations of the embodiment are described by reference to FIG.1 in addition to Figures 2 and 3. The display controller 7 (Figure 3) divides data signals for display into two groups for the upper and the lower screens and transfers them to the liquid crystal driver 15. A data signal for the upper screen and a data signal for the lower screen synchronize with a shift clock pulse SCP and shift into the shift registers 30A and 30B for the signal electrodes for the upper and the lower screens 10A and 10B, respectively. After the completion of the shift of data corresponding to all signal electrodes Y1, Y2, Y3, ... Yn for one display line into the shift registers 30A and 30B for the signal electrodes, the data is transferred to the driver output stages 20A and 20B by means of a latch pulse LP and the driver output stages 20A and 20B synchronize with the latch pulse LP and simultaneously output an electric potential according to the data for one display line corresponding to all signal electrodes Y1, Y2, Y3, ... Yn. The latch pulse LP is input simultaneously not only to the driver output stages 20A and 20B, but also to a clock input of the shift register 40 for the scanning electrodes. A frame pulse FP is input to a data input of the shift register 40 for the scanning electrodes and shifted within the shift register 40 for the scanning electrodes each time a latch pulse LP is inputted. Thus, by means of a first latch pulse LP after a rise of a frame pulse FP, a first scanning electrode X1 (scanning electrode for a first display line) is turned on by the driver output stage 45, then by means of a second latch pulse LP, a second scanning electrode X2 is turned on, and finally a scanning electrode Xm, the same as above, is turned on. That is, the scanning electrodes X1, X2, X3, ... Xm are sequentially turned on synchronously with latch pulses LP. In other words, each time data for one display line is sequentially transferred to one of the signal electrodes Y1 to Yn by means of a latch pulse LP input to clock inputs of the shift registers 30A and 30B for the signal electrodes, the respective scanning electrodes X1, X2, X3, ... Xm of the upper and the lower screens 10A and 10B are sequentially turned on by means of latch pulses LP input to the clock input of the shift register 40 for the scanning electrodes. In the following description, the operation of the lower screen 10B is mainly described, and unless otherwise stated, the same description holds true for the upper screen 10A. At the completion of the transfer of data for the last display line (scanning electrode Xm) to the shift register 30B, by the shift latch pulse LP, an electric potential corresponding to the data for the last display line is applied to the signal electrodes Y1 to Yn from the driver output stage 20B and the scanning electrode Xm is turned on. As a result, the last display line is displayed. Also after the last display line has been displayed, latch pulses LP are transmitted one after another during the time interval of predetermined horizontal duration from the display controller 7. However, the following frame pulse FP is not transmitted from the display controller 7 immediately after the last display line has been displayed. Accordingly, until the following frame pulse FP is provided to the scanning electrode X1 (for the first display line) by means of a latch pulse LP, no scanning electrode is turned on. This period of time for which no scanning electrode is turned on is referred to as a vertical blanking term. In a conventional LCD apparatus, data for the first display line of the following frame is transferred to the shift register for the signal electrodes at a horizontal duration immediately before the following frame pulse FP and output to the signal electrodes at the next horizontal duration. However, in the present embodiment, during a horizontal duration followed by a horizontal duration immediately before a horizontal duration in the conventional LCD apparatus, data for the first display line of the following frame is transferred to the data side driver block 17B or the shift register 30B for the signal electrodes and during the next horizontal duration, the data is output to the signal electrodes Y1 to Yn. The data transferred to the shift register 30B is output, by means of the first latch pulse LP after the completion of the transfer of the data, from the driver output stage 20B in an electric potential accordance to the data to pre-charge the signal electrodes Y1 to Yn. At this point, none of the scanning electrodes X1 to Ym is turned on and thus a vertical blanking term continues. When, immediately before the completion of a horizontal duration during which the signal electrodes Y1 to Yn are pre-charged with a potential according to the data for the first display line, the following frame pulse FP is transmitted to the shift register 40 for the scanning electrodes from the display controller 7, the first scanning electrode X1 is turned on, by means of the first latch pulse LP after rise of the frame pulse FP, to display the first display line. Then a potential according to data for the second display line (scanning electrode X2) previously transferred to the shift register 30A is output, by means of the following latch pulse LP, to the signal electrodes Y1 to Yn to display the second display line. The remaining display lines are sequentially displayed in the same manner as above. In the present embodiment, the period of time for which a data signal for the first display line is provided to a signal electrode corresponds to two horizontal durations, although the period of time corresponds to one horizontal duration the same as in data signals for other display lines in the case of the conventional LCD apparatus. Therefore, before the first scanning electrode X1 is turned on and the first display line is displayed, the signal electrodes Y1 to Yn are given sufficient time to change electric potential in response to new data and thus data for the last display line has no effect on the first display line. In the present embodiment, during a horizontal duration followed by a horizontal duration immediately before a frame pulse FP, data for the first display line is transferred to the shift register 30B for the signal electrodes. However, it will be appreciated that during the horizontal duration immediately before the frame pulse FP, the data for the first display line may be transferred again to the shift register 30B for the signal electrodes. If the data for the first display line is transferred to the shift register 30B for the signal electrodes during the horizontal duration followed by the horizontal duration immediately before the frame pulse FP, whether or not the data for the first display line is transferred again to the shift register 30B for the signal electrodes at a horizontal duration immediately before the following frame pulse FP, an electric potential according to the data for the first display line is applied to the signal electrodes Y1 to Yn during two horizontal durations. It will be appreciated also that data for the first display line may be transferred earlier to the shift register 30B for the signal electrodes. For example, it will be appreciated that during a horizontal duration immediately after data for the last display line of the preceded frame has been transferred to the shift register 30B for the signal electrodes, data for the first display line of the following frame may be transferred to the shift register 30B for the signal electrodes to output to a signal electrode. As described above, the present invention has an advantage that data for the last display line has no effect on the first display line. To remove the effect of data for the last display line on the first display line, prior to a horizontal duration immediately before a data signal for the last display line is transferred to a driver and then a scanning electrode for the first display line of the following frame is turned on, signal electrodes are pre-charged with an electric potential according to data for the first display line of the following frame. By this, data for the first display line of the next frame is used to pre-charge the signal electrodes at least one horizontal duration before the first display line is displayed.

Claims: A liquid crystal display controller (7) characterized in that during a vertical blanking term, data and a control signal or signals are output so that signal electrodes (Y1-Yn) of a liquid crystal display module (10) are pre-charged with a potential which is in accordance with the first display line data of the following frame. A liquid crystal display controller according to claim 1, wherein the module (10) is split and data for the upper screen (10A) and data for the lower screen (10B) are transferred. Information processing apparatus including apparatus according to claim 1 or 2, drivers for driving the expective electrodes (Y1-Yn), a video memory for storing data to be displayed on the LCD cell (10), a CPU for writing data into the memory, and a liquid crystal display controller (7) for transferring data in the video memory and a control signal or signals to the drivers. A method of operating liquid crystal display apparatus comprising pre-charging signal electrodes (Y1-Yn) of a liquid crystal display cell (10) with a potential in accordance with data for a first display line in a vertical blanking term at least one horizontal duration before the first scanning electrode is turned on.

Inventors: HAYASHIGUCHI FUMIEI, TOKUDA TAKAYUKI, HAYASHIGUCHI, FUMIEI
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: G09G3
IPC codes: G02F1, G09G3