Wireless coordinate reader and switch state detection system for coordinate indicator

Claim 1

A wireless coordinate reader comprising: (a) a sense line plate having a plurality of excitation lines and a plurality of sense lines arranged thereon; (b) excitation signal delivery means for delivering an alternating excitation signal to a selected excitation line; (c) a coordinate indicator including a resonant circuit having a resonance frequency value selected to cause said resonant circuit to induce a signal in at least one sense line in response to an excitation signal in an excitation line and composed of a coil and a first capacitor, and a switch circuit connected in parallel with said resonant circuit, said switch circuit comprising at least one series circuit composed of a switch and a resistor, and a second capacitor connected in series with one end of said series circuit; (d) an amplification detection circuit for detecting the magnitude of an induction signal which is induced on a selected sense line when the excitation signal is applied to a selected excitation line and said coordinate indicator is placed above said sense line plate at a location at least approximately above the selected lines and for outputting an amplitude signal representative of the magnitude of the induction signal; (e) a phase detection circuit for receiving a signal derived from the excitation signal and the induction signal induced on the selected sense line for outputting a phase signal representing the phase of said induction signal relative to the phase of said excitation signal; and (f) a controller connected for receiving said amplitude signal and said phase signal, correcting height data extracted from said amplitude signal by coordinate data calculated on the basis of said amplitude signal, correcting said phase signal on the basis of the corrected height data, and determining the state of said switch from the corrected phase signal.