An Efficient Pulse Width Measurement Method using Multiphase Clock Signals for Capacitive Touch Switches

Abstract

-We propose an efficient method to measure a pulse width using multiphase clock signals generated from a ring oscillator. These clocks, which have the same frequency and are evenly spaced, give multiple rising edges within a clock cycle. Thus, it is possible to measure a pulse width more accurately than with existing single clock-based methods. The proposed method is applied to a capacitive touch switch. Experimental results show that the capacitive touch switch with the proposed method gives a 118 fF resolution, which is 6.4 times higher than that of the touch switch with a single clock-based pulse width measurement method.

Capacitive sensor supporting multiple touch switches using a resistor string

“…The efficiency of a pin is defined as the inverse of the average number of pins needed to implement a functional block. The pin efficiency of commercial or previous capacitive touch switches [1,10] is less than or equal to one.…”

“…Capacitive touch switches are widely used in many electronic devices such as MP3 players, TVs, computers, lamps, and wall switches because they give a soft touch feeling and allow the design of slim products [1].…”

“…A simple method to measure rise time in digital form is to use a digital counter [10]. A more accurate rise time measurement based on multiphase clock signals is proposed in [1].…”

“…A capacitive touch switch measures the capacitance induced between a finger and an electrode plate, and then determines if the finger touched the plate [10]. A simple but effective way to measure capacitance is to measure the rise time of the voltage across the capacitance while it is being charged [1,10,11].…”

“…A capacitive touch switch is an application of a capacitive sensor that converts capacitance to voltage [2][3][4][5], frequency [6,7], or time [1,[8][9][10][11] and then measures the converted values. A capacitive touch switch measures the capacitance induced between a finger and an electrode plate, and then determines if the finger touched the plate [10].…”