Patent Publication Number: US-2007111765-A1

Title: Portable electronic apparatus with azimuth measuring function, magnetic sensor suitable for the apparatus, and azimuth measuring method for the apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a Continuation of application Ser. No. 10/190,525 filed on Jul. 9, 2002 and is based on Japanese Patent Application No. 2002-115250, filed on Apr. 17, 2002, and Japanese Patent Application No. 2001-210053, filed on Jul. 10, 2001, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      A) Field of the Invention  
      The present invention relates to a portable electronic apparatus having a communication device with permanent magnets and a direction (azimuth) measuring device, to a magnetic sensor unit suitable for the apparatus, and to a direction measuring method for the apparatus.  
      B) Description of the Related Art  
      A magnetic sensor unit is known which detects geomagnetism and measures a direction. Recent studies are directed to adding a navigation function to a portable electronic apparatus typically a portable phone having the communication device and the magnetic sensor unit capable of detecting geomagnetism, the communication device including a speaker, a microphone, a transceiver circuit, a display device and the like.  
      The communication device including a speaker, a microphone, a display device and the like has permanent magnets. The magnetic sensor unit outputs a signal corresponding to the synthesized magnetic field of geomagnetism and a magnetic field of permanent magnets. There arises therefore a problem that the direction determined from the signal output from the magnetic sensor unit is not precise. The magnetic field of a permanent magnet changes with the temperature of the magnet. If the signal of the magnetic sensor unit is corrected only by the influence of the magnetic field of the magnet detected at one temperature on the signal of the magnetic sensor, and the direction is determined from the corrected signal, the determined direction is not correct when the temperature of the magnet changes.  
     SUMMARY OF THE INVENTION  
      An object of this invention is to provide a portable electronic apparatus capable of measuring a direction at high precision even if the temperature of permanent magnets changes, and a magnetic sensor unit suitable for the apparatus.  
      Another object of the invention is to provide a direction measuring method capable of measuring a direction at high precision by estimating the influence of the magnetic field of permanent magnets of the apparatus upon the magnetic sensor unit with simple user operations.  
      According to one aspect of the present invention, there is provided a portable electronic apparatus comprising: a casing; a communication device accommodated in the casing and having permanent magnets; and a direction measuring device accommodated in the casing for measuring a direction by utilizing geomagnetism, wherein the direction measuring device comprises: magnetic sensors for outputting signals corresponding to an external magnetic field; a temperature sensor for detecting a temperature; a corrector for estimating influence of the magnetic field of the permanent magnets upon the signals output from the magnetic sensors in accordance with the detected temperature, and correcting the signals output from the magnetic sensors in accordance with the estimated influence; and a direction determining device for determining a direction in accordance with the corrected signals. The detected temperature corresponds to the temperature of permanent magnets. Detecting the temperature also includes estimating the temperature.  
      The influence of the magnetic field of the permanent magnets upon the outputs of the magnetic sensors is estimated from the detected temperature. The outputs of the magnet sensor unit are corrected by the estimated influence. The direction is determined from the corrected outputs of the magnetic sensors. Accordingly the direction can be measured and determined at high precision even if the temperature of the permanent magnets changes and the influence of the magnetic field of the permanent magnets upon the outputs of the magnetic sensor unit changes.  
      The influence of the magnetic field of the permanent magnets upon the outputs of the magnetic sensors can be estimated, for example, in the following manner. First, the portable electronic apparatus is placed on a desk and signals output from the magnetic sensor unit are measured as first values. Next, signals output from the magnetic sensor unit are measured as second values in the state that the portable electronic apparatus is rotated by 180° on the desk. A sum of the first and second values is divided by 2 (an average of the first and second values is obtained). This estimation requires the user to rotate the portable electronic phone by 180° on the desk and perform other operations. These operations are cumbersome for the user so that the number of such operations is desired as small as possible.  
      It is preferable that the correcting means measures at a first temperature and a second temperature different from the first temperature the influence of the magnetic field of the permanent magnets contained in the signals output from the magnetic sensors, and estimates the influence of the magnetic field of the permanent magnets from the influences at the first and second temperatures, and the present temperature detected with the temperature sensor.  
      By measuring the influences of the magnetic field of the permanent magnets at the first and second temperatures, the influence at another temperature can be estimated. Accordingly, the direction can be measured at high precision while the number of operations to be performed by the user for the influence estimation is reduced. According to experiments, the magnetic field of the permanent magnets of a portable electronic apparatus is approximately in proportion to the temperature of the permanent magnets. Accordingly, the influence at the temperature of the present time can be estimated easily through linear interpolation or extrapolation of the influences at the first and second temperatures relative to the temperature.  
      Measurements of the influence of the magnetic field of the permanent magnets inevitably contain a measurement error. Therefore, if a difference between the first and second temperatures is too small when the influence of the magnetic field of the permanent magnets at another temperature is estimated from the influences of the magnetic field of the permanent magnets at the first and second temperatures, there is a fear that the measurement error of the influence at each temperature may greatly degrade the estimation precision of the influence at another temperature.  
      To avoid this, the corrector is preferably provided with an initialization prompting device for prompting a user of the portable electronic apparatus to perform an operation of acquiring the influence at the second temperature when a difference between the first temperature and a temperature detected with the temperature sensor after measuring the influence at the first temperature becomes a predetermined temperature or higher. This initialization prompting device may be a device for displaying such effect on the display unit of the portable electronic apparatus or a device for producing sounds of a message of such effect from a sound producing device of the portable electronic apparatus.  
      If the influences at the first and second temperatures are acquired in the above manner, it is possible to prevent the measurement error contained in the measurements of the influences from greatly degrading the estimation precision of the influence at another temperature. Since the user is notified the time when the influence is measured at the second temperature, it is possible to avoid unnecessary initialization operations.  
      According to another aspect of the present invention, there is provided a magnetic sensor comprising: a substrate; a magnetic sensor element formed on the substrate for outputting a signal corresponding to the direction and amplitude of an external magnetic field; and a temperature sensor formed on the substrate for sensing a temperature.  
      It is possible to provide a magnetic sensor which is compact, inexpensive and capable of compensating the influence of the magnetic field of the permanent magnet upon the direction measurement relative to the temperature of the permanent magnets, and is suitable for the portable electronic apparatus having permanent magnets.  
      It is preferable that the magnetic sensor includes a plurality of magnetic sensor elements and that the magnetic sensor element is a magnetoresistive effect element having a pinned layer with a fixed magnetization direction and a free layer whose magnetization direction changes with the external magnetic field and the magnetic sensor element changes its resistance value in accordance with an angle between the magnetization direction of the pinned layer and the magnetization direction of the free layer, and that the magnetization directions of the pinned layers of at least two elements among the plurality magnetoresistive effect elements are crossed.  
      A magnetic sensor capable of measuring a direction at high precision can therefore be provided by using a giant magnetoresistive effect (GMR) element or a magnetic tunneling effect (TMR) element.  
      It is also preferable that the magnetic sensor includes a digital signal processing circuit formed on the same substrate.  
