Abstract: In a coordinate detection period, a first electrode drive circuit sequentially selects source electrodes Sn of the TFT LCD panel to connect the same to an x-signal current amplifying circuit, and to apply an AC voltage to the non-selected electrodes. The x-signal current amplifying circuit detects a detection current flowing through the selected electrode, the magnitude of which corresponds to a distance to a finger (or a conductor pen), to obtain an x-coordinate detection voltage. A second electrode drive circuit and a y-signal current amplifying circuit operate in similar fashion to obtain a y-coordinate detection voltage. A coordinate detection circuit then obtains the x- and y-coordinate values of the designated position based on the x-coordinate detection voltage and the y-coordinate detection voltage, thus enabling pointing by a codeless pen and a finger.

Abstract: A coordinate input device includes (1) a pen switch sensing circuit for providing a power supply to a pen switch in a position sensing pen, and for sensing that the pen switch is turned on and off and (2) a pen switch control circuit for controlling a timing of the power supply. The power supply is provided to the pen switch only when coordinate sensing operations are not affected. The power supply is stopped in order to avoid a voltage change happening to the pen switch when the coordinate sensing operation is affected. Therefore, the pen switch is neither turned on nor turned off during a coordinate sensing period. Consequently, it is possible to avoid a wrong sensing of a coordinate indicated with the position sensing pen.

Abstract: During a display period (Y-coordinate detection period), an image is written to an electro-optical material so as to form a display by driving row electrodes and column electrodes by using a row-electrode driving circuit and a column-electrode driving circuit so as to selectively operate TFTs. In this case, a shift data signal having a width corresponding to two clocks of a horizontal sync signal is supplied to the row-electrode driving circuit by using a start-pulse-width expanding circuit; therefore, two row-electrode scanning signals are simultaneously inputted to two of the corresponding row electrodes. Moreover, the row-electrode scanning signals have periods in which they instantaneously turn "off" respectively, and the turning-off timing is coincident in the respective signals.

Abstract: An induction voltage signal induced at a detection electrode of a detection pen is binarized by a slice voltage "e.sub.s ". The number of pulses existing in the binary signal is counted by a counting device. The resulting count value obtained by the counting device and the number of times of electrode scanning in an x-coordinate detection period or a y-coordinate detection period are compared with each other by means of a comparator. When the number of pulses is greater, an error detection alarm signal is output by an alarm device. Thus the existence of the pulses based on the error detection signal is detected in the binary signal based on the induction voltage signal from the detection electrode to output the error detection alarm signal. The above arrangement allows the coordinates at the tip end of the detection pen to be detected in a stable and correct manner even when external noise is detected.