JP2018005572A - Electronic instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a function attached with a time limit from being unusable.SOLUTION: An electronic instrument 100 equipped with a function attached with a time limit comprises: a main power source 11; an auxiliary power source 15 to be charged by a power supply from the main power source 11; a voltage monitor unit 18 that measures a voltage value of the auxiliary power source 15; a clock circuit 17 that creates time information, is operative by a power supply from the main power source 11 upon turning on the main power source 11, and is operative by a power supply from the auxiliary power source 15 upon turning off the main power source 11; a storage unit 20 that stores the time information and the voltage value; and a control unit that determines whether the function attached with the time limit is valid by a second time based on time information on a first time, a first voltage value at the first time, and a second voltage value after the first time and measured prior to a start of charging from the main power source 11 to the auxiliary power source 15 when the main power source 11 is switched from ON to OFF.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electronic device including a control unit that determines whether or not a function with a time limit is valid.

  Since recent electronic devices have a communication function and a network connection function, there is an increasing interest in security technology. For example, a commercial wireless communication device is equipped with a function with a time limit (encryption key with a time limit) for the purpose of preventing unauthorized use. By setting a time limit in advance for such a device, the device cannot be used after the expiration.

  However, if the clock function inside the device becomes inoperable due to a power supply voltage drop, time information may be lost. In that case, the device cannot be used even within the time limit. As means for solving such a problem, Patent Document 1 proposes a technique of storing time information in a non-volatile memory and automatically resetting the time after power is restored.

  Some electronic devices having a time limit function include a clock circuit and a backup auxiliary power source so that time limit management can be performed even after the main power is turned off. In such a device, even after the main power supply is turned off, the auxiliary power supply maintains the operation of the clock circuit and continues the time limit management.

JP 2006-52962 A

  However, the capacity of the auxiliary power supply decreases due to discharge, and the output voltage gradually decreases accordingly. When the output voltage of the auxiliary power source falls below the operating voltage of the timepiece circuit, the timepiece circuit stops operating and time information disappears. When the time information is lost, the function with a time limit becomes unusable even if it is within the time limit.

  The present invention has been made to solve such a problem, and an object of the present invention is to prevent the time-limited function from being disabled.

An electronic device having a time-limited function according to the present invention includes a main power source, an auxiliary power source charged by power supply from the main power source, a voltage monitoring unit that measures a voltage value of the auxiliary power source,
A timepiece circuit that generates time information and operates by power supply from a main power supply when the main power supply is on, and operates by power supply from the auxiliary power supply when the main power supply is off;
A storage unit for storing the time information and the voltage value;
The time information of the first time, the first voltage value at the first time, and when the main power source is switched from off to on after the first time, the main power source to the auxiliary power source A controller that determines whether or not the time-limited function is valid at a second time based on a second voltage value measured prior to starting charging to
Is provided.

  By this invention, it can prevent that a function with a time limit becomes unusable.

2 is a block diagram according to Embodiment 1. FIG. 3 is a discharge characteristic diagram of an auxiliary power supply according to Embodiment 1. FIG. 3 is a flowchart according to the first embodiment. 3 is a flowchart according to the first embodiment. 3 is a flowchart according to the first embodiment. It is a block diagram concerning Embodiment 2 and Embodiment 3. 6 is a flowchart according to the second embodiment. 6 is a flowchart according to the second embodiment. 10 is a flowchart according to the third embodiment.

Embodiment 1
First, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an electronic device 100 according to the first embodiment. The electronic device 100 includes a main power supply 11, a power switch 12, a power supply circuit 13, a switching circuit 14, an auxiliary power supply 15, a power supply mixing circuit 16, a clock circuit 17, a voltage monitoring unit 18, a control circuit 21, and an electronic circuit unit 23. The control circuit 21 includes a control unit 19 and a storage unit 20, and the switching circuit 14, the clock circuit 17, the voltage monitoring unit 18, the control unit 19, and the storage unit 20 are connected to the communication bus 22. The electronic device 100 includes a time-limited encryption key, and the user can set a time limit in advance. The electronic device 100 is limited in its predetermined functions after a set time limit. For example, if the electronic device 100 is a wireless communication device and the electronic circuit unit 23 is a wireless communication unit, the electronic device 100 disables the wireless communication unit. Details of each block will be described below.

