Abstract:
An electromagnetic pressure-sensitive pointer is disclosed. The electromagnetic pointer comprises a pen core and an oscillating circuit. The pointer core has a pointer tip. The oscillating circuit comprises a plurality of conductive terminals and a conductor. A capacitance of the oscillating circuit is increased through the contact between the conductive terminals and the conductor as a force is applied upon the pointer tip. When pointer&#39;s tip is been pressed, the pointer&#39;s frequency will have an obvious transition while its pressure-sensitivity is maintained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The entire contents of Taiwan Patent Application No. 101145019, filed on Nov. 30, 2012, from which this application claims priority, are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a pressure-sensitive electromagnetic pointer and the control method thereof, and more particularly to an electromagnetic pointer and the control method thereof which use the writing force applied upon to suddenly alter the emitting frequency. 
     2. Description of the Related Art 
     The operation principle of electromagnetic type input technology is performed by a circuit board with a plurality of antennas or sensor coils arranged along axial directions and an electromagnetic pen which can emit electromagnetic signals. The sensing plane of an electromagnetic input apparatus includes a plurality of antennas or sensor coils. The antennas or sensor coils are arranged under a work surface or a display panel of the electromagnetic input apparatus. The coordinates of the electromagnetic pen are obtained through the transmitting and receiving of electromagnetic waves between the circuit of the electromagnetic pen and the antennas or the sensor coils. 
     Input apparatuses which use the electromagnetic type input technology comprise smart mobile devices (Smart Phone), digitizers or tablets or e-books/green books and are used with electromagnetic pens or styluses. 
     The circuit of the electromagnetic pen usually comprises an inductor, a capacitor and relative components enclosed in a case. The inductor constituted by a ferrite core winded with a metal coil and the capacitor constitute the circuit to transmit and receive electromagnetic signals with the antennas or sensor coils. The frequency of the circuit is determined according to the capacitance and the inductance connected in parallel. When a user utilizes the electromagnetic pen to perform writing function on the input apparatus, the tip of the electromagnetic pen being pressed would induce the continuous changes of the inductance or capacitance as well as the frequency of the circuit. The input apparatus detects and calculates the frequency of the electromagnetic signals received to obtain pressure levels of the electromagnetic pen via an internal circuit. 
     For example, in U.S. Pat. No. 5,206,785, the main design principle involves continuous variation of capacitance induced by pressing the pen tip. While in U.S. Pat. No. 5,565,632, the disclosure involves continuous variation of inductance caused by pressing the pen tip. The designer of the electromagnetic pen can choose either the inductor or capacitor to be variable or adjustable according to the requirement and determine the variation ranges of the capacitance or inductance. 
     Both the two principles set forth have a disadvantage. Since both principles involve continuous variations of inductance or capacitance, the frequency variations caused by the displacement of the pen core structure may not be detected or the frequency variations are very tiny such that the input apparatus is unable to detect the frequency variations when the pen tip of the electromagnetic pen is pressed by a relatively small force. These design principles are not able to allow a user to start writing just after slightly touching the pen tip. 
     In US Pub. 20110175845, the operation principle involves a structure of activating a micro switch via a pen tip. The electromagnetic pen lacks a structure of variable inductor or capacitor varying according to the displacement of the pen tip. Thus the frequency of the oscillation circuit of the electromagnetic pen is unable to vary along with the displacement of the pen tip. The electromagnetic pen with such design structure does not have a pressure-sensitive level function of pen tip. 
     In view of the above shortcomings of the previous designs, the invention provides a new electromagnetic pointer and the control method thereof. When a user utilizes the electromagnetic pointer with an input apparatus, the tip of the electromagnetic pointer with a micro force applied upon suddenly alters capacitance connected in parallel with the oscillation circuit of the electromagnetic pointer so as to discontinuously change the emitting frequency of the electromagnetic pointer at the beginning of the pressing of the tip through the pressure-sensitive level function of the tip of the electromagnetic pointer. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a pressure-sensitive electromagnetic pointer and the control method thereof which allow a user to trigger the function through a tiny force applied upon the tip of the electromagnetic pointer. The force applied on the tip of the electromagnetic pointer suddenly changes the frequency of the electromagnetic pointer to activate the writing function of the electromagnetic pointer accompanying with the tip pressure-sensitive level of electromagnetic pointer. 
