BACKGROUND OF THE INVENTION 
     The invention relates to an electronic apparatus whose power is supplied by a supply voltage, comprising a memory which contains electrically erasable data and a supply voltage detection device for detecting a variation of the supply voltage. 
     In heavily industrialized countries, a current tendency seeks to miniaturize the dimensions of portable telecommunication equipment intended for the public at large and for current use. The makers of this electronic equipment are thus exposed to heavy competition regarding the weight-to-volume ratio and the range of their apparatus. As the latest integrated circuit technologies currently permit of considerably reducing the volume of the electronic circuits, the range of this portable apparatus has become the factor which restricts this course to miniaturization. Indeed, generally intended for mobile use, this apparatus is fed by power supply batteries whose range is proportional to the weight in a given technology. In consequence, an apparatus fed by a battery is all the lighter as its range is smaller, which explains why the battery of a portable apparatus frequently turns out to be discharged. 
     Thus, to avoid certain phenomena which are likely to damage the apparatus, it is very important to anticipate during the discharging the instant at which the supply voltage will be cut off. Notably, the mobile radio telephones or other equipment comprising processors and electrically erasable memories of the EEPROM type (Electrically Erasable Programmable Read-only Memory) cannot withstand sudden power cuts. If the power supply is not cut off until its voltage drops below a threshold defined as critical by the manufacturer, the contents of the EEPROMs may be corrupted. In effect, the processor powered by an insufficient voltage no longer controls its addressing system. It runs the risk of writing an erroneous address in the EEPROM and overwriting the data which are found at that address. Data may then be lost, which is very harmful to the use of the radio telephone. 
     In a known electronic apparatus, a supply voltage detection device is used to periodically sound the supply voltage produced by the battery in order to detect the instant at which said voltage drops below the critical voltage and cut off the power supply so as not to change the data of the EEPROM. 
     An apparatus comprising such a supply voltage detection device, however, thus has a major drawback that questions its reliability. In effect, based on a periodic sounding technique, the detection precision and thus the reliability of this device directly depend on the sounding period used. As the conventional discharging curve of a battery drops very rapidly towards the end of the discharge, the power supply is frequently cut off which is undetected by the current device of which the sounding period is very large compared to the rapidity with which the voltage drops at the end of the discharge. 
     Diminishing the sounding period for enhancing the chances to detect the crossing of the critical threshold forms a too expensive solution which necessitates a faster and more powerful processor than that used in the known device. 
     SUMMARY OF THE INVENTION 
     The present invention describes a simple and economic solution for largely remedying these drawbacks. The invention notably has for its object to provide a supply power detection device which is more reliable than the known device for detecting the supply voltage, without notably augmenting the complexity of the electronic circuits used. The device according to the invention is furthermore capable of instantaneously detecting the crossing of a voltage threshold defined as critical for cutting off the power supply of the apparatus so as not to change the digital data stored in the EEPROM memory. 
     Therefore, an apparatus of the type defined in the opening paragraph is characterized in that the detection device for detecting the supply voltage comprises: 
     a continuous monitoring device for the supply voltage for forming a first signal which indicates a drop of supply voltage, 
     backup means for backing up the data of the memory when said signal appears. 
     This continuous monitoring device for the supply voltage has the advantage of using the processor only when the supply voltage crosses a fixed threshold. Said processor is thus available for carrying out other operations, notably for managing the communication between the mobile station and the base station in the case of a radio telephone. 
     A first characteristic feature of the invention according to which an electronic apparatus of the type defined above comprises a supply voltage detection device characterized in that the continuous monitoring device produces a second signal for indicating a supply voltage level provides the advantage of permitting the user to control the charging level of his battery to anticipate the instant at which the battery needs recharging. 
     A second characteristic feature of the invention according to which the supply voltage detection device characterized in that the continuous monitoring device produces a third signal to indicate an overload provides the additional advantage of preventing the battery from exploding as a result of a power supply being cut off when the third signal appears. 
     According to another interesting characteristic feature of the invention, an electronic apparatus of the type already mentioned additionally comprising a control element which has at least one interrupt input is characterized in that said signals described above are applied to said interrupt input for respectively generating an interruption of the control element and for triggering an interrupt program in response to said signals. 
