Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
2000-12-08
2004-08-31
Eisen, Alexander (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S204000
Reexamination Certificate
active
06784880
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a malfunction-free electro-optical device, a clock signal adjusting method therefor, a clock signal adjusting circuit therefor, a producing method therefor, and electronic equipment using the electro-optical device.
2. Description of Related Art
A conventional electro-optical device, for example, a conventional active matrix liquid crystal display device includes a device substrate on which a switching device is provided at each of pixel electrodes arranged like a matrix, an opposing substrate on which color filters are formed, and liquid crystal filled between the substrates. In such a configuration, when a scanning signal is applied to a switching device through a scanning line, the switching device is conducted. When an image signal is applied to a pixel electrode through a data line during the switching device is in such a conducting state, a predetermined amount of electric charge is stored in a liquid crystal layer between the pixel electrode and an opposing electrode (or common electrode). Even when the switching device is turned off after the charge is stored, the stored charge is maintained in the case that the resistance of the liquid crystal layer is sufficiently high. Controlling an amount of stored charge by driving each switching device in this manner causes a change in the alignment of crystal liquid molecules at each pixel. This enables the display to indicate predetermined information.
At that time, the operation of storing charge in the liquid crystal layer of each pixel requires only part of processing time. Thus, a time division multiplexing driving method, according to which each of scanning lines and data lines is shared by a plurality of pixels, is implemented by first sequentially selecting scanning lines by means of a scanning line driving circuit, and second sequentially selecting one or more data lines by means of a data line driving circuit in a scanning line selecting time, and third sampling image signals and supplying the sampled image signals to the selected data lines.
Incidentally, generally, the scanning line driving circuit and the data line driving circuit are constituted by shift register circuits. According to a signal transferred by each of these shift register circuits, the scanning line driving circuit performs a vertical scanning operation, while the data line driving circuit performs a horizontal scanning operation.
Meanwhile, the scanning line driving circuit and the data line driving circuit may be formed on the aforementioned device substrate, in addition to an image display area including the scanning lines, data lines and switching devices. In such a case, a thin film transistor (hereunder referred to as TFT) is usually used as an active device constituting each of the scanning line driving circuit and the data line driving circuit.
Incidentally, in the case of some process for forming TFTs, there is variation in threshold voltage of a TFT. Especially, in the case of using a glass substrate as the device substrate, there is a large variation in the threshold voltage of a TFT.
On the other hand, each of the shift register circuits includes series-connected unit circuits, each of which includes a clocked inverter and a latch circuit. Each of the shift register circuits sequentially shifts a start pulse according to a clock signal and an inverted clock signal.
However, as described above, there is variation in threshold voltages of TFTs of the shift registers. Thus, when there is a certain difference between the threshold voltage and a designed value, each of the shift registers malfunctions. The operating speed of a TFT varies with the electric current value of an ON-current thereof. Thus, when there is a certain difference between the electric current value of the ON-current and a designed value, each of the shift registers malfunctions.
In such cases, even when the image display area normally operates, the liquid crystal panel should be rejected as a defective on the whole. Consequently, the conventional device has a drawback in that the yield of the liquid crystal panel is reduced.
SUMMARY OF THE INVENTION
The present invention is accomplished in view of the aforementioned circumstances. Accordingly, an object of the present invention is to provide a clock signal adjusting method and circuit, which can prevent the shift registers from malfunctioning, and to provide an electro-optical device and electronic equipment, to which the clock signal adjusting method and circuit are applied. Further, another object of the present invention is to provide an electro-optical devices producing method that can enhance the yield of the electro-optical devices when such devices are produced.
To achieve such objects, according to the present invention, there is provided a method of adjusting a clock signal in an electro-optical device having a display portion, which has a plurality of scanning lines, a plurality of data lines, and pixels provided respectively corresponding to intersections between the scanning lines and the data lines, and also having a shift register that sequentially shifts a start pulse according to a clock signal and an inverted clock signal. This method can be employed in the electro-optical device adapted to generate each of signals, which are supplied to the plurality of scanning lines and the plurality of data lines, according to each output signal of the shift register, and that the phases of the clock signal and the inverted clock signal to be supplied to the shift register are adjusted. Moreover, this method of the present invention comprises the steps of detecting a threshold voltage of each of transistors of the shift register, and adjusting the relative phases of the clock signal and the inverted clock signal according to the detected threshold voltage.
The turning on and off of the transistors of the shift register are controlled according to the clock signal and the inverted clock signal. It is determined by the threshold voltage of a transistor and a voltage supplied to the control terminal thereof whether the transistor is turned on or off. When the threshold voltage has a value that is higher or lower than a target value, the timing of switching between the on and off of the transistor shifts from intended timing. Even in such a case, according to the method of the present invention, the relative phases of the clock signal and the inverted clock signal are adjusted according to the threshold voltage of the transistor. Thus, the shift register can be normally operated.
Incidentally, preferably, at the step of detecting the threshold voltage of the transistor, a threshold voltage of a test transistor produced by the same manufacturing process as that of manufacturing the transistors of the shift register is measured. Moreover, the threshold voltage of each of transistors is detected according to a result of the measurement. The threshold voltages of the transistors produced by the same manufacturing process are equal to each other. Thus, the threshold voltage of the transistor of the shift register can be known by measuring the threshold voltage of such a test transistor. The present invention eliminates the necessity for directly measuring the threshold voltage of the transistor of the shift register. The threshold voltage of the transistor of the shift register can easily be detected by placing the test transistor in such a manner as to facilitate the measurement of the threshold voltage thereof.
Further, when the transistors of the shift register are a P-channel TFT and an N-channel TFT, preferably, at the step of measuring the threshold voltage, a first threshold voltage of the P-channel TFT and a second threshold voltage of the N-channel TFT are measured. Moreover, preferably, at the step of adjusting the phases of the clock signal and the inverted clock signal, the relative phases are adjusted according to the first and second threshold voltages. Thus, the relative phases can be adjusted according to the characteristics of
Fujita Shin
Ozawa Tokuro
Eisen Alexander
Oliff & Berridg,e PLC
Seiko Epson Corporation
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