Preventing noise in a coordinate input device by reducing or suspending backlight oscillation voltage

摘要

A d.c. voltage is supplied to an inverter power source to drive a light source. Light from the light source is directed uniformly from the back side of a liquid crystal panel. When an image is displayed on the liquid crystal panel during a display period and a position detection pen is in close proximity to the liquid crystal panel at a position detection period, coordinates are calculated according to an induced voltage by the position detection pen and the timing of applying a scanning voltage to an electrode of the liquid crystal panel. The driving of the light source by the inverter power source is suspended or reduced during the position detection period, so that noise from the inverter power source is prevented from being mixed into the position detection pen.