     According to again another characteristic feature of the invention, an electronic apparatus is provided of the type already defined above, characterized in that the continuous monitoring device additionally comprises a comparing device for comparing the value of the supply voltage to set values and to produce said respective signals. This characteristic feature provides a means for defining detection thresholds as a function of the type of battery used, on the one hand, and of the needs of the user on the other hand. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawings: 
     FIG. 1 represents an example of an electronic apparatus according to the invention, 
     FIG. 2 shows a conventional discharging curve of a power supply battery representing the supply voltage plotted against time, 
     FIG. 3 shows a block diagram of a circuit of the apparatus shown in FIG. 1, 
     FIG. 4 shows a preferred embodiment of the device according to the invention, 
     FIG. 5 represents the table  32   a  of the EEPROM  32  indicated in FIG. 3, 
     FIG. 6 shows: 
     FIG. 6A shows a timing diagram illustrating the logic states of three ports of the element R 2  represented in FIG. 4, for enhancing the value of the resistance of R 2 , 
     FIG. 6B shows a timing diagram illustrating the logic states of these three gates, for diminishing the value of the resistance of R 2 , 
     FIG. 7 represents an automaton illustrating the operation of the device shown in FIG.  4  and 
     FIG. 8 illustrates another embodiment of the device according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The electronic apparatus shown in FIG. 1 is a radio telephone, but the supply voltage detection device may be applied to any other electronic apparatus whose power is supplied by a supply voltage. 
     The radio telephone shown in FIG. 1 comprises a housing  10  enclosing a power supply battery  11  and electronic circuits among which that represented in FIG. 3, a screen  12 , a keyboard  13 , an earphone  14 , a microphone  15  and a transceiver device (not shown) cooperating with an antenna  16  for exchanging radio signals with a base site  17 . 
     The discharging curve of the battery (Vbat) plotted against time (t) represented in FIG. 2 shows the acceleration of the lowering of the voltage at the end of the discharge. The instants denoted t 1 , t 2 , t 3 , t 4  and t 5  represent examples of samples recorded by the periodic sounding device known from the prior art for measuring the supply voltage Vbat. 
     SS represents the critical threshold whose detection by the prior art device signals a voltage drop Vbat that is large enough to prohibit any writing in the EEPROM memory, because the risks of address errors become too high. SC represents the power cut-off threshold the crossing of which by the supply voltage causes the supply power to be cut off. It is thus indispensable for detecting the threshold SS before the supply voltage crosses the threshold SC. 
     However, between the samples t 4  and t 5 , the supply voltage curve crosses the power supply cut off threshold SC without the detection device detecting the crossing of the threshold SS because the thresholds SS and SC are crossed between two successive samplings. This example perfectly illustrates the lack of reliability of the known device. 
     The block diagram of FIG. 3 represents the various elements used in the device according to the invention. The control element  30  is formed by a processor  31 , an EEPROM electrically erasable programmable read-only memory  32 , a random-access memory  33  RAM and a read-only memory  34  ROM. 
     The main operation program of the apparatus is stored in the read-only memory  34 . The random-access memory  33  is used in parallel with the read-only memory  34  for storing the useful variables. The erasable data are stored in the EEPROM  32  to be modified while the apparatus is being used. The set values corresponding to the voltage thresholds to be detected are arranged in a table  32   a  of the memory  32  so as to be accessible to the control element  30 . 
     A bus network  35  contains an interrupt bus for transmitting the signals from the keyboard  13  to the processor  31  and a serial IIC (InterIntegrated Circuit) bus intended for the signals in transit between the processor  31 , the EEPROM  32  and the screen  12 . A transceiver device  36  communicates with the devices described above to connect the user of the radio telephone to the mobile telephone network. 
     A power supply detection circuit  37  cooperates with the control element  30  and the power supply battery  11  to utilize the supply voltage detection device according to the invention. 
     A preferred embodiment of the device  37  is represented in FIG. 4 in the form of an electronic circuit. The power supply battery  11  produces a supply voltage Vbat whose value lies, for example, between 3 and 5.5 volts and which varies as a function of the charge/discharge of the battery. R 0  and R 1  represent resistive elements whose respective resistances are fixed. R 2  is a resistive element whose resistance is variable and is intended to be programmed by the control element  30  as a function of the value of the voltage Vbat and of the voltage threshold one wishes to detect. Such an element R 2  whose resistance is digitally programmable is commercially available. For example, a potentiometer DS1804 NV by Dallas Semiconductor could be used. 
     According to the preferred embodiment of the invention, R 2  is controlled by the element  30  of which one output, referred to as a command output CMD, is directly connected thereto for real-time control of the value of its resistance notably as a function of the supply voltage Vbat which occurs in the formula of the voltage divider bridge linking R 1  and R 2  via the equation: 
     