      It is possible to provide a magnetic sensor which is more compact, capable of processing signals in the form of digital signals, and is suitable for the portable electronic apparatus.  
      According to a further aspect of the present invention, there is provided a direction measuring method comprising steps of: preparing a portable electronic apparatus comprising a casing having a first plane, a communication device accommodated in the casing and having permanent magnets, magnetic sensors accommodated in the casing and outputting signals corresponding to an external magnetic field, and an input device formed on the first plane for inputting an operation signal; measuring signals output from the magnetic sensors as first values when the operation signal is input, in a state that the first plane of the portable electronic apparatus is turned upside; measuring signals output from the magnetic sensors as second values when the operation signal is input, in a state that the first plane of the portable electronic apparatus is turned upside and the portable electronic apparatus is rotated by 180° after the first values are measured; estimating an influence of a magnetic field by the permanent magnets upon the signals output from the magnetic sensors in accordance with the first and second values; correcting the signals output from the magnetic sensors in accordance with the estimated influence; and determining a direction in accordance with the corrected signals of the magnetic sensors.  
      The geomagnetism of the same amplitude and opposite directions is applied to the magnetic sensors before and after the portable electronic apparatus is rotated by 180°. Therefore, each sum of the outputs of the magnetic sensors before and after the portable electronic apparatus is rotated by 180° is independent from the geomagnetism, and corresponds to the influence of the magnetic field of the permanent magnets upon the outputs of the magnetic sensors. By using this sum, the influence of the magnetic field of the permanent magnets can be estimated easily and at high precision. By determining the direction in the above manner, the direction can be measured easily and at high precision. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a front view of a portable phone having a magnetic sensor unit according to the invention.  
       FIG. 2  is a block diagram showing the structure of electronic circuits of the portable phone shown in  FIG. 1 .  
       FIG. 3  is a plan view (component layout) of the magnetic sensor unit shown in  FIG. 2 .  
       FIG. 4  is a graph showing the output characteristics of the X-axis components of an external magnetic field H detected with an X-axis magnetic sensor shown in  FIG. 2 .  
       FIG. 5  is a graph showing the output characteristics of the Y-axis components of the external magnetic field H detected with a Y-axis magnetic sensor shown in  FIG. 2 .  
       FIG. 6  is an equivalent circuit diagram of the X-axis magnetic sensor shown in  FIG. 2 .  
       FIG. 7  is a schematic plan view of a first magnetic tunneling effect element group shown in  FIG. 6 .  
       FIG. 8  is a schematic cross sectional view of the first magnetic tunneling effect element group shown in  FIG. 7  and taken along line  1 - 1  shown in  FIG. 7 .  
       FIG. 9  is a schematic partial plan view of the first magnetic tunneling effect element group shown in  FIG. 7 .  
       FIG. 10  is a graph showing the resistance change characteristics relative to an external magnetic field of the first magnetic tunneling effect element group shown in  FIG. 7 .  
       FIG. 11  is a diagram showing the positional relation between the X-axis magnetic sensor and Y-axis magnetic sensor shown in  FIG. 2  and their electrical connection.  
       FIG. 12  is an equivalent circuit of a temperature sensor shown in  FIG. 2 .  
       FIG. 13  is a graph showing outputs of the X-axis magnetic sensor and Y-axis magnetic sensor shown in  FIG. 2  relative to the direction.  
      FIGS.  14  to  16  are graphs showing the temperature characteristics of magnetic fields of different permanent magnets in the portable phone shown in  FIG. 1 .  
       FIG. 17  is a graph showing the output characteristics of the X-axis magnetic sensor shown in  FIG. 2  relative to geomagnetism.  
       FIG. 18  is a graph showing the output characteristics of the Y-axis magnetic sensor shown in  FIG. 2  relative to geomagnetism.  
       FIG. 19  is a vector diagram showing the relation between geomagnetism and a leak magnetic field of permanent magnets applied to the magnetic sensor unit shown in  FIG. 2 .  
      FIGS.  20  to  23  are flow charts illustrating routines to be executed by CPU shown in  FIG. 2 .  
       FIG. 24  is a circuit diagram showing another example of the temperature sensor.  
       FIG. 25  is a graph showing the temperature characteristics of the circuit shown in  FIG. 24 .  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      A portable electronic apparatus according to an embodiment of the invention will be described with reference to the accompanying drawings, by using a portable phone as an example of the portable electronic apparatus. As shown in the schematic plan view of  FIG. 1 , the portable phone  10  has a casing  11 , an antenna unit  12 , a speaker unit  13 , a liquid crystal display unit  14 , an operation unit (operation signal input unit)  15  and a microphone unit  16 . The casing  11  is generally rectangular having sides extending along orthogonal X- and Y-axes as viewed in plan in  FIG. 1 . The antenna unit  12  is disposed at the upper right or left side of the casing  11 . The speaker unit  13  is disposed at the uppermost front side of the casing  11 . The liquid crystal display unit  14  is disposed at the front side of the casing  11  under the speaker unit  13 . The liquid crystal display unit  14  is used for displaying characters and graphic. The operation unit  15  is disposed at the front side of the casing  11  under the liquid crystal display unit  14 . The operation unit  15  is used for entering a telephone number and other command signals. The microphone unit  16  is disposed at the lowermost front side of the casing  11 . Some or all of the antenna unit  12 , speaker unit  13 , liquid crystal display unit  14 , operation unit  15  and microphone unit  16  constitute a communication device including permanent magnets.  
       FIG. 2  is a block diagram showing the outline of electronic circuits of the portable phone  10 . The portable phone  10  has a CPU  21 , a ROM  22 , a RAM  23  and a nonvolatile RAM  24  interconnected by a bus. CPU  21  executes various programs stored in ROM  22 . RAM  23  temporarily stores data and the like necessary for CPU  21  to execute programs. Data is written in the nonvolatile RAM  24  in response to an instruction from CPU  21  while the main power source of the portable phone  10  is turned on, and this written data is stored and retained even during the turn-off period of the main power source. In response to a request from CPU  21  during the turn-on period of the main power source, the retained data is supplied to CPU  21 . The nonvolatile RAM  24  may be replaced by an EEPROM.  
      The antenna unit  12  has a transceiver antenna  12   a , a transceiver circuit  12   b  connected to the antenna  12   a , and a modem circuit  12   c  connected to the transceiver circuit  12   b . The modem circuit  12   c  demodulates a signal received by the transceiver circuit  12   b , and modulates a signal to be transmitted and supplies it to the transceiver circuit  12   b . The speaker unit  13  has a speaker  13   a  including a permanent magnet and a sound generator circuit  13   b  connected to the speaker  13   a  for generating a signal which is supplied to the speaker  13   a  to reproduce a corresponding sound. The liquid crystal display unit  14  has a liquid crystal display panel  14   a  and a display circuit  14   b  connected to the liquid crystal display panel  14   a . The liquid crystal display panel  14   a  is disposed at the front side of the casing  11  of the portable phone  10 . The display circuit  14   b  generates a signal which is supplied to the liquid crystal display panel  14   a  to display corresponding data. The operation unit  15  has a plurality of push buttons  15   a  and a detector circuit  15   b  connected to the push buttons  15   a  for detecting an on/off state of each push button  15   a . The microphone unit  16  has a microphone  16   a  and an amplifier circuit  16   b  connected to the microphone  16   a  for amplifying a sound signal input from the microphone  16   a . Of these units, the modem circuit  12   c , sound generator circuit  13   b , display circuit  14   b , detector circuit  15   b  and amplifier circuit  16   b  are connected via the bus to CPU  21  and controlled by CPU  21 .  
      The portable phone  10  has also a magnetic sensor unit  30  for outputting a signal corresponding to the direction and amplitude of an external magnetic field. The magnetic sensor unit  30  has an X-axis magnetic sensor  31 , a Y-axis magnetic sensor  32 , a temperature sensor  33 , and a control circuit (digital signal processor)  34 . As shown in  FIG. 3  which is a schematic plan view of the magnetic sensor unit  30 , these X-axis magnetic sensor  31 , Y-axis magnetic sensor  32 , temperature sensor  33  and control circuit  34  as well as a plurality of pads  35  are formed on a single chip of generally a square shape. The magnetic sensor unit  30  is held in the portable phone  10  generally in parallel to the plane (front side of the casing) of the liquid crystal display panel  14   a  as indicated by broken lines in  FIG. 1 .  
