Electromagnetic induction pen and position detecting device
An electromagnetic induction pen of an electromagnetic induction system (EMR) system that can achieve a multilevel pen signal. The electromagnetic induction pen includes a resonance circuit that transmits a pen signal by using power obtained from an alternating magnetic field sent out by a position detecting device, and a control circuit that causes the resonance circuit to transmit the pen signal phase-modulated according to information to be transmitted to the position detecting device, by temporarily changing a resonance frequency of the resonance circuit from a first frequency to a second frequency different from the first frequency based on the information and then returning the resonance frequency of the resonance circuit 23 to the first frequency.
1 . An electromagnetic induction pen comprising:
a resonance circuit that, in operation, transmits a pen signal by using power obtained from an alternating magnetic field sent out by a position detecting device, wherein the resonance circuit includes:
a node;
a first capacitor connected between the node and a grounding terminal;
a selector including a common terminal and a plurality of selection terminals, wherein the selector, in operation, receives a selection signal and selectively connects the common terminal to one of the selection terminals based on the selection signal;
a switch element connected between the node and the common terminal of the selector; and
a plurality of second capacitors, wherein each of the second capacitors is connected between a corresponding one of the selection terminals of the selector and a ground terminal; and
a control circuit that, in operation, causes the resonance circuit to transmit the pen signal phase-modulated according to information to be transmitted to the position detecting device, by supplying to the switch element a control signal that is generated based on the information, wherein the control signal causes the switch element to switch between an on state and an off state, and by generating the selection signal based on the information and supplying the selection signal to the selector, wherein the selection signal causes the selector to change a resonance frequency of the resonance circuit from a first frequency to a second frequency different from the first frequency and then return the resonance frequency of the resonance circuit to the first frequency.
2 . The electromagnetic induction pen according to claim 1 , wherein
the pen signal phase-modulated according to the information is a signal that oscillates at the first frequency.
3 . The electromagnetic induction pen according to claim 1 , wherein
the control circuit, in operation, changes the resonance frequency of the resonance circuit from the first frequency to the second frequency based on the information by changing a combined capacitance of the first capacitor and the plurality of the second capacitors from a first capacitance to a second capacitance different from the first capacitance based on the information after sending of the alternating magnetic field by the position detecting device is ended.
4 . The electromagnetic induction pen according to claim 3 , wherein
the control circuit includes a processing circuit that, in operation, controls the selection signal based on the information.
5 . The electromagnetic induction pen according to claim 4 , wherein
the resonance circuit includes a coil that is connected to the node,
the first capacitor is connected in parallel with the coil, and
the control circuit includes a modulation circuit that, in operation, changes the combined capacitance from the first capacitance to the second capacitance by turning on the switch element and changes the combined capacitance from the second capacitance to the first capacitance by turning off the switch element.
6 . The electromagnetic induction pen according to claim 5 , wherein
the modulation circuit, in operation, performs on-off control of the switch element at a timing at which the pen signal becomes 0.
7 . The electromagnetic induction pen according to claim 4 , wherein
the resonance circuit includes a coil that is connected to the node,
the resonance circuit includes a decoupling capacitor that is coupled to the node, wherein the decoupling capacitor, in operation, adds a direct-current component to an output of the resonance circuit and supplies the output to which the direct-current component is added to the processing circuit, and
the control circuit includes a modulation circuit that, in operation, changes the combined capacitance from the first capacitance to the second capacitance by turning on the switch element and changes the combined capacitance from the second capacitance to the first capacitance by turning off the switch element.
8 . The electromagnetic induction pen according to claim 3 , wherein
the control circuit includes a processing circuit that, in operation, controls a length of a period during which the combined capacitance is maintained at the second capacitance based on the information.
9 . The electromagnetic induction pen according to claim 8 , wherein
the resonance circuit includes a decoupling capacitor that is coupled to the node, wherein the decoupling capacitor, in operation, adds a direct-current component to an output of the resonance circuit and supplies the output to which the direct-current component is added to the processing circuit, and
the control circuit includes a modulation circuit that, in operation, changes the combined capacitance from the first capacitance to the second capacitance by turning off the switch element and changes the combined capacitance from the second capacitance to the first capacitance by turning on the switch element.
10 . The electromagnetic induction pen according to claim 9 , wherein
the processing circuit, in operation, controls the length of the period during which the combined capacitance is maintained at the second capacitance based on the information, by controlling a length of a period during which the modulation circuit maintains the switch element in an off state based on the information.
11 . The electromagnetic induction pen according to claim 8 , wherein
the resonance circuit includes a decoupling capacitor that is coupled to the node, wherein the decoupling capacitor, in operation, adds a direct-current component to an output of the resonance circuit and supplies the output to which the direct-current component is added to the processing circuit, and
the control circuit includes a modulation circuit that, in operation, changes the combined capacitance from the first capacitance to the second capacitance by turning on the switch element and changes the combined capacitance from the second capacitance to the first capacitance by turning off the switch element.
12 . The electromagnetic induction pen according to claim 11 , wherein
the processing circuit controls the length of the period during which the combined capacitance is maintained at the second capacitance based on the information, by controlling a length of a period during which the modulation circuit maintains the switch element in an on state based on the information.
13 . The electromagnetic induction pen according to claim 3 , wherein
the control circuit includes:
a modulation circuit that, in operation, returns the combined capacitance to the first capacitance after a predetermined time after the combined capacitance is changed to the second capacitance, and
a processing circuit that, in operation, controls a timing at which the modulation circuit changes the combined capacitance to the second capacitance based on the information.
14 . The electromagnetic induction pen according to claim 13 , wherein
the resonance circuit includes a coil that is connected to the node,
the first capacitor is connected in parallel with the coil,
the first capacitor, in operation, is connected in parallel with one of the plurality of second capacitors is connected in parallel with the first capacitor via the switch element and the selector, and
the modulation circuit, in operation,
charges the combined capacitance by turning on the switch element before the sending of the alternating magnetic field by the position detecting device is ended, and
temporarily changes the combined capacitance from the first capacitance to the second capacitance by signaling the control circuit to change the selection signal after the sending out of the alternating magnetic field by the position detecting device is ended.
15 . The electromagnetic induction pen according to claim 14 , wherein
the processing circuit, in operation, controls a timing at which the modulation circuit changes the combined capacitance to the second capacitance based on the information, by controlling a timing at which the modulation circuit signals the control circuit to change the selection signal based on the information.
16 . A position detecting device, comprising:
a plurality of loop coils; and
a sensor controller that, in operation, sends out an alternating magnetic field of a first frequency by using at least one of the loop coils, and after stopping sending out the alternating magnetic field, demodulates information transmitted by an electromagnetic induction pen, based on a phase of a signal of the first frequency detected within a pen signal detection period using a table that stores information regarding the phase of the signal in association with information indicating a plurality of bits.
17 . The position detecting device, according to claim 16 , wherein
the plurality of bits consists of at least four bits.