       
           VR 2 =Vbat×R 2/( R 1+ R 2).  
       
     
     A transistor T arranged as an interrupter allows of generating an interruption IT of the control element  30  notably as a function of the voltage applied to its base, equal to VR 2 , to trigger the execution of an interrupt program. The fact that the resistive elements R 1  and R 2  work together with the transistor T realizes the continuous test of the condition of the conduction of the transistor T utilized in the continuous monitoring device for the supply voltage. 
     As an advantage of the invention, this test takes place naturally and continuously without the intervention of the control element  30 . It makes use of an electronic phenomenon linked with the difference of potential Vbe between the base and the emitter of the transistor T, also called conduction threshold of said transistor. When the potential of the base of the transistor (VR 2 ) is higher than the conduction threshold Vbe (generally Vbe=0.6 volt), the transistor T conducts according to the conduction condition of the transistor T. When this potential is lower than Vbe, the transistor will be blocked depending on the reverse blocking condition. 
     These conditions are actually verified in FIG.  4 . As long as VR 2 &lt;Vbe, the transistor T is blocked and the voltage on the collector is equal to Vbat. The value of R 2 , fixed by the element  30 , remains constant, an augmentation of the supply voltage Vbat causes the value of the voltage VR 2  to augment. When VR 2 &gt;Vbe, the transistor T will be conductive and the voltage on the collector will become substantially zero. 
     The device has been arbitrarily programmed so that during the charging of the battery, a zero voltage at the collector (T conducts) triggers an interruption of the control element  30  and that during the discharging, an interruption is triggered when the voltage on the collector is close to Vbat (T blocked). The interrupt port of the control element  30  is thus normally active at the high logic level during the discharging of the battery and at the low logic level during charging. 
     Thus, according to circuit  37 , the value of the voltage VR 2  on the terminals of R 2  determines the interrupt thresholds of the control element  30  representing the detection thresholds of the supply voltage Vbat. The principle of the invention thus consists of fixing the value of the resistance of R 2  as a function of the threshold of the supply voltage which one wishes to detect. Therefore, the element  30  utilizes its control output CMD. 
     In the example of table  32   a  represented in FIG. 5, the resistance of the element R 2  could only assume set values ρ0 to ρ5, in a logical order as a function of the previously detected thresholds, that is to say, in an order of decreasing values during the charging and increasing values during the discharging. The largest value of the table  32   a  which can be given to the resistance of R 2  corresponds to the overload threshold (detected via the value ρ0) and the lowest value to the power cut off threshold (detected via the value ρ5). The table  32   a  contains as many set values as there are thresholds to be detected for generating a respective interruption of the control element  30  and triggering an interrupt program. 
     The following table gives examples of digital values which correspond to the set values ρ0 to ρ5 according to the preferred embodiment of the invention. The digital values have been obtained by taking for R 2  a 100-adjustment-step potentiometer DS1804 NV by Dallas Semiconductor and for R 1  a 400 kohm resistor. The voltage Vbat and the resistance of R 2  are indicated in volts and kohm, respectively. The number of steps of the potentiometer is a unitless number between 0 and 100 according to the manufacturer&#39;s specification. 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Vbat 
                  5.5 
                  5 
                  4.5 
                  4 
                  3.5 
                  3 
               