      Reverting to  FIG. 2 , the control circuit  34  has an A/D converter (ADC)  34   a  and a d.c. constant voltage circuit  34   b . The control circuit  34  has a function of processing the signals output from the X-axis magnetic sensor  31 , Y-axis magnetic sensor  32  and temperature sensor  33  and outputting digital signals. The A/D converter  34   a  is connected via the bus to CPU  21 . The A/D converter  34   a  A/D converts the signals output from the X-axis magnetic sensor  31 , Y-axis magnetic sensor  32  and temperature sensor  33  connected to the A/D converter  34   a  and supplies A/D converted digital data to CPU  21 . The d.c. constant voltage circuit  34   b  supplies a constant voltage to the X-axis magnetic sensor  31 , Y-axis magnetic sensor  32  and temperature sensor  33  connected to the d.c. constant voltage circuit  34   b.    
       FIG. 4  is a graph showing the relation between the X-axis components of an external magnetic field H and an output Sx of the X-axis magnetic sensor, and  FIG. 5  is a graph showing the relation between the Y-axis components of the external magnetic field H and an output Sy of the Y-axis magnetic sensor. The X-axis magnetic sensor  31  in the state mounted on the portable phone  10  outputs a signal value proportional to the X-axis components of the external magnetic field. Similarly, the Y-axis magnetic sensor  32  in the state mounted on the portable phone  10  outputs a signal value proportional to the Y-axis components of the external magnetic field. The X-axis magnetic sensor  31  and Y-axis magnetic sensor  32  have the same structure that a signal value proportional to the amplitude of a magnetic field along each predetermined direction is output, and are disposed on the single chip of the magnetic sensor unit  30  in such a manner that the predetermined directions (magnetic field detection directions) are perpendicular. The magnetic sensor unit  30  is mounted on the portable phone  10  in such a manner that the X-axis magnetic sensor  31  and Y-axis magnetic sensor  32  output signal values proportional to the magnetic field amplitudes along the directions in parallel to the X- and Y-axes of the casing  11  of the portable phone  10 .  
       FIG. 6  is an equivalent circuit of the X-axis magnetic sensor  31 . The structure of the X-axis magnetic sensor  31  will be described in detail. The structure of the Y-axis sensor  32  is similar to that of the X-axis magnetic sensor  31 . The X-axis magnetic sensor  31  has first to fourth magnetic tunneling effect element groups  31   a ,  31   b ,  31   c  and  31   d  connected to form a full-bridge circuit.  
      Each of the first to fourth magnetic tunneling effect element groups  31   a ,  31   b ,  31   c  and  31   d  has the same structure. The structure of the first magnetic tunneling effect element group  31   a  will be described as a representative example of these elements.  
       FIG. 7  is an enlarged plan view of the first magnetic tunneling effect element group  31   a . The first magnetic tunneling effect group  31   a  is constituted of a plurality of serially connected magnetic tunneling effect elements (in this example, twenty elements).  
       FIG. 8  is a partial cross sectional view of the first magnetic tunneling effect element group  31   a  taken along line  1 - 1  shown in  FIG. 7 . The magnetic element tunneling effect group has a plurality of lower electrodes  31   a   1  of a rectangular shape formed on a substrate  30   a . The lower electrodes  31   a   1  are disposed laterally (along the X-axis direction) in rows at a predetermined interval. The lower electrode  31   a   1  is made of conductive nonmagnetic metal material of Cr (or Ta, Ti) and has a film thickness of about 30 nm. On each lower electrode  31   a   1 , an antiferromagnetic film  31   a   2  having the same plan shape as the lower electrode  31   a   1  is stacked. The antiferromagnetic film  31   a   2  is made of PtMn and has a film thickness of about 30 nm.  
      On each antiferromagnetic film  31   a   2 , a pair of ferromagnetic films  31   a   3  made of NiFe and having a film thickness of about 10 nm is stacked with some gap between the films  31   a   3 . The ferromagnetic film  31   a   3  has a rectangular shape as viewed in plan and their longer sides are disposed in parallel.  
       FIG. 9  is a partial plan view of the first magnetic tunneling effect element group  31   a  shown in  FIG. 7 . The ferromagnetic film  31   a   3  constitutes a pinned layer whose magnetization direction is pinned along an arrow direction (a positive X-axis direction, i.e., a short side direction) by the antiferromagnetic film  31   a   2 .  
      Reverting to  FIG. 8 , on each ferromagnetic film  31   a   3 , an insulating layer  31   a   4  having the same plan shape as that of the ferromagnetic film  31   a   3  is stacked. This insulating layer  31   a   4  is made of insulating material of Al 2 O 3 (Al—O) and has a film thickness of 1 nm.  
      On the insulating layer  31   a   4 , a ferromagnetic film  31   a   5  having the same plan shape as that of the insulating layer  31   a   4  is stacked. The ferromagnetic film  31   a   5  is made of NiFe and has a film thickness of about 40 nm. This ferromagnetic film  31   a   5  constitutes a free layer (free magnetization layer) whose magnetization direction changes so as to approximately coincide with the direction of an external magnetic field. The ferromagnetic film  31   a   5 , insulating film  31   a   4  and ferromagnetic film  31   a   3  or pinned layer constitute a magnetic tunneling junction structure. One magnetic tunneling effect element (excepting electrodes) is constituted of the antiferromagnetic film  31   a   2 , ferromagnetic film  31   a   3 , insulating layer  31   a   4  and ferromagnetic film  31   a   5 .  
      On each ferromagnetic film  31   a   5 , a dummy film  31   a   6  having the same plan shape as that of the ferromagnetic film  31   a   5  is stacked. This dummy film  31   a   6  is made of conductive nonmagnetic metal material of Ta and has a film thickness of about 40 nm.  
      An interlayer insulating layer  31   a   7  is formed covering the substrate  30   a , lower electrodes  31   a   1 , antiferromagnetic films  31   a   2 , ferromagnetic films  31   a   3 , insulating layers  31   a   4 , ferromagnetic films  31   a   5  and dummy films  31   a   6 . This interlayer insulating layer  31   a   7  electrically insulates a plurality of lower electrodes  31   a   1  and antiferromagnetic films  31   a   2 , and also electrically insulates pairs of ferromagnetic films  31   a   3 , insulating layers  31   a   4 , ferromagnetic films  31   a   5  and dummy films  31   a   6 , respectively formed on the antiferromagnetic films  31   a   2 . The interlayer insulating layer  31   a   7  is made of SiO 2  and has a film thickness of about 250 nm.  
      Contact holes CH reaching the dummy films  31   a   6  are formed through the interlayer insulating layer  31   a   7 . On this interlayer insulating layer  31   a   7 , upper electrodes  31   a   8  are formed burying the contact holes CH and electrically connecting ones of the dummy films  31   a   6  formed above different lower electrodes  31   a   1  and antiferromagnetic films  31   a   2 . For example, the upper electrode  31   a   8  is made of Al and has a film thickness of 300 nm. Adjacent pairs of ferromagnetic films  31   a   5  (and dummy films  31   a   6 ) and antiferromagnetic films  31   a   2  are, therefore, alternately and sequentially connected electrically by the lower electrodes  31   a   1  and antiferromagnetic films  31   a   2 , and upper electrodes  31   a   8 . In this manner, the magnetic tunneling effect element group  31   a  can be formed which has twenty serially connected magnetic tunneling junction structures having the pinned layers with the same magnetization direction. Although not shown, a passivation film of SiO and SiN is formed covering the upper electrodes  31   a   8 .  
       FIG. 10  is a graph showing the relation between an external magnetic field H and a resistance R 1  of the first magnetic tunneling effect element group  31   a  formed as described above. The resistance R 1  changes in proportion to the external magnetic field H in the range where the absolute value of the external magnetic field H is small (i.e., in the range of saturated magnetic fields −Hc to +Hc), the external magnetic field changing its amplitude along the magnetization direction of the pinned layer. Namely, the resistance R 1  is given by the following equation (1): 
   R 1=−(Δ R/Hc )· H+R 0  (1)  
      As shown in  FIG. 6 , the X-axis magnetic sensor  31  has four magnetic tunneling effect element groups. The magnetization direction of the pinned layers of the magnetic tunneling effect element groups  31   a  to  31   d  are shown in  FIG. 6  by arrows. The magnetization direction of the pinned layers of the first and fourth magnetic tunneling effect element groups  31   a  and  31   d  is the positive X-axis direction, whereas the magnetization direction of the pinned layers of the second and third magnetic tunneling effect element groups  31   b  and  31   c  is the negative X-axis direction. The resistance R 1  of the first and fourth magnetic tunneling effect element groups  31   a  and  31   d  changes in accordance with the equation (1), whereas the resistance R 2  of the second and third magnetic tunneling effect element groups  31   b  and  31   c  changes in accordance with the following equation (2): 
 
