Drive current calibration for an analog resistive touch screen

摘要

An analog resistive touch screen is powered by a current source responsive to a stored digital control value. The current source is calibrated by comparing the excitation voltage across the touch screen to a desired voltage to produce a comparison signal, and adjusting the digital control value in response to the comparison signal so that the excitation voltage becomes substantially equal to the desired voltage. The power supply can therefore be easily constructed in a low-voltage CMOS integrated circuit having a minimal power consumption without sacrificing touch screen resolution. Preferably the comparison of the excitation voltage to the reference voltage is performed by an analog-to-digital converter that is later used in a normal conversion mode for digitizing the touch screen coordinates. Moreover, because the touch screen presents a different load resistance depending on whether a first resistive sheet is energized for measuring an X coordinate or a second resistive sheet is energized for measuring a Y coordinate, a separate calibration is performed for each of the two resistive sheets, and the digital control value from each calibration is stored in a separate register for selective retrieval to measure either an X coordinate or a Y coordinate.