Capacitive touch systems for finger touch have become widely used in mobile devices such as smartphones, tablets, and so on. Beyond ordinary touch functions, some devices adopt an extra electromagnetic resonance (EMR) system [1] to support pens for advanced user experiences; these devices have been successfully commercialized for high-end devices [2]. Such devices offer a realistic and accurate pen-based drawing experience for consumers using a battreryless, light, and pressure-sensitive pen; this is possible because the EMR system excites a passive pen via magnetic coupling and senses the pen's returning signal that contains coordinate and fine pen-pressure information. As shown in Fig. 6.8.1 (top), an EMR system, however, requires an additional costly sensor board made with a flexible printed circuit board beneath a display panel to find coordinates of the EMR pen via a magnetic field and a dedicated controller, and it also consumes excessive power. If a capacitive touch system could cover the function of the EMR system, it would be a cost-effective and compact solution in terms of re-utilizing an existing system without the additional sensor board and EMR controller. In this paper, a capacitive touch system sensing a passive pen with pen pressure as well as a finger is introduced as an alternative to an EMR system.
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