 R 2=(Δ R/Hc )· H+R 0  (2) 
 
      In the X-axis magnetic sensor  31 , one end of the first magnetic tunneling effect element group  31   a  is connected to one end of the second magnetic tunneling effect element group  31   b , and the other ends of the first and second magnetic tunneling effect element groups  31   a  and  31   b  are connected respectively to the positive and negative electrodes of the d.c. constant voltage circuit  34   b . Similarly, one end of the third magnetic tunneling effect element group  31   c  is connected to one end of the fourth magnetic tunneling effect element group  31   d , and the other ends of the third and fourth magnetic tunneling effect element groups  31   c  and  31   d  are connected respectively to the positive and negative electrodes of the d.c. constant voltage circuit  34   b . A difference between a potential at the connection point between the first and second magnetic tunneling effect element groups  31   a  and  31   b  and a potential at the connection point between the third and fourth magnetic tunneling effect element groups  31   c  and  31   d  is picked up and supplied to the A/D converter  34   a  as an output Vout of the X-axis magnetic sensor  31 .  
      The X-axis magnetic sensor  31  constructed as above detects the X-axis components Hx of the external magnetic field H in the X-axis direction, and outputs a signal Vout (=Sx) given by the following equation (3): 
 
 Sx=V in·(Δ R/R 0)·( Hx/Hc )  (3) 
 
 where Vin is a voltage of the d.c. constant voltage circuit  34   b.  
 
      As shown in  FIG. 11 , the Y-axis magnetic sensor  32  having the same structure as that of the X-axis magnetic sensor  31  is disposed perpendicular to the X-axis sensor  31 . The Y-axis magnetic sensor  32  detects the Y-axis components Hy of the external magnetic field H in the Y-axis direction, and outputs a signal Vout (=Sy) given by the following equation (4): 
 
 Sy=V in·(Δ R/R 0)·( Hy/Hc )  (4) 
 
      The temperature sensor  33  is made of a band gap reference circuit. This circuit is a well known bias circuit one example of which is shown in  FIG. 12 . As shown, this circuit is constituted of a current source I without temperature dependency, four transistors Q 1  to Q 4  and three resistors R 10  to R 30 . The connection of these components will be described. The current source I is connected between a voltage source Vcc and the collector of the transistor Q 1 . The emitter of the transistor Q 1  is grounded, and the base thereof is connected to the connection point between one end of the resistor R 10  and the collector of the transistor Q 2 . The emitter of the transistor Q 2  is grounded via the resistor R 20 , and the base thereof is connected to the base and collector of the diode-connected transistor Q 3 . The emitter of the transistor Q 3  is grounded, and the collector and base thereof are connected via the resistor R 30  to the other end of the resistor R 10  and the emitter of the transistor Q 4 . The base of the transistor Q 4  is connected to the collector of the transistor Q 1 , and the collector thereof is connected to the voltage source Vcc. The voltage source Vcc is accommodated in the control circuit  34 .  
      In this circuit shown in  FIG. 12 , the emitter area ratio of the transistor Q 3  to the transistor Q 2  is set to a predetermined value N larger than “1”. An output voltage Vbg of the band gap reference circuit is given by the following equation (5): 
 
 Vbg=VBE   Q3   +VT ·ln( N )· R 100/ R 200  (5) 
 
 where VBE Q3  is a base-emitter voltage of the transistor Q 3 , VT is a thermal voltage, R 100  is a resistance of the resistor R 10 , and R 200  is a resistance of the resistor R 20 . 
 