     The invention provides a control method for an electromagnetic pointer, and the method comprises the following step. First of all, an electromagnetic pointer and an input apparatus are provided. The electromagnetic pointer comprises a core and an oscillation circuit. The oscillation circuit comprises a plurality of conductive terminals and a conductive device, a capacitance of the oscillation circuit is increased through the core pushing the conductive device to connect the conductive terminals. Then a tip of the core is applied against the input apparatus to push the conductive device to connect the conductive terminals and to increase the capacitance of the oscillation circuit so as to suddenly change a frequency of the oscillation circuit. The frequency from the electromagnetic pointer will discontinuously vary during the initial stage which the tip is pressed to activate the writing function of the electromagnetic pointer accompanying with the process of dynamically adjusting tip pressure-sensitive levels of electromagnetic pointer and maintaining the tip pressure-sensitive level of electromagnetic pointer. 
     The invention provides an electromagnetic pointer, and the electromagnetic pointer comprises a core having a tip and an oscillation circuit. The oscillation circuit comprises a plurality of conductive terminals and a conductive device, a capacitance of the oscillation circuit is increased through the core pushing the conductive device to connect the conductive terminals so as to suddenly change a frequency of the oscillation circuit. 
     The invention further provides a method for determining a frequency variation mode of an electromagnetic pointer, and the method comprises the following step. First of all, a frequency f n  of an electromagnetic pointer is detected. Then whether the frequency f n  is smaller than a tip-on threshold frequency f on  which a writing trace starts to appear is judged. Next whether the frequency f n  is smaller than an upper limit frequency f UB  which is designated as the frequency when a first stage of a switch of the electromagnetic pointer is turned on is judged. Then whether the frequency f n  is smaller than the lower limit frequency f LB  which is designated as the frequency when a second stage of a switch of the electromagnetic pointer is turned on is judged. 
     The invention further provides a method for dynamically adjusting tip pressure-sensitive levels of an electromagnetic pointer, and the method comprises the following step. First of all, a frequency f n  and a minimum pressure-sensitive level frequency f 1  of the electromagnetic pointer are compared. Then whether the frequency f n  larger than the minimum pressure-sensitive level frequency f 1  is judged. Next whether the frequency f n  is larger than a maximum pressure-sensitive level frequency f 2  is judged. Then the maximum pressure-sensitive level frequency f 2  is refreshed so that the frequency f n  is a new maximum pressure-sensitive level frequency if the frequency f n  is larger than the maximum pressure-sensitive level frequency f 2 . Tip pressure-sensitive levels of the electromagnetic pointer are calculated according to the new maximum pressure-sensitive level frequency and the minimum pressure-sensitive level frequency f 1 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an electromagnetic pointer according to one embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the electromagnetic pointer shown in  FIG. 1  according to one embodiment of the present invention. 
         FIG. 2A  shows a schematic diagram of a switch formed by the conductive device and the conductive terminals of the circuit board according to one embodiment of the invention. 
         FIG. 2B  shows a LC oscillation circuit of an electromagnetic pointer according to one embodiment of the invention. 
         FIG. 3  shows an equivalent circuit of a LC oscillation circuit of an electromagnetic pointer according to one embodiment of the invention. 
         FIGS. 4A and 4B  show two abrupt and discontinuous frequency variation modes of two LC oscillation circuits according to two embodiments of the invention respectively. 
         FIG. 5A  shows a method for determining a frequency variation mode of an electromagnetic pointer. 
         FIG. 5B  shows a method for dynamically adjusting tip pressure-sensitive level of an electromagnetic pointer. 
         FIG. 6  shows a relation between the frequency and the tip pressure-sensitive level of electromagnetic pointer according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment of this invention will be described in detail below. However, in addition to as described below, and this invention can be broadly implemented in the other cases the purpose and scope of this invention is not affected by the application of qualified, claim after its prevail. Furthermore, to provide a description clearer and easier to understand the invention, the pieces within the schema and not in accordance with their relative size of drawing, compared to certain dimensions to other scales have been exaggerated; details not related nor completely drawn in part in order to schematic simplicity. 
       FIG. 1  shows an electromagnetic pointer according one embodiment of the present invention. As shown in  FIG. 1 , the electromagnetic pointer  100  comprises a switch  102 , pointer cases  104   a  and  104   b  and a pointer core  106 . 