  The main power supply 11 supplies power by closing the power switch 12 to operate the electronic device 100. The main power source 11 is a DC stabilized power source or a rechargeable secondary battery that uses a commercial power source as a power source. Examples of rechargeable secondary batteries include lithium ion batteries and automobile batteries. The user switches whether the power switch 12 supplies the power received from the main power supply 11 to the power supply circuit 13 or not. The power supply circuit 13 stabilizes the voltage from the main power received via the power switch 12 and supplies power to the switching circuit 14, the power supply mixing circuit 16, the control circuit 21, and the electronic circuit unit 23.

  The switching circuit 14 receives an instruction from the control circuit 21 and switches whether to supply the power supplied from the power supply circuit 13 to the auxiliary power supply 15. The switching circuit 14 is connected to the power supply circuit 13, the auxiliary power supply 15, and the communication bus 22, respectively.

  The auxiliary power supply 15 is a rechargeable secondary battery, such as a lithium ion battery or a nickel metal hydride battery. When the main power supply 11 is a rechargeable secondary battery, the battery capacity of the auxiliary power supply 15 is smaller than that of the main power supply 11. The auxiliary power supply 15 is charged by supplying power from the main power supply 11 when receiving power from the switching circuit 14. When the supply of power from the switching circuit 14 is stopped, the auxiliary power supply 15 supplies power to the control circuit 21 and the clock circuit 17 via the power supply mixing circuit 16. The auxiliary power supply 15 is connected to the voltage monitoring unit 18, and the voltage is monitored by the voltage monitoring unit 18.

  The power supply mixing circuit 16 is configured by, for example, a voltage comparator and a switching circuit. When power is supplied from the power supply circuit 13, the power supply mixing circuit 16 supplies this power to the clock circuit 17. When power is not supplied from the power supply circuit 13, the power supply mixing circuit 16 supplies power received from the auxiliary power supply 15 to the timepiece circuit 17.

  The clock circuit 17 is a circuit that receives power supplied from the power supply mixing circuit 16 and generates time information. The clock circuit 17 is generally called RTC (Real Time Clock). The clock circuit 17 has a function of recording an error history, and records that it is impossible to generate time information continuously when power supply is cut off. For example, the clock circuit 17 stores the error history as a flag. The initial value of the error flag is “0”, and “1” when an error occurs. The clock circuit 17 is connected to the communication bus 22 and outputs time information and error history to the control circuit 21 as necessary.

  The voltage monitoring unit 18 measures the voltage value of the auxiliary power supply 15. The voltage monitoring unit 18 includes an A / D converter (Analog to Digital converter), and outputs the measured voltage value to the communication bus 22. The voltage monitoring unit 18 is connected to the auxiliary power supply 15 and the communication bus 22, respectively. The voltage value corresponds to the remaining battery capacity of the auxiliary power supply 15.

  The control circuit 21 includes a control unit 19 and a storage unit 20. The control circuit 21 is supplied with power from the power supply circuit 13.

  The control unit 19 reads time information and error history generated by the clock circuit 17, and further reads a voltage value measured by the voltage monitoring unit 18. Then, the control unit 19 accesses the storage unit 20, writes the read time information and voltage value, and reads the written information. Further, the control unit 19 determines whether or not the time-limited encryption key is valid according to the read information. Specifically, the control unit 19 calculates elapsed time from each data based on the principle shown in FIG. 2, and determines whether or not the time-limited encryption key is valid.