      In the equation (5), it is known that VBE Q3  has a negative temperature coefficient (−2 mV/K) and VT has a positive temperature coefficient (0.085 mV/K). As apparent from the equation (5), by properly selecting the resistance values R100 and R200, the temperature dependency of the output signal Vbg can be eliminated. In this embodiment, therefore, the resistance values R100 and R200 are selected so that the temperature dependency of the output signal Vbg can be eliminated. The temperature sensor  33  supplies a voltage (Vbg−VBE Q3 ) across the resistance R 30  to the A/D converter  34   a.    
      Next, a direction measuring method by the portable phone  10  constructed as described above will be described on the assumption that the external magnetic field H applied to the magnetic sensor unit  30  is only geomagnetism. A direction of the portable phone  10  is defined as the direction of a vector directing from a distal portion (e.g., microphone unit  16 ) to the proximal portion (e.g., speaker unit  13 ) of the portable phone  10 , i.e., a vector directing along the positive Y-axis direction, under the condition that the front side of the casing  11  of the portable phone  10  is generally horizontal and the front side is turned upside. In this specification, as shown in  FIG. 13  the direction is defined on the assumption that the reference of the direction a is 0° (west), and takes 90°, 180°, and 270° as the direction a is rotated in the order from the north, east, to the south.  
       FIG. 13  is a graph showing the relation between the direction a of the portable phone  10  and sensor output signals Sx and Sy of the X- and Y-axis magnetic sensors  31  and  32 .  
      Geomagnetism is a magnetic field directed from the south to north. If the front side of the casing  11  of the portable phone  10  is generally horizontal and the front side is turned upside, the output signals of the X- and Y-axis magnetic sensors  31  and  32  of the magnetic sensor unit  30  change cosinusoidally and sinusoidally relative to the direction a of the portable phone  10 , as shown in  FIG. 13 . The values of the sensor output signals Sx and Sy shown in  FIG. 13  are normalized values. More specifically, the actual output signal Sx of the X-axis magnetic sensor  31  is divided by a half of the difference between the maximum and minimum values of the output signal Sx which are obtained during the 360° rotation of the portable phone  10  under the condition that the front side of the casing  11  of the portable phone  10  is generally horizontal and the front side is turned upside. The actual output signal Sx divided by a half of the difference is used as the normalized value of the output signal value Sx. Similarly, the actual output signal Sy of the Y-axis magnetic sensor  32  is divided by a half of the difference between the maximum and minimum values of the output signal Sy which are obtained during the 360° rotation of the portable phone  10  under the condition that the front side of the casing  11  of the portable phone  10  is generally horizontal and the front side is turned upside. The actual output signal Sy divided by a half of the difference is used as the normalized value of the output signal value Sy.  
      As seen from the graph shown in  FIG. 13 , the direction a of the portable phone  10  can be obtained by taking the following four cases (1) to (4) into consideration: 
 
If Sx&gt; 0and| Sx|&gt;|Sy|,a =tan −1 ( Sy/Sx )  (1) 
 
If Sx&lt; 0and| Sx|&lt;|Sy|,a= 180°+tan −1 ( Sy/Sx )  (2) 
 
If Sx&gt; 0and| Sx|&gt;|Sy|,a= 90°−tan −1 ( Sy/Sx )  (3) 
 
If Sx&lt; 0 and| Sx|&lt;|Sy|,a= 270°−tan −1 ( Sy/Sx )  (4) 
 
      If the direction obtained in any one of the four cases (1) to (4) is negative, 360° is added to the direction a to use this result as the direction a. If the direction obtained is 360° or larger, 360° is subtracted from the direction a to use this result as the direction a.  
      The portable phone  10  has many permanent magnets of the speaker  13   a  and the like. The permanent magnet generates a leak magnetic field. FIGS.  14  to  16  are graphs showing the temperature characteristics of a leak magnetic field from permanent magnets in the portable phone  10 . The strength of the leak magnetic field depends on the strength of the permanent magnet at the temperature at the time of measuring, and the distance between the permanent magnet and a measuring point.  FIGS. 14-16  are graphs showing the temperature dependent characteristics of leak magnetic fields of different permanent magnets on the condition that the distance between the permanent magnet and a measuring point is constant. In the graphs, the abscissa represents temperature, and the ordinate represents the strength of the leak magnetic field. Provided that the distance between the magnet and the measuring point is constant, the strength of the leak magnetic field has a relation with negative coefficient with respect to the temperature. Therefore, the leak magnetic field (external magnetic field other than geomagnetism) from these permanent magnets having an amplitude approximately proportional to the temperature of these permanent magnets and approximately the same direction is applied to the magnetic sensor unit  30  disposed at the predetermined position in the portable phone  10 .  
      As shown in the graph of  FIG. 17 , an output of the X-axis magnetic sensor  31  is shifted (parallel motion) by an offset amount OFx corresponding to the leak magnetic field. Similarly, as shown in the graph of  FIG. 18 , an output of the Y-axis magnetic sensor  32  is shifted by an offset amount OFy corresponding to the leak magnetic field. As described above, since the leak magnetic field changes in approximate proportion to the temperature of the permanent magnets, the offset amounts OFx and OFy also change in approximate proportion to the temperature of the permanent magnets. These offset amounts OFx and OFy can be regarded as the influence amounts by the permanent magnets upon the outputs of the magnetic sensor unit  30 .  
       FIG. 19  is a vector diagram showing geomagnetism and leak magnetic field from the permanent magnets applied to the magnetic sensor unit  30  by using the magnetic sensor unit  30  as a reference.  
      First, the geomagnetism TH 0  and leak magnetic field LH from the permanent magnets applied to the magnetic sensor unit  30  are drawn in this diagram in the state that the front side of the portable phone  10  is turned upside and the direction of the portable phone  10  is set to a predetermined (desired) direction. Next, the geomagnetism TH 180  and leak magnetic field LH from the permanent magnets applied to the magnetic sensor unit  30  are drawn when the direction of the portable phone  10  is rotated by 180°. As seen from  FIG. 19 , the leak magnetic field LH from the permanent magnets having the same direction and amplitude is always applied to the magnetic sensor unit  30  irrespective of the direction of the portable phone  10 . In contrast, the geomagnetism having the same amplitude and opposite direction is applied to the magnetic sensor unit  30  when the portable phone  10  is rotated by 180°. The offset amount OFx of the X-axis magnetic sensor  31  can be given by the following equation (6): 
 
 OFx =( S 1 x+S 2 x )/2  (6) 
 
 where S 1   x  is an output of the X-axis magnetic sensor  31  when the direction of the portable phone  10  is set to an optional direction θ, and S 2   x  is an output of the X-axis magnetic sensor  31  when the direction of the portable phone  10  is rotated by 180° (i.e., at a direction θ+180°). 
 