               
                   
                 R2 
                 48.98 
                 54.54 
                 61.54 
                 70.59 
                 82.76 
                 100 
               
               
                   
                 Step 
                 43 
                 49 
                 57 
                 67 
                 80 
                 100 
               
               
                   
                 Set 
                 ρ0 
                 ρ1 
                 ρ2 
                 ρ3 
                 ρ4 
                 ρ5 
               
               
                   
                   
               
             
          
         
       
     
     The programming of R 2  takes place by specific buses connecting the output CMD of the control element  30  formed by three output ports to three input ports of R 2  denoted {overscore (INC)}, U/{overscore (D)} and {overscore (C)}S respectively, while reference is made to FIGS. 6A and 6B. FIG. 6A shows a timing diagram of the states of the three ports mentioned above for augmenting the value of the resistance of R 2  in four steps. FIG. 6B shows the states of these ports for diminishing said value in three steps. {overscore (CS)} is placed at the low logic level to modify the value of the resistance of R 2 . To increase this value, U/{overscore (D)} is at the high level and to diminish this value, U/{overscore (D)} is at the low level. {overscore (INC)}, which is active at the low level, is used for increasing or diminishing the value of the resistance of R 2  step by step. 
     An interruption IT of the control element  30  is generated during the charging and discharging of the battery upon detection of each voltage threshold that corresponds to a set value. Depending on the detected set value, an interrupt program cuts off the power supply (detection at ρ5), stops the charging (detection at ρ0), or indicates a supply voltage level (detection at ρ1, ρ2, ρ3 or ρ4). 
     Any device that permits of the transmission of a specific message to the user can be used according to the invention to indicate the supply voltage level. For example, if the apparatus has a control screen as in the example shown in FIG. 3, the supply voltage level can be indicated by a display device which permits of displaying on screen  12  an icon  12   a  or a particular message which symbolizes the charging/discharging level of the battery  11 . The icon  12   a , which continues to be displayed on the screen, represents bars which are suitable for appearing on or disappearing from the screen  12  as soon as supply voltage thresholds are detected during the charging and discharging of the battery. If the apparatus has an earphone  14 , as in the example shown in FIG. 1, a device emitting a sound signal may also be used for indicating the charging level of the battery. 
     The operation of the supply voltage detection device shown in FIG. 4 will now be described with reference to FIG.  7 . 
     The automaton comprises twenty stable states which represent various states of the device according to the invention, determined by two binary variables (CH, IT) and one variable (ρ) that is capable of assuming the 6 set values. 
     Each time the apparatus is switched on while lying on its charger, the main program sets the variables CH=1 and activates the interruption IT by way of exception at the high level (IT normally being active at the low level during the charging) in order to test the charging level of the battery and assign to the resistance of R 2  the adequate set value for starting or proceeding with the charging depending on whether the battery is completely discharged or already partly charged. 
     Therefore, the control element  30  scans the values of the table  32   a  in decreasing order until an interruption is provoked (active at high level by way of exception). The device thus describes the states C 0  to C 4  (characterized by a set value ρ4 to ρ0), passing from one state to another when a timing τ, which is sufficient for detecting the possible appearance of an interruption (IT=1), expires, which brings the device to one of the states C 5  to C 9 . The interrupt program then stores the set value ρ0 to ρ4 detected for displaying the charging level of the battery. The interruption is reprogrammed at low level. Then, at each new interruption (IT=0) during the charging (CH=1), the device prescribes the following states according to the order indicated by the arrows. The interrupt program updates the level of the supply voltage and diminishes the set value when it passes from one of the states C 6  to C 9  to one of the states C 5  to C 8  or deactivates the charger to stop charging when state C is reached from state C 5 . 