  The memory | storage part 20 memorize | stores time information and a voltage value, and keeps this, even after supply of electric power is cut off. The storage unit 20 is a nonvolatile storage device such as an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory, or a FeRAM (Ferroelectric Random Access Memory). The storage unit 20 is connected to the communication bus 22.

  The communication bus 22 connects the switching circuit 14, the clock circuit 17, the voltage monitoring unit 18, and the control circuit 21. The communication bus 22 is an in-machine communication protocol such as I2C (Inter-Integrated Circuit) or UART (Universal Asynchronous Receiver Transmitter).

  Next, the relationship between the voltage and time of the auxiliary power supply 15 according to the first embodiment will be described with reference to FIG.

  The vertical axis in FIG. 2 indicates voltage, and the horizontal axis indicates time. A curve 30 represents discharge characteristics when a lithium ion secondary battery used as a general backup auxiliary power source is employed. The electronic device 100 operates by receiving power from the main power supply 11. The auxiliary power supply 15 continues to be charged during that time and maintains a substantially constant voltage V1.

  Here, at time t1, the power switch 12 is turned off, and charging of the auxiliary power supply 15 is stopped. Then, the electronic device 100 uses the auxiliary power supply 15 to switch the power supply to a preset circuit such as the clock circuit 17 for example. The clock circuit 17 continues to operate with power supplied from the auxiliary power supply 15. The auxiliary power supply 15 consumes the stored energy, and the voltage gradually decreases. When the auxiliary power supply 15 consumes energy, the internal resistance of the battery itself increases. When the inflection point 31 is reached, the voltage drops rapidly.

  The operation guarantee voltage of the clock circuit 17 is V2, and when it falls below V2, the clock circuit 17 does not operate. The auxiliary power supply 15 gradually decreases the voltage and reaches the voltage V2 at time t2. Eventually, the voltage of the auxiliary power supply falls below the voltage V2, and the operation of the clock circuit 17 stops.

  After the clock circuit 17 stops operating, the impedance of the clock circuit 17 increases and the discharge current of the auxiliary power supply 15 decreases, so that the voltage drop becomes gentle (inflection point 32). Thereafter, the voltage of the auxiliary power source 15 gradually decreases due to the dark current flowing through the clock circuit 17 that has stopped operating.

  The relationship between the discharge time and the voltage represented by the curve 30 is determined by the specifications of the auxiliary power supply 15 and the circuit configuration of the electronic device 100. Therefore, if the curve 30 is grasped in advance by actual measurement or the like and the time tx and the voltage Vx before the operation guarantee voltage V2 of the clock circuit 17 are stored are stored, the time ty is predicted by measuring the voltage Vy. It becomes possible to do. The first embodiment uses this principle to solve this problem. Note that the graph shown in FIG. 2 may be measured by the actual electronic device 100 or may be measured by using an apparatus having the same type of circuit configuration.

  Next, processing of electronic device 100 according to Embodiment 1 of the present invention will be described with reference to the flowcharts shown in FIGS. FIG. 3 is a flowchart showing an interrupt operation before the main power supply 11 is turned off. FIG. 4 is a flowchart showing processing after the main power supply 11 of the electronic device 100 is turned off and before the main power supply 11 is turned on again. FIG. 5 is a flowchart showing processing after the main power supply 11 is turned on again.

  The control circuit 21 stores the time information generated by the clock circuit 17 and the voltage value measured by the voltage monitoring unit 18 at the time determined based on the timing when the main power supply 11 is switched from on to off. Here, the control circuit 21 stores the time information generated by the clock circuit 17 as an interrupt process and the voltage value measured by the voltage monitoring unit 18 at that time before the main power supply 11 of the electronic device 100 is turned off. (S100). The control circuit 21 can periodically repeat this interrupt process to overwrite the time information and the voltage value. Here, in the first embodiment, the last stored time is Txa and the voltage value at time Txa is Vxa before the main power supply 11 is turned off.