      Similarly, the offset amount OFy of the Y-axis magnetic sensor  32  can be given by the following equation (7): 
 
 OFy =( S 1 y+S 2 y )/2  (7) 
 
 where S 1   y  is an output of the Y-axis magnetic sensor  32  when the direction of the portable phone  10  is set to the optional direction θ, and S 2   y  is an output of the Y-axis magnetic sensor  32  when the direction of the portable phone  10  is rotated by 180° (i.e., at the direction θ+180°). 
 
      These offset amounts OFx and OFy are proportional to the temperature of the permanent magnets. The offset amount OFx of the X-axis magnetic sensor  31  at a temperature T is given by the following equation (8): 
 
 OFx =( OF 2 x−OF 1 x )·( T−T 1)/( T 2 −T 1)+ OF 1 x   (8) 
 
 where OF 1   x  is the offset amount of the X-axis magnetic sensor  31  at a temperature T 1  and OF 2   x  is the offset amount of the X-axis magnetic sensor  31  at a temperature T 2  different from T 1 . 
 
      Similarly, the offset amount OFy of the Y-axis magnetic sensor  32  at the temperature T is given by the following equation (9): 
 
 OFy =( OF 2 y−OF 1 y )·( T−T 1)/( T 2− T 1)+ OF 1 y   (9) 
 
 where OF 1   y  is the offset amount of the Y-axis magnetic sensor  32  at the temperature T 1  and OF 2   y  is the offset amount of the Y-axis magnetic sensor  32  at the temperature T 2 . 
 