     Each time the apparatus is switched on, and when it is removed from its charger, the main program sets the variable CH=0 and activates the interruption IT by way of exception at low level (IT normally being active at high level during the discharging) in order to test the discharging level of the battery and assign the proper set value to the resistance of R 2 . 
     Therefore, it scans the set values of the table  32   a  in growing direction so as to provoke an interruption (by way of exception active at low level). The device thus describes the states D 0  to D 4  (characterized by a set value ρ1 to ρ5) until an interruption (IT=0) places the device in one of the states D 5  to D 9 . The interrupt program then stores the detected set value ρ1 to ρ5 for displaying the charging level of the battery. The interruption is reprogrammed at high level. Then, with each new interruption (IT=1) during the discharging (CH=0), the device describes the following states in the order indicated by the arrows. The interrupt program updates the supply voltage level and increases the set value when passing from one of the states D 6  to D 9  to one of the states D 5  to D 8  or cuts off the power supply when passing from the state D 5  to the state D. 
     According to an advantage of the invention, it is also possible to switch from the discharge mode (CH=0) to the charge mode (CH=1), and vice versa, without affecting the whole process of the start of the charging or discharging intended to assign the correct set value to the resistance of R 2 . It is sufficient to place the variable CH=0 for passing to the discharge mode from the charging states C 6  to C 9 , or place the variable CH=1 for passing to the charge mode from the discharging states D 6  to D 9 . The resistive element R 2  retains its set value ρ1 to ρ4, the charging level of the battery is stored and the interruption IT is reprogrammed at the adequate logic level (low by way of exception for switching to the discharge mode, high by way of exception for switching to the charge mode). 
     Another embodiment of the invention is shown in FIG. 8 in the form of a simplified electronic circuit. It comprises the control element  30 , a digital/analog converter  80  and a comparator  81 . 
     The control element  30  computes and stores in the table  32   a  the set values corresponding to the voltage thresholds one wishes to detect. It transmits the values via its output  82  to the digital/analog converter  80  to convert them into voltage thresholds to be detected Vp, which can be comprehended by the other analog elements of the circuit. As in the device described with reference to FIG. 4, the element  30  sets the voltages Vp in a logical order to follow the charging/discharging of the battery. The comparator  81  compares the voltages Vp and the supply voltage Vbat applied to its inputs  83  and  84  respectively, to generate an interruption IT of the control element  30  as a function of the result  85  of the comparison. If Vp=Vbat, an interruption IT is generated for producing the signal which indicates that the supply voltage Vbat exceeds a detection threshold. 
     Thus, an electronic apparatus is described comprising a simple and economic supply voltage detection device capable of instantaneously detecting the crossing of a threshold of a supply voltage to cut off the power supply without modifying digital data, indicate at any moment the charge level of the power supply battery and stop the charging if there is an overload. 
     Obviously, the invention is not restricted to the embodiments that have just been described and represented. Other variants of embodiments of the invention will be obvious to a person of ordinary skill in the art, which variants do not lie outside the scope of the invention. For example, the arrangement of the resistive elements R 1  and R 2  for forming the voltage divider bridge may be modified. Also the transistor T could be replaced by any other switching device.
The invention relates to an electronic apparatus whose power is supplied by a battery ( 11) having a memory ( 32) which contains electrically erasable data ( 32 a) and a supply voltage detection device ( 37) for detecting a variation of the supply voltage (Vbat). The invention notably has for its object to provide a detection device ( 37) which is capable of instantaneously detecting various voltage levels to cut off the power supply (Vbat) without the contents of the memory ( 32) being changed and capable of indicating the charging level of a battery ( 11) or an overload. The invention may be used in portable telephones and other electronic equipment whose power is supplied by a power supply battery.