       FIG. 2  is a cross-sectional view of the electromagnetic pointer shown in  FIG. 1  according to one embodiment of the present invention. The electromagnetic pointer shown in  FIGS. 1 and 2  is merely an example of one embodiment of the invention, not a limitation. The embodiment of the invention is able to be applied any other suitable electromagnetic pointer. 
     The electromagnetic pointer  100  shown in  FIG. 2  comprises the switch  102 , the pointer cases  104   a  and  104   b , the pointer core  106 , ferrite cores  108  and  110 , a buffer  109 , an elastomer  112 , a touch pin  114 , a conductor device  116  and a circuit board  118 . The pointer cases  104   a  and  104   b  accommodate the pointer core  106 , the ferrite cores  108  and  110 , the buffer  109 , the elastomer  112 , the touch pin  114 , the conductor device  116  and the circuit board  118 . A conductive coil is configured to wind the ferrite core  108 , the conductive coil is configured to be connected to the circuit of the circuit board  118 . The conductive coil, the ferrite cores  108  and  110  constitute a variable inductor. The conductive device  116  comprises a cone spring or a conductive rubber, but is not limited to a cone spring or a conductive rubber. 
     In this embodiment of the invention, the ferrite core  108  is hollow. The pointer core  106  penetrates through the ferrite core  108  so that the pointer core  106  is able to slide through the ferrite core  108 . The ferrite core  110  includes a groove or a trench or a concave to accommodate one end of the pointer core  106 . When the pointer core  106  is moved axially inside the ferrite core  108 , the end of the pointer core  106  configured in the groove of the ferrite core  110  pushes the ferrite core  108  to generate a relative displacement between the ferrite core  110  and the ferrite core  108  with the winding coil. The elastomer  112  is located between the ferrite core  110  and the touch pin  114 . When the ferrite core  110  is pushed by the pointer core  106  to generate a relative displacement between the ferrite core  110  and the ferrite core  108  with the winding coil, the elastomer  112  will be compressed and push the touch pin  114  and the touch pin  114  will push the conductive device  116 . 
     The circuit board  118  is configured to be secured in the pointer cases  104   a  and  104   b . The end of the circuit board  118  toward the tip of the electromagnetic pointer has a plurality of conductive terminals. The conductive terminals of the circuit board  118  are electrically connected through the conductive device  116  after the touch pin  114  pushes the conductive device  116 . Detail content will be further described in the following description. The switch  102  is configured to be located on the circuit board  118  and extend out from the pointer case  104   a . The switch  102  is used as a switch of the circuit board  118 . 
     In one embodiment of the invention, the circuit board  118  comprises a printed circuit board with an oscillation circuit and a controller so that the electromagnetic pointer is able to transmit electromagnetic signals to an electromagnetic input apparatus, or to receive electromagnetic signals from electromagnetic antenna loops or sensor coils of the electromagnetic input apparatus. The electromagnetic input apparatus comprises a smart mobile communication device, a digitizer or a tablet or an eBook. 
       FIG. 2A  shows a schematic diagram of a switch formed by the conductive device and the conductive terminals of the circuit board according to one embodiment of the invention. In one embodiment which the conductive device  116  is a cone spring, when one end of the touch pin  114  pushes the cone spring, the center of the cone spring will contact the conductive terminals on one end of the circuit board  118  to electrically connect the conductive terminals and change the circuitry of the circuit board  118  so as to alter the frequency of the LC oscillation circuit. Detail content will be further described in the following description. The arrangement with a distance between the cone spring and the conductive terminals is just for clearer and easier to understand. In actual arrangement, the cone spring is configured to contact the conductive terminals while only the center portion of the cone spring maintains a distance from the central conductive terminal  119  when the touch pin  114  does not push the cone spring. 
       FIG. 2B  shows a LC oscillation circuit of an electromagnetic pointer according to one embodiment of the invention. CL 1  and CL 2  are designated as two terminals of the equivalent inductor of the LC oscillation circuit. Whether capacitors VC 1 , C 1 , TC 1  and J 1  are connected with capacitors MC 1 -MC 7  and M 1 -M 5  in parallel is determined via the connection between conductive terminal SL 1 , SL 2  and SL 3 . Referring to  FIG. 2A , the conductive terminal SL 2  is equivalent to the central conductive terminal  119 . When the center portion of the cone spring contacts the central conductive terminal  119 , which is equivalent to the conductive terminal SL 2  connecting the conductive terminal SL 1  and SL 3 , the capacitors VC 1 , C 1 , TC 1  and J 1  will connect with capacitors MC 1 -MC 7  and M 1 -M 5  in parallel. According to the equation, 
     