  Next, the user turns off the power switch 12 (S101). Then, the supply of power from the main power supply 11 is stopped, and the control circuit 21 is stopped (S102). Subsequently, the power supply mixing circuit 16 switches the output voltage to the auxiliary power supply 15 (S103).

  After step S103, the clock circuit 17 continues to operate by receiving power from the auxiliary power supply 15. The auxiliary power supply 15 drives the clock circuit 17 while gradually reducing the capacity. The output voltage of the auxiliary power supply 15 decreases with time. When the voltage received by the timepiece circuit 17 falls below the operation guarantee voltage of the timepiece circuit 17, the timepiece circuit 17 eventually stops operating (S104).

  Even after the clock circuit 17 stops operating, the auxiliary power supply 15 continues to discharge (S105).

  Next, processing after the user turns on the power switch 12 again will be described with reference to FIG. The user turns on the power switch 12 again (S201). Then, a voltage is supplied to the control circuit 21. Thereby, the control circuit 21 starts operation again (S202).

  Subsequently, the power supply mixing circuit 16 switches the power supplied to the clock circuit 17 from the auxiliary power supply 15 to the main power supply 11 (S203). At this time, the clock circuit 17 records as an error history whether or not the operation has stopped while receiving power from the auxiliary power supply 15. Specifically, for example, the clock circuit 17 switches the error flag from 0 to 1.

  Next, the control circuit 21 reads the voltage value of the auxiliary power supply 15 measured by the voltage monitoring unit 18 (S204). In the first embodiment, this voltage value is Vya.

  Subsequently, the control circuit 21 sends an instruction to the switching circuit 14 to start charging to the auxiliary power supply 15 (S205). That is, the control circuit 21 reads the voltage value of the auxiliary power supply 15 before starting charging of the auxiliary power supply 15.

  Next, the control circuit 21 reads the error history of the clock circuit 17 and determines whether or not the time information generated by the clock circuit 17 is valid (S206). Specifically, the control circuit 21 reads the value of the error flag of the clock circuit 17. When the error flag is 0, the control circuit 21 determines that the time information is valid (S206: Yes). In this case, the control circuit 21 proceeds to a step (S207) for determining whether or not the time limit of the time-limited encryption key is valid. When the error flag is 1, the control circuit 21 does not determine that the time information is valid (S206: No). In this case, the control circuit 21 proceeds to a step (S210) for determining whether or not the elapsed time can be calculated.

  The control circuit 21 determines whether or not the elapsed time can be calculated (S210). First, the control circuit 21 reads the time information of the clock circuit 17 stored before the power switch 12 is turned off in step S100 and the voltage value of the auxiliary power supply 15 at that time. That is, in the first embodiment, the control circuit 21 reads the time information Txa and the voltage value Vxa. Next, the voltage value Vya of the auxiliary power supply 15 measured in step S204 is read. When the voltage value Vya is within a predetermined range, the control circuit 21 can calculate the elapsed time based on the principle described with reference to FIG. For example, in the first embodiment, referring to the graph shown in FIG. 2, ty-tx is the elapsed time. Therefore, the current time information can be obtained by adding the elapsed time ty-tx to the time information Txa. The elapsed time calculation method may be a method based on a predetermined calculation formula stored in the storage unit 20 or a method of storing a predetermined table in the storage unit 20 and applying an approximate value. After the control circuit 21 calculates the elapsed time (S210: Yes), the control circuit 21 updates the time information (S211), and proceeds to step S207. On the other hand, if the voltage value is not measured within the predetermined range in step S204, the control circuit 21 cannot calculate the elapsed time (S210: No). In that case, the control circuit 21 initializes time information of the clock circuit. Then, the control circuit 21 proceeds to a step (S212) in which the user inputs time information. For example, a display unit (not shown) displays a message that prompts input of time information. When the user inputs time information via a display unit and an input unit (not shown), the control circuit 21 proceeds to step S207.

