1. Field of the Invention
The present invention relates to a method for driving a display made up of a current-driving type light-emitting device which displays various pieces of information, results of measurement, moving pictures, or still pictures and to a circuit employing the above method and to portable electronic devices incorporating the circuit and more particularly to the method for driving the display which is used as a display device of computers such as notebook computers, palm-sized computers, pocket computers, or a like, of portable electronic devices such as a PDA (Personal Digital Assistant), a portable cellular phone, or a PHS (Personal Handy-phone System) and to the circuit employing the above method and to the portable electronic devices being equipped with driving circuits for the display.
The present application claims priority of Japanese Patent Application No.2001-285838 filed on Sep. 19, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
Some types of displays are made up of current-driving type light-emitting devices. The displays of this kind conventionally include a display made up of an EL (Electroluminescnece) device, a display made up of an LED (Light-Emitting Diode), a VFD (Vacuum Fluorescent Display) including, in particular, an FED (Field Emission Display) being one of types of the VFDs, a PDP (Plasma Display Panel), or a like. Hereinafter, this type of the display is called a “self-emissive type display”.
Generally, the self-emissive type display tends to draw current more than a voltage-driving type liquid crystal display. That is, in the voltage-driving type liquid crystal display, since its liquid cell is a capacitive load, an amount of current consumed is as little as several mA. However, since the self-emissive type display emits light for every pixel and therefore consumes current every time it emits light, an amount of current consumed reaches 200 mA or more when high current volumes are consumed, for example, when an image is displayed at a high value of luminance. Therefore, when the self-emissive type display is used in a displaying section of a portable electronic devices to which power is supplied by a battery, dry cell, or a like, to keep operating time as long as possible, an amount of current consumed has to be reduced to a minimum. The portable electronic devices include notebook-type, palm-type, or pocket-type computers, PDAs, or portable cellular telephones, PHSs, or a like.
The portable cellular phone or the PHS has a waiting mode in which, though power is provided, a user does not perform any operation while waiting for an incoming call. The displaying section provides a waiting screen corresponding to a waiting mode.
Not only the portable cellular phone or the PHS but also other portable electronic devices when, though power is provided, and a specified time has elapsed without any operations performed by a user, as shown in FIG. 8, is put into a screen save mode in which a specified character or diagram is displayed randomly on each portion on the display at every specified time and a moving object is displayed on the display. FIG. 8 shows that a current time indicated by the self-emissive type display in the portable cellular phone or the PHS is displayed in the screen save mode in a manner that a place for displaying time is changed at every specified time. The screen save mode is used to prevent occurrence of a phenomenon called an “image burn-in” state in which same characters or same diagrams continue to be displayed for a long time, even if power is turned OFF, a trace of the character or the diagram is left. The user, when such portable electronic devices are in the waiting mode or in the screen save mode, does not view the screen of a display carefully.
However, conventionally, even when the user drives a screen of a display in a waiting mode or lowers luminance of each pixel in a screen save mode, the user employs the same driving method as is used for driving the ordinary screen on which the user views the screen carefully. For this reason, in the former case, power is drawn wastefully. Moreover, in the latter case, the display becomes dark as a whole and are hard to view and, therefore, if the user views the display unexpectedly, the user cannot confirm contents of the display immediately and misunderstands, in some cases, that power has not been provided.
In some cases, as shown in FIG. 9, the display screen of the conventional portable cellular phone or the PHS is made up of, for example, an upper display portion 1, a central display portion 2, and a lower display portion 3. In the upper display portion 1, a battery mark 1a indicating a charging state of a battery, an antenna mark 1b indicating whether or not the portable cellular phone or the PHS being presently used is in a service area of a wireless telephone system of a mobile communication network, or a like are displayed. In the central display portion 2, sentences of electronic mail, images being attached to the electronic mail, and images indicating contents being provided from various contents providers in a WWW (World Wide Web) server or a like are displayed. FIG. 9 shows an example in which map data is displayed in the central display portion 2. In the lower display portion 3, a menu key used to select a menu is displayed. Then, generally, in the central display portion 2 is displayed a detailed image, while in the upper display portion 1 and in the lower display portion 3 are displayed a character and/or a mark in a simplified manner. This is because, even if the character and/or mark is simplified, information can be fully transferred to the user of the portable cellular phone or the PHS.
However, conventionally, even in the case of the upper display portion 1 and in the lower display portion 3 in which such simplified characters or marks are displayed, a same driving method as is used to display in the central display portion 2 in which detailed images are displayed is employed. This causes wasteful power consumption. The same inconveniences as described above occur in other portable electronic devices to which power is supplied by the battery or the dry cell such as the notebook-type, the palm-type, and pocket-type computers, the PDA or the like, though contents and portions displayed therein (for example, in a window), or a like are different.