       
         
           
             f 
             = 
             
               1 
               
                 2 
                 ⁢ 
                 π 
                 ⁢ 
                 
                   LC 
                 
               
             
           
         
       
     
     The frequency f will be decreased when the capacitor C is increased, wherein L is the inductance. 
     Switch SW 1  shown in  FIG. 2B  is equivalent to the switch on the circuit board  118  activated by the switch  102 . Whether capacitors SC 1 , SC 2  and JS 1  are connected with capacitors MC 1 -MC 7  and M 1 -M 5  in parallel is determined via the switch SW 1  to vary the frequency of the oscillation circuit of the electromagnetic pointer so as to perform functions of the electromagnetic pointer. The functions comprise simulating mouse left and right click switches, a writing function switch or an erasing (eraser) function switch. 
       FIG. 3  shows an equivalent circuit of a LC oscillation circuit of an electromagnetic pointer according to one embodiment of the invention. It is noted that the equivalent circuit in  FIG. 3  is not entirely the same with the LC oscillation circuit in  FIG. 2B , but both are used to illustrate the embodiments of the invention. Inductor L is designated as the equivalent inductance of the LC oscillation circuit, capacitor C 1  is designated as the equivalent capacitance of LC oscillation circuit and capacitor C tip  is controlled by switch S. In one embodiment of the invention, in connection with the embodiment shown in  FIG. 2A , the switch S comprising the conductive device  116  and the conductive terminal  119  is activated to connect the capacitor C tip  with the capacitor C 1  in parallel, the parallel connection between the capacitor C tip  and the capacitor C 1  will increase the new equivalent capacitance and cause a sudden or abrupt decrease of the frequency of the LC oscillation circuit. In connection with the embodiment shown in  FIG. 2 , when the conductive device  116  is pushed by the touch pin  114  to connect the conductive terminals of the circuit board  118 , the gradual variation of the frequency of the LC oscillation circuit will suddenly be altered to an abrupt drop. The abrupt drop of frequency variation is able to be used as a signal to notify an input apparatus to activate the writing function of the electromagnetic pointer. 
       FIGS. 4A and 4B  show two abrupt and discontinuous frequency variation modes of two LC oscillation circuits according to two embodiments of the invention respectively. The discontinuous frequency variation mode of the LC oscillation circuit shown in  FIG. 4A  involves a gradual decrease of an equivalent inductance of the LC oscillation circuit and a gradual increase of a frequency of the LC oscillation circuit when the tip or pointer core of the electromagnetic pointer is being pressed. As shown in  FIG. 4A , as the tip or of the electromagnetic pointer continues to be pressed, the equivalent inductance of the LC oscillation circuit continues to decrease and the frequency continues to increase until an abrupt and discontinuous frequency variation occurs. One embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 2A , wherein the touch pin  114  pushes the conductive device  116  to connect the conductive terminals of the circuit board  118  to connect an additional capacitor in parallel with the LC oscillation circuit. Another embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 2B , wherein the capacitors VC 1 , C 1 , TC 1  and J 1  are connected with the capacitors MC 1 -MC 7  and M 1 -M 5  in parallel via the connection between the conductive terminal SL 1 , SL 2  and SL 3 . Still another embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 3 , wherein the capacitor C tip  is connected with the capacitor C 1  in parallel by the switch S to increase the new equivalent capacitance and cause a sudden or abrupt decrease of the frequency of the LC oscillation circuit. 