      In this embodiment, after the offset amounts OFx and OFy are calculated, the offset amounts OFx and OFy are subtracted from the actual sensor outputs Sx and Sy to obtain the corrected sensor outputs Sx and Sy. The direction a is determined in accordance with the corrected sensor outputs Sx and Sy and each of the direction calculation methods classified into the four cases (1) to (4). In this manner, the direction a can be determined at a high precision without the influence of the leak magnetic field of the permanent magnets. The principle of the direction determining method by the portable phone  10  has been described above.  
      Next, the operation of the direction determining method by CPU  21  of the portable phone  10  in accordance with the above-described principle will be described with reference to FIGS.  20  to  23 . FIGS.  20  to  23  are flow charts illustrating the programs (routines) to be executed by CPU  21  each time a predetermined time lapses.  
      When a user purchased the portable phone  10  uses it at the first time and turns the power on, CPU  21  starts at a predetermined timing an initializing prompt display routine (accomplishing the function of an initialization prompting device) shown in  FIG. 20  at Step  1700 . Next, at Step  1705  it is checked whether a first initialization flag F 1  is “0”. The value of the first initialization flag F 1  was set to “0” by an initialization routine which was performed immediately after the manufacture of the portable phone  10 . Therefore, CPU  21  judges “Yes” at Step  1705  to advance to Step  1710  whereat a message (initializing prompt message) for prompting the user of the portable phone  10  to perform an initialization operation is displayed on the liquid crystal display panel  14   a . Thereafter, at Step  1795  this routine is once terminated. The initialization prompt message includes a message of prompting the user to depress a specific offset data acquisition button among the plurality of push buttons  15   a  to change the state of the button to an “on” state.  
      CPU  21  starts at a predetermined timing an offset data acquisition routine shown in  FIG. 21  at Step  1800 . Then, at Step  1805  it is checked whether the state of the offset data acquisition button changes from an “off” state to an “on” state. If not, it is judged as “No” at Step  1805  to advance to Step  1895  and repeat the above process.  
      When the user responds to the initialization prompt message displayed on the liquid crystal display panel  14   a  and the state of the offset data acquisition button is changed from the “off” state to the “on” state, CPU  21  judges as “Yes” at Step  1805  to advance to Step  1810 . At Step  1810  an explanation for a “first operation method” is displayed on the liquid crystal display panel  14   a . The explanation for the first operation method includes a message of prompting the user to place the portable phone  10  on a desk by turning the front side of the casing  11  upside (i.e., by setting the front side approximately horizontal) and depress a specific offset button among the plurality of push buttons  15   a  to thereby change the state of the button to the “on” state. Next, at Step  1815  CPU  21  monitors whether the state of the offset button changes from the “off” state to the “on” state.  
      When the user responds to the explanation for the first operation method and changes the state of the offset button from the “off” state to the “on” state, CPU  21  judges as “Yes” at Step  1815  to advance to Step  1820 . At Step  1820  it is checked whether the absolute value of the output Sx of the X-axis magnetic sensor  31  is larger than the measurable maximum value Smax or whether the absolute value of the output Sy of the Y-axis magnetic sensor  32  is larger than the measurable maximum value Smax. If the absolute value of the output Sx of the X-axis magnetic sensor  31  is larger than the measurable maximum value Smax or if the absolute value of the output Sy of the Y-axis magnetic sensor  32  is larger than the measurable maximum value Smax, CPU  21  judges as “Yes” at Step  1820  to advance to Step  1825 . At Step  1825  an alarm message to the effect that the initialization failed is displayed on the liquid crystal display panel  14   a  to advance to Step  1895  and this routine is once terminated.  
      If at Step  1820  the absolute value of the output Sx of the X-axis magnetic sensor  31  is equal to or smaller than the measurable maximum value Smax and if the absolute value of the output Sy of the Y-axis magnetic sensor  32  is equal to or smaller than the measurable maximum value Smax, CPU  21  judges as “No” at Step  1820  to advance to Step  1830 . At Step  1830  the output Sx of the X-axis magnetic sensor  31  is stored as a first X-axis sensor output S 1   x  and the output Sy of the Y-axis magnetic sensor  32  is stored as a first Y-axis sensor output S 1   y.    
      At Step  1835 , CPU  21  displays an explanation for a “second operation method” on the liquid crystal display panel  14   a . The explanation for the second operation method includes a message of prompting the user to depress the offset button again after the portable phone  10  is rotated by 180° on the desk with the front side thereof being turned upside and change the state of the button to the “on” state. At Step  1840  CPU  21  monitors again whether the state of the offset button changes from the “off” state to the “on” state.  
      When the user responds to the explanation for the second operation method and changes the state of the offset button from the “off” sate to the “on” state after rotating the portable phone  10  by 180°, CPU  21  judges as “Yes” at Step  1840  to advance to Step  1845 . At Step  1845  it is checked whether the absolute value of the output Sx of the X-axis magnetic sensor  31  is larger than the measurable maximum value Smax or whether the absolute value of the output Sy of the Y-axis magnetic sensor  32  is larger than the measurable maximum value Smax. If the absolute value of the output Sx of the X-axis magnetic sensor  31  is larger than the measurable maximum value Smax or if the absolute value of the output Sy of the Y-axis magnetic sensor  32  is larger than the measurable maximum value Smax, CPU  21  judges as “Yes” at Step  1845  to advance to Step  1825 . At Step  1825  an alarm message to the effect that the initialization failed is displayed to advance to Step  1895  and this routine is once terminated.  
      If at Step  1845  the absolute value of the output Sx of the X-axis magnetic sensor  31  is equal to or smaller than the measurable maximum value Smax and if the absolute value of the output Sy of the Y-axis magnetic sensor  32  is equal to or smaller than the measurable maximum value Smax, CPU  21  judges as “No” at Step  1845  to advance to Step  1850 . At Step  1850  the output Sx of the X-axis magnetic sensor  31  is stored as a second X-axis sensor output S 2   x  and the output Sy of the Y-axis magnetic sensor  32  is stored as a second Y-axis sensor output S 2   y.    
      Next, at Step  1855  CPU  21  checks whether the value of the first initialization flag F 1  is “0”. In this case, since the value of the first initialization flag F 1  remains “0”, CPU  21  judges as “Yes” at Step  1855  to advance to Step  1860 . At Step  1860  the first X-axis offset amount OF 1   x  of the X-axis magnetic sensor  31  and the first Y-axis offset amount OF 1   y  of the Y-axis magnetic sensor  32  are calculated. More specifically, a sum of the first X-axis sensor output S 1   x  and second X-axis sensor output S 2   x  is divided by 2 (i.e., an average value is calculated) and the obtained value is used as the first X-axis offset amount OF 1   x . A sum of the first Y-axis sensor output Sly and second Y-axis sensor output S 2   y  is divided by 2 and the obtained value is used as the first Y-axis offset amount OF 1   y . The first X-axis offset amount OF 1   x  and first Y-axis offset amount OF 1   y  are stored in the nonvolatile RAM  24 .  
      At Step  1865  CPU  21  reads the temperature Temp of the temperature sensor  33  and stores it in the nonvolatile RAM  24  as the first temperature T 1 . At Step  1870  the value of the first initialization flag F 1  is set to “1” to advance to Step  1895  whereat this routine is once terminated.  
      In this state, as CPU  21  starts the initializing prompt display routine shown in  FIG. 20  at Step  1700  and advances to Step  1705 , since the value of the first initialization flag F 1  was set to “1”, CPU  21  judges as “No” to advance to Step  1715  whereat it is checked whether the value of a second initialization flag F 2  is “0”. The value of the second initialization flag F 2  was also set to “0” by the initialization routine described earlier. Therefore, CPU  21  judges as “Yes” at Step  1715  to advance to Step  1720  whereat the temperature Temp of the temperature sensor  33  is read and stored as a present temperature Tc. It is checked at Step  1725  whether the absolute value of a difference between the first temperature T 1  and the present temperature Tc is larger than a predetermined temperature (threshold temperature) Tth. It is necessary to measure the temperature and geomagnetism at two temperatures having some difference in order to ensure the measurement precision of the temperature and geomagnetism amplitude with the temperature sensor and magnetic sensors. If a temperature difference is too small, it is difficult to obtain a correct temperature coefficient and make a proper correction. However, the smaller the threshold temperature Tth (≧0° C.) is, the direction measurement becomes more precise. In addition, a smaller threshold temperature Tth is preferable in the case that the sensors and external magnetic field change abruptly with the temperature. From these reasons, it is preferable that the threshold temperature Tth is selected from the range of 5-25° C. The threshold temperature Tth is preferably set by considering the above-described conditions. For example, Tth is 10° C.  
      Since the present time is immediately after the first temperature T 1  was acquired, the absolute value of a difference between the first temperature T 1  and present temperature Tc is smaller than the threshold temperature Tth. Therefore, CPU  21  judges as “No” at Step  1725  to advance to Step  1795  whereat this routine is once terminated.  
      These processes are repeated until the absolute value of a difference between the first temperature T 1  and present temperature Tc becomes larger than the threshold temperature Tth. The initialization prompt message will not be displayed again until such time.  
