1. Field of Invention
The present invention relates to a method of driving a display device such that a gray-scale image is displayed by using pulse width modulation with reduced electrical power consumption. The present invention also relates to a driver circuit based on such a method, a display device, and an electronic device.
2. Description of Related Art
In general, a portable electronic device includes a display device for presenting various kinds of information to a user. In such display devices, information is displayed using an electrooptical change in an electrooptical material. For example, liquid crystal display devices are widely used for this purpose. In recent years, it has become desirable that display devices be not only capable of simply providing an achromatic display (ON/OFF or two-value black and white), but also be capable of representing a large number of gray levels so that images of intermediate gray levels can be displayed.
However, in portable electronic devices which are powered by a battery, it is very important that the portable electronic devices operate with small power consumption. As is well known, extremely greater power consumption is needed to display a gray-scale image relative to that which is needed to display a simple black-and-white image. That is, in display devices for use in portable electronic devices, it is necessary to meet the requirements of having the capability of displaying a gray-scale image and having small power consumption, which often conflict with each other.
In view of the above, it is an object of the present invention to provide a method of driving a display device so as to display a gray-scale image without causing a significant increase in power consumption, a driver circuit for implementing the method and a display device using such a method, and an electronic device using such a display device.
According to a first aspect of the present invention, there is provided a method of driving a display device so as to display a gray-scale image by driving pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns. The method comprising the steps of: selecting a single scanning line from the plurality of scanning lines during one horizontal scanning period and applying a selection voltage to the scanning line during one of half periods of the horizontal scanning period; and selecting a single scanning line adjacent to the previously selected scanning line during following one horizontal scanning period and applying a selection voltage to the adjacent scanning line during the other one of the half periods of the horizontal scanning period; while at the same time applying a turn-on or turn-off voltage to a pixel at a location corresponding to a selected scanning line via a corresponding data line such that the turn-on voltage is applied during a period with a length corresponding to a gray level in the period during which the selection voltage is applied and the turn-off voltage is applied during the remaining period. In this first aspect of the present invention, a reduction is achieved in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage during an operation of displaying pixels having intermediate gray levels, and thus it is possible to reduce electric power consumed in switching the voltage.
In this first aspect of the present invention, in the case where only white or black pixels are displayed and no intermediate gray levels are displayed, the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage does not decrease, and thus an increase in power consumption can occur. To avoid such a problem, in this first aspect of the present invention, it is specified whether or not the mode should be changed. In the case where it is specified that the mode should be changed, when the adjacent scanning line is selected in the following one horizontal scanning period, the selection voltage is preferably applied to the adjacent scanning line during one of half periods of the horizontal scanning period. That is, when an intermediate level is not displayed, the mode is changed to prevent an increase in power consumption.
A command to change the mode may be issued by an application, or a user. Alternatively, the gray level data associated with the pixels are examined, and a command to change the mode may be issued depending upon a result of the examination. In this case, it is preferable that the mode shift be specified when the number of pixels successively aligned along a column in which pixels will be displayed in a single color of either black or white exceeds a predetermined number of pixels located along a single scanning line that will be selected. This prevents an increase in power consumption.
Furthermore, in the first aspect of the present invention, it is preferable that the mode shift be prevented when pixels the number of pixels successively aligned along a column in which black and white pixels will be displayed alternately exceeds a predetermined number of pixels located along a single scanning line that will be selected. This is because if the mode is changed in such a case, an increase occurs in the number of times the voltage applied to the data line of the pixels is switched between the turn-on voltage and the turn-off voltage, and thus power consumption increases.
According to a second aspect of the present invention, in order to achieve the above-described object, there is provided a driver circuit for driving a display device so as to display a gray-scale image by driving pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns. The driver circuit comprising: a scanning line driver circuit for selecting out of said plurality of scanning lines a single scanning line during a horizontal scanning period and applying a selection voltage to the scanning line during one of two half periods that said horizontal scanning period has been divided into, and for selecting a single scanning line adjacent to the previously selected scanning line during the subsequent horizontal scanning period and applying a selection voltage to the adjacent scanning line during the other of the two half periods of said horizontal scanning period; and a data line driver circuit for applying a turn-on or turn-off voltage to a pixel at a location corresponding to the scanning line selected the scanning line driver circuit via a corresponding data line such that the turn-on voltage is applied during a period with a length corresponding to a gray level in the period during which said selection voltage is applied and the turn-off voltage is applied during the remaining period. According to this second aspect of the present invention, as with the first aspect of the present invention, a reduction can be achieved in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage, and thus it is possible to reduce electric power consumed in switching the voltage.
According to a third aspect of the present invention, to achieve the above-described object, there is provided a display device for displaying a gray-scale image by driving pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns. The display device comprising: a scanning line driver circuit for selecting out of the plurality of scanning lines a single scanning line during a horizontal scanning period and a selection voltage to the scanning line during one of two half periods that the horizontal scanning period has been divided into, and selecting a single scanning line adjacent to the previously selected scanning line during the subsequent horizontal scanning period and applying a selection voltage to the adjacent scanning line during the other of the two half periods of the horizontal scanning period; and a data line driver circuit for applying a turn-on or turn-off voltage to a pixel at a location corresponding to the scanning line selected the scanning line driver circuit via a corresponding data line such that the turn-on voltage is applied during a period with a length corresponding to a gray level in the period during which said selection voltage is applied and the turn-off voltage is applied during the remaining period. According to this third aspect of the present invention, as with the first or the second aspect of the present invention, a reduction is achieved in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage, and thus it is possible to reduce electric power consumed in switching the voltage.
In this third aspect of the present invention, the pixel preferably includes a switching element and a capacitor driven by the switching element. In this construction, a selected pixel and a non-selected pixel is electrically isolated from each other by the switching element, and thus good contrast and response can be obtained and a high-quality image can be displayed.
In this construction, the switching element is a thin film diode having a conductor/insulation/conductor structure. In this case, one end of the thin film diode is connected to either a scanning line or a data line, and the other end thereof is connected to the capacitor. When the thin film diode is used as the switching element, the production process becomes simpler. Besides, principally, no short-circuited path is created between a scanning line and a data line.
According to a fourth aspect of the present invention, there is provided an electronic device including a display device according to the previous aspect of the invention. The electronic device according to the present invention is capable of displaying a gray-scale image with reduced power consumption.