     When the frequency of the electromagnetic pointer is lower than a tip-on threshold frequency f on  on which a writing trace starts to show, a writing function of the electromagnetic pointer is activated. As the frequency of the LC oscillation circuit of the electromagnetic pointer achieves a minimum pressure-sensitive level frequency f 1 , the electromagnetic pointer is able to draw the minimum width of writing trace. As the tip of the electromagnetic pointer is continued being pressed, the equivalent inductance of the LC oscillation circuit continue to decrease to gradually increase the frequency of the LC oscillation circuit so that the width of writing trace will also gradually increase until the frequency of the LC oscillation circuit achieves a maximum pressure-sensitive level frequency f 2 . It is noted that the adjustment of the pressure-sensitive level of the tip of the electromagnetic pointer is dynamic. In one embodiment of the invention, the adjustment range of the pressure-sensitive level of the tip of the electromagnetic pointer is between the tip-on threshold frequency f on  and an upper limit frequency f UB  of the switch  102 . The upper limit frequency f UB  and a lower limit frequency f LB  of the switch  102  depend on the circuit design of the electromagnetic pointer. 
     The discontinuous frequency variation mode of the LC oscillation circuit shown in  FIG. 4B  involves a gradual increase of an equivalent inductance of the LC oscillation circuit and a gradual decrease of a frequency of the LC oscillation circuit when the tip or pointer core of the electromagnetic pointer is being pressed. As shown in  FIG. 4B , as the tip or of the electromagnetic pointer continues to be pressed, the equivalent inductance of the LC oscillation circuit continues to increase and the frequency continues to decrease until an abrupt and discontinuous frequency variation occurs. One embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 2A , wherein the touch pin  114  pushes the conductive device  116  to connect the conductive terminals of the circuit board  118  to connect an additional capacitor in parallel with the LC oscillation circuit. Another embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 2B , wherein the capacitors VC 1 , C 1 , TC 1  and J 1  are connected with the capacitors MC 1 -MC 7  and M 1 -M 5  in parallel via the connection between the conductive terminal SL 1 , SL 2  and SL 3 . Still another embodiment triggering the abrupt and discontinuous frequency variation is shown in  FIG. 3 , wherein the capacitor C tip  is connected with the capacitor C 1  in parallel by the switch S to increase the new equivalent capacitance and cause a sudden or abrupt decrease of the frequency of the LC oscillation circuit. 
     When the frequency of the electromagnetic pointer is lower than a tip-on threshold frequency f on  on which a writing trace starts to show, a writing function of the electromagnetic pointer is activated. As the frequency of the LC oscillation circuit of the electromagnetic pointer achieves a minimum pressure-sensitive level frequency f 1 , the electromagnetic pointer is able to draw the minimum width of writing trace. As the tip of the electromagnetic pointer is continued being pressed, the equivalent inductance of the LC oscillation circuit continue to increase to gradually decrease the frequency of the LC oscillation circuit so that the width of writing trace will also gradually increase until the frequency of the LC oscillation circuit achieves a maximum pressure-sensitive level frequency f 2 . It is noted that the adjustment of the pressure-sensitive level of the tip of the electromagnetic pointer is dynamic. In one embodiment of the invention, the adjustment range of the pressure-sensitive level of the tip of the electromagnetic pointer is between the tip-on threshold frequency f on  and an upper limit frequency f UB  of a side switch such as the switch  102 . The upper limit frequency f UB  and the lower limit frequency f LB  are designated as the frequencies when first and second stages of the switch of the electromagnetic pointer are turned on respectively. 
     Whether the equivalent inductance of the LC oscillation circuit is decreased or increased when the tip of the electromagnetic pointer is pressed depends the configuration of the ferrite core and the winding coil and the internal structure of the electromagnetic pointer. It is noted that the embodiments of the invention focus on the function of the electromagnetic pointer rather than the configuration thereof. Thus the function of the embodiments of the invention is able to be applied upon any suitable configuration of the electromagnetic pointer according to various requirements. The configuration to which the function of the embodiments of the invention is applied depends on requirement. 
