1. Field of the Invention
The present invention relates to a display device. Further, the invention relates to an electronic appliance having the display device for a display portion.
2. Description of the Related Art
In recent years, a thin display device having pixels formed using self-luminous light emitting elements has attracted attention. As a light emitting element, an organic light emitting diode (OLED) or an EL (electroluminescent) element has attracted attention, and have been used for an organic EL display or the like.
As a driving method for expressing a multi-gray scale image of a display device using the aforementioned light emitting element, there are an analog driving method (analog gray scale method) and a digital driving method (digital gray scale method).
The analog driving method is a method in which current magnitude flowing in a light emitting element is continuously controlled to obtain a gray scale. Whereas, the digital driving method is a method in which a light emitting element is driven by only two states of an ON state (a lighting state with the luminance of approximately 100%) and an OFF state (a state where the luminance is approximately 0%, that is, a non-lighting state).
Next, brief description is made of an example of a pixel structure of a display device employing the time gray scale method and drive thereof. A circuit shown in FIG. 1 includes transistors 201 and 202, and a light emitting element 203. A gate electrode, a first electrode, and a second electrode of the transistor 201 are connected to a gate signal line 205, a source signal line 204, and a gate electrode of the transistor 202 respectively. A first electrode and a second electrode of the transistor 202 are connected to a power source line 206 and a first electrode of the light emitting element 203 respectively. A second electrode of the light emitting element 203 is connected to a counter electrode.
Note that it is difficult to define a source electrode and a drain electrode of a thin film transistor (hereinafter referred to as TFT) due to a structure thereof. Here, one of a source electrode and a drain electrode is referred to as a first electrode, and the other is referred to as a second electrode. In general, a lower potential side electrode is a source electrode and a higher potential side electrode is a drain electrode in an n-channel transistor, whereas a higher potential side electrode is a source electrode and a lower potential side electrode is a drain electrode in a p-channel transistor. Accordingly, in the case where there is description concerning a gate-source voltage or the like in description of circuit operation, the aforementioned basis is referred.
Subsequently, description of FIG. 1 is made with reference to a timing chart in FIG. 2. The source signal line 204 to be selected is determined by an SWE211 (source writing/erasing select signal). Further, the gate signal line 205 to be selected is determined by a GIWE212 (gate writing select signal) and a G2WE213 (gate erasing select signal). Whether the light emitting element 203 emits light or no light is determined by signals of the source signal line 204 and the gate signal line 205. Here, as for an arbitrary wiring, a digital signal “1” is referred to as H (High level), whereas “0” is referred to as L (Low level). It is to be noted that “0” means not only a ground potential but a common potential. A state where a potential is higher than an arbitrary threshold voltage may be H, whereas a state where a potential is lower than an arbitrary threshold voltage may be L.
Black is written when the source signal 214 is H. However, if the gate signal 215 is not H at that time, such data is not reflected to the light emitting element 203. Meanwhile, white, that is, data is written when the source signal 214 is L. However, if the gate signal 215 is not H, such data is not reflected to the light emitting element 203.
Subsequently, the digital driving method is described. With the digital driving method alone, only 2 gray scales can be expressed. Therefore, it is suggested that the digital driving method be used in combination with a driving method for expressing multi gray scales, such as an area gray scale method or a time gray scale method. The area gray scale method is a method in which gray scale is expressed depending on the size of a light emitting area of a sub-pixel provided in a pixel (for example, see Patent Document 1). Further, the time gray scale method is a method in which gray scale is expressed by controlling a light-emitting period and light-emitting frequency (for example, see Patent Documents 2 and 3).    [Patent Document 1] Japanese Published Patent Application No. H11-73158    [Patent Document 2] Japanese Published Patent Application No. 2001-5426    [Patent Document 3] Japanese Published Patent Application No. 2001-343933