      CPU  21  starts at a predetermined timing an offset determining routine shown in  FIG. 22  at Step  1900 . It is checked at Step  1905  whether the value of the second initialization flag F 2  is “0”. In this case, since the value of the second initialization flag F 2  is maintained “0”, CPU  21  judges as “Yes” at Step  1905  to advance to Step  1910 . At Step  1910  the first X-axis offset amount OF 1   x  and first Y-axis offset amount OF 1   y  calculated already are set as the offset amount OFx of the X-axis magnetic sensor  31  and the offset amount OFy of the Y-axis magnetic sensor  32 . Thereafter, this routine is once terminated at Step  1995 .  
      CPU  21  starts a direction calculating routine (constituting a direction determining device) shown in  FIG. 23  at Step  2000 . At Step  2005 , the output Sx of the X-axis magnetic sensor  31  subtracted by the offset amount OFx of the X-axis magnetic sensor  31  is set as the corrected output Sx of the X-axis magnetic sensor  31 , and the output Sy of the Y-axis magnetic sensor  32  subtracted by the offset amount OFy of the Y-axis magnetic sensor  32  is set as the corrected output Sy of the Y-axis magnetic sensor  32 . CPU  21  judges at Step  2010  which one of the cases (1) to (4) is to be adopted. In accordance with the judgement result, the flow advances one of Steps  2015  to  2030  whereat the direction a is calculated by using the equation shown in each Step. Next, CPU  21  determines the final direction in the following manner. Namely, if the calculated direction a is negative at Step  2035 , the direction a added with 360° is used as the final direction a at Step  2040 , whereas if the calculated direction a is equal to or larger than 360° at Steps  2035  and  2045 , the direction a subtracted by 360° is used as the final direction a at Step  2050 . Thereafter, this routine is once terminated at Step  2095 .  
      Next, the description will be given for the case that the temperature of the permanent magnets in the portable phone  10  rises and the absolute value of a difference between the first temperature T 1  and present temperature Tc becomes larger than the threshold value Tth (takes the second temperature T 2 ). In this case, at Step  1725  after Steps  1700 ,  1705 ,  1715  and  1720 , CPU judges as “Yes” to advance to Step  1710  whereat the initialization prompt message is again displayed on the liquid crystal panel  14   a.    
      When the user responds to this and depresses the offset data acquisition button to change the state to the “on” state, CPU  21  judges as “Yes” at Step  1805  shown in  FIG. 21  to advance to Step  1810  and following Steps. At Step  1830  the output Sx of the X-axis magnetic sensor  31  and the output Sy of the Y-axis magnetic sensor  32  in the state that the direction of the portable phone  10  takes an arbitrary direction θ are stored as the first X-axis sensor output S 1   x  and first Y-axis sensor output Sly, respectively. At Step  1850  the output Sx of the X-axis magnetic sensor  31  and the output Sy of the Y-axis magnetic sensor  32  in the state that the direction of the portable phone  10  takes a direction θ+180° are stored as the second X-axis sensor output S 2   x  and second Y-axis sensor output S 2   y , to thereafter advance to Step  1855 .  
      Since the value of the first initialization flag F 1  was set to “1” at Step  1870 , CPU  21  judges as “No” at Step  1855  to advance to Step  1875 . At Step  1875  the second X-axis offset amount OF 2   x  of the X-axis magnetic sensor  31  and the second Y-axis offset amount OF 2   y  of the Y-axis magnetic sensor  32  are calculated. Specifically, an average value of the first X-axis sensor output S 1   x  and the second X-axis sensor output S 2   x  is used as the second X-axis offset amount OF 2   x , and an average value of the first Y-axis sensor output Sly and the second Y-axis sensor output S 2   y  is used as the second Y-axis offset amount OF 2   y . The second X-axis offset amount OF 2   x  and second Y-axis offset amount OF 2   y  are stored in the nonvolatile RAM  24 .  
      Next, at Step  1880  CPU  21  reads the temperature Temp of the temperature sensor  33  and stores it in the nonvolatile RAM  24  as the second temperature T 2 . After the value of the second initialization flag F 2  is set to “1” at Step  1885 , this routine is once terminated at Step  1895 .  
      In this state, as CPU  21  starts the initializing prompt display routine shown in  FIG. 20  at Step  1700 , since the values of the first and second initialization flags F 1  and F 2  were both set to “1”, CPU  21  judges as “No” at both Steps  1705  and  1715  to advance to Step  1795  whereat this routine is once terminated. The initialization prompt message will not be displayed thereafter.  
      In this state, as the offset determining routine shown in  FIG. 22  starts, since the value of the second initialization flag F 2  was changed to “1”, CPU  21  judges as “No” at Step  1905  to advance to Step  1915  whereat the temperature Temp of the temperature sensor  33  is read and stored as the present temperature Tc.  
      Next, at Step  1920  CPU  21  linearly interpolates relative to the temperature the first X-axis offset amount OF 1   x  at the first temperature T 1  and the second X-axis offset amount OF 2   x  at the second temperature T 2  in accordance with the above-described equation (8) to thereby obtain the X-axis offset amount OFx at the present temperature Tc. Similarly, at Step  1925  CPU  21  linearly interpolates relative to the temperature the first Y-axis offset amount OF 1   y  at the first temperature T 1  and the second Y-axis offset amount OF 2   y  at the second temperature T 2  in accordance with the above-described equation (9) to thereby obtain the Y-axis offset amount OFy at the present temperature Tc. This routine is once terminated at Step  1995 . In the above manner, the offset values OFx and OFy represent the influence of the magnetic field of the permanent magnets upon the magnetic sensor outputs estimated from the temperature of the permanent magnets.  
      In the following processes, CPU  21  executes the direction calculating routine shown in  FIG. 23  so that at Step  2005  the output Sx of the X-axis magnetic sensor  31  is corrected by the offset amount OFx and the output Sy of the Y-axis magnetic sensor  32  is corrected by the offset amount OFy. Step  2005  constitutes a portion of correcting device. At Step  2010  and following Steps, the direction a is calculated (measured and determined) from the outputs Sx and Sy of the X- and Y-axis magnetic sensors  31  and  32 .  
      As described above, in the portable phone  10  according to the embodiment of the invention, the influence of the magnetic field of the permanent magnets used as the components of the portable phone  10  upon the magnetic sensor outputs is estimated as the offset amounts OFx and OFy from the temperature of the permanent magnets. The magnetic sensor outputs are corrected by using the estimated offset amounts OFx and OFy. The direction is measured from the corrected magnetic sensor outputs so that the measurement precision of the direction can be improved considerably. Since a user is prompted to perform the initialization operations at proper timings (when the temperature takes the first temperature T 1  and second temperature T 2 ), the user is prevented from performing unnecessary initialization operations. The difference between the first temperature T 1  and second temperature T 2  is larger than the threshold temperature Tth. Therefore, the influence of an estimation error contained in the offset amount obtained at each temperature is hard to appear in the offset amount at the present temperature Tc obtained through linear interpolation or extrapolation or the like of the offset amounts. The measurement precision of a direction can be improved further. The magnetic sensor unit  30  has the X- and Y-axis magnetic sensors  31  and  32 , temperature sensor  33  and control circuit  34  formed on a single substrate. This magnetic sensor unit  30  is therefore compact and inexpensive and suitable for portable electronic apparatuses having permanent magnets such as portable phones.  
      The invention is not limited only to the above embodiment, but various modifications are possible without departing from the scope of the invention. For example, in the above embodiment, although the X- and Y-axis magnetic sensors  31  and  32  are magnetic tunneling effect element groups, other magnetic sensors capable of outputting a signal corresponding to a magnetic field such as giant magnetoresistive effect elements may also be used. In the embodiment, the offset button and offset data acquisition button are used for the initialization operation. Instead, the same functions of these buttons may be realized by adding menus in the liquid crystal display panel  14   a  and selecting each menu by a specific operator of the operation unit  15 . In addition to the X-axis magnetic sensor  31  and Y-axis magnetic sensor  32 , a Z-axis magnetic sensor may be used which detects the magnetic field along the Z-axis perpendicular to the X- and Y-axes.  
      The band gap reference circuit as the temperature sensor  33  may have the structure shown in  FIG. 24 . A difference ΔVbe of the base-emitter voltages Vbe of transistors Tr 1  and Tr 2  is given by the following equation (10) and the output Vbg is given by the following equation (11) using Vbe and VT multiplied by a constant K 1 . The constant K 1  is given by the following equation (12). 
 
Δ Vbe=V   T ·ln{( Ic 1/ Ic 2)·( A 2/ A 1)}  (10) 
 
 Vbg=Vbe ( Q 1)+ K 1 ·VT   (11) 
 
 K 1=( R 3/ R 2)·ln{( Ic 1/ Ic 2)−( A 2/ A 1)}  (12) 
 
 where V T =KT/q, A 1  and A 2  are emitter areas of the transistors Tr 1  and Tr 2 , and Vbe(Q 1 ) is a base-emitter voltage of the transistor Tr 1 . 
 
      The constant K 1  is properly selected to eliminate the temperature dependency of Vbg. A voltage across a resistor R 3  is supplied to an A/D converter  34   a  as an output of the temperature sensor  33 .  
       FIG. 25  is a graph showing the temperature characteristics of the circuit shown in  FIG. 24 . As will be understood from  FIG. 25 , the circuit arrangement shown in  FIG. 24  can realize the temperature sensor  33  having the temperature characteristics of 2 mV/° C.  
      The present invention has been described in connection with the preferred embodiment. The invention is not limited only to the above embodiment. It is apparent that the various modifications, improvements, combinations, and the like can be made by those skilled in the art.