       FIGS. 5A and 5B  show a method for determining a frequency variation mode of electromagnetic pointer and a method for dynamically adjusting tip pressure-sensitive level of electromagnetic pointer respectively. As shown in  FIG. 5A , first of all, a frequency f n  of an electromagnetic pointer is detected in step  502 . Then whether the frequency f n  is smaller than the tip-on threshold frequency f on  is judged in step  504 . If the frequency f n  is not smaller than the tip-on threshold frequency f on , then a status of the electromagnetic pointer that a switch is not turned on is determined in step  506 . If the frequency f n  is smaller than the tip-on threshold frequency f on , then whether the frequency f n  is smaller than the upper limit frequency f on  is judged in step  508 . If the frequency f n  is not smaller than the upper limit frequency f UB , then a status of the electromagnetic pointer that the tip is being pressed (tip on) is determined in step  510  and a process for dynamically adjusting tip pressure-sensitive level of electromagnetic pointer is performed in step  512 . If the frequency f n  is smaller than the upper limit frequency f UB , then whether the frequency f n  is smaller than the lower limit frequency f LB  is judged in step in step  514 . If the frequency f n  is not smaller than the lower limit frequency f LB , then a status of the electromagnetic pointer that a first stage of the switch is turned on is determined in step  516 . If the frequency f n  is smaller than the lower limit frequency f LB , then a status of the electromagnetic pointer that a second stage of the switch is turned on is determined in step  518 . 
       FIG. 5B  shows the method for dynamically adjusting tip pressure-sensitive levels of an electromagnetic pointer corresponding to the embodiment shown in  FIG. 4A . As shown in  FIG. 5B , the frequency f n  and the minimum pressure-sensitive level frequency f 1  are compared in step  519 . Then whether the frequency f n  is larger than the minimum pressure-sensitive level frequency f 1  is judged in step  520 . If f n  is not larger than f 1 , then f 1  which the electromagnetic pointer is able to draw the minimum width of writing trace is refreshed in step  522  such that the refreshed f 1  is equal to f n , or f 1 =f n . Then the tip pressure-sensitive level of electromagnetic pointer is calculated according to the refreshed f 1  and the maximum pressure-sensitive level frequency f 2  in step  524 . If f n  is larger than f 1 , then whether the frequency f n  is larger than the maximum pressure-sensitive level frequency f 2  is judged in step  526 . If f n  is not larger than f 2 , then f 1  and f 2  are not refreshed in step  528 . The tip pressure-sensitive level of electromagnetic pointer is calculated according to f 1  and f 2  in step  530 . If f n  is larger than f 2 , then f 2  which the electromagnetic pointer is able to draw the maximum width of writing trace is refreshed in step  532  such that the refreshed f 2  is equal to f n , or f 2 =f n . Then the tip pressure-sensitive level of electromagnetic pointer is calculated according to the refreshed f 2  and f 1  in step  534 . 
       FIG. 6  shows a relation between the frequency and the tip pressure-sensitive level of an electromagnetic pointer according to one embodiment of the invention. As shown in  FIG. 6 , the minimum pressure-sensitive level frequency f 1  and the maximum pressure-sensitive level frequency f 2  are dynamically refreshed while f n  is the current frequency of electromagnetic pointer. The minimum pressure-sensitive level frequency f 1  and the maximum pressure-sensitive level frequency f 2  correspond to the minimum pressure-sensitive level  1  and the maximum pressure-sensitive level  4096  respectively and f n  corresponds to the current pressure-sensitive level for example. If f 1  is 461538 Hz and f 2  is 470588 Hz and the number of total pressure-sensitive levels is 4096, then the current pressure-sensitive level can be calculated by the following equation,
 
pressure-sensitive level=4095*[( f   n −461538)/(470588−451538)].
 
     The invention provides an electromagnetic pointer and the control method thereof which allow an user to trigger the function through a tiny force applied upon the tip of the electromagnetic pointer. The force applied on the tip of the electromagnetic pointer alters the equivalent capacitance of the oscillation circuit to suddenly change the frequency of the electromagnetic pointer. The frequency from the electromagnetic pointer will discontinuously vary during the initial stage which the tip is pressed to activate the writing function of the electromagnetic pointer accompanying with the tip pressure-sensitive level of electromagnetic pointer. 
     Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments; but only by the scope of the appended claims.