Capacitive sensor supporting multiple touch switches using a resistor string


Abstract: In a low-cost integrated circuit (IC) chip, the packaging cost is a considerable portion of the total manufacturing cost of the chip. As the packaging cost is directly related to the total number of I/O pins, it is highly desirable to minimize the number of I/O pins while maintaining the same functionality. We propose a capacitive sensor that supports multiple touch switches with only two IC pins. The electrode plates of the touch switches are placed at the ends of a resistor string and between every two adjacent resistors in the resistor string. The proposed method measures rise times at each side of the resistor string while driving the opposite side. The capacitive sensor can recognize an electrode plate contacted by a finger using the two rise times and the proposed rise time model. Experimental results with four touch plates show that the proposed capacitive sensor gives 181 fF resolution and can handle four touch switches.

Keywords: Capacitive sensor, capacitive touch switch, pin efficiency, resistor string

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