  Subsequently, the control circuit 21 determines whether the time information is within the expiration date of the preset encryption key (S207). When the control circuit 21 determines that the time information is within the validity period of the encryption key (S207: Yes), the control circuit 21 validates the function of the electronic circuit unit 23 that is a wireless communication unit (S208). That is, the electronic device 100 is in a state where communication using the encryption key is possible, and the control circuit 21 accepts an operation by the user. On the other hand, when the control circuit 21 does not determine that the time information is within the validity period of the encryption key (S207: No), the control circuit 21 invalidates the function of the electronic circuit unit 23 that is a wireless communication unit (S209). ). That is, communication using the encryption key is invalid in the electronic device 100, and the control circuit 21 does not accept an operation by the user.

  As described above, according to the present embodiment, it is possible to prevent the time-limited function from being disabled while suppressing power consumption in a state where the main power source is turned off.

Embodiment 2
Next, a second embodiment according to the present invention will be described. FIG. 6 is a block diagram of Embodiment 2 according to the present invention. In order to avoid redundant description, the same blocks as those in the first embodiment are given the same numbers.

  Differences between the electronic device 200 and the electronic device 100 are a power supply circuit 40, a power supply mixing circuit 41, and a control circuit 44. The differences will be described below.

  The power circuit 40 stabilizes the voltage from the main power received via the power switch 12 and supplies power to the switching circuit 14 and the power mixing circuit 41.

  When receiving power from the power supply circuit 40, the power supply mixing circuit 41 supplies this to the control circuit 44 and the clock circuit 17. The power supply mixing circuit 41 is configured by, for example, a voltage comparator and a switching circuit. The power supply mixing circuit 41 supplies the power received from the auxiliary power supply 15 to the control circuit 44 and the timepiece circuit 17 when no power is supplied from the power supply circuit 40.

  The control circuit 44 includes a control unit 42 and a storage unit 43. The control circuit 44 receives power from the power supply mixing circuit 41.

  The control unit 42 reads the time information and error history generated by the clock circuit 17 and further reads the voltage value measured by the voltage monitoring unit 18. And the control part 42 accesses the memory | storage part 43, writes the read time information and voltage value, and reads the written information. Further, the control unit 42 determines whether or not the time-limited encryption key is valid according to the read information. Specifically, the control unit 42 calculates the elapsed time from each data based on the principle shown in FIG. 2, and determines whether or not the time-limited encryption key is valid.

  Due to the differences described above, the electronic device 200 supplies the power of the auxiliary power supply 15 to the control circuit 44 and the timepiece circuit 17 when the power switch 12 is turned off.

  Next, processing of the electronic device 200 according to Embodiment 2 of the present invention will be described with reference to the flowcharts shown in FIGS. FIG. 7 is a flowchart showing processing after the main power is turned off and before the main power is turned on again. FIG. 8 is a flowchart showing processing after the main power supply is turned on again.

  First, the details of the flowchart of FIG. 7 will be described. The user turns off the power switch 12 (S301). Then, the supply of power from the main power supply 11 to the power supply circuit 40 and later stops. Accordingly, the power supply mixing circuit 41 switches the power supply output to the control circuit 44 and the clock circuit 17 from the main power supply 11 to the auxiliary power supply 15 (S302).

  Next, the control circuit 44 stores the time information generated by the clock circuit 17 and the voltage value measured by the voltage monitoring unit 18 at the time determined based on the timing when the main power supply is switched from on to off. To do. Here, the control circuit 44 reads the time information generated by the clock circuit 17 and determines whether or not a predetermined time set in advance elapses (S303). Here, the predetermined time set in advance refers to a time during which the clock circuit 17 can operate by the auxiliary power supply 15. Such time is determined based on circuit design, experimental data, and the like. If the predetermined time has not elapsed (S303: No), the control circuit 44 further reads time information. When the predetermined time has passed (S303: Yes), the control circuit 44 reads the time information and the voltage value of the auxiliary battery at that time, and stores them in the storage unit 43 (S304). Here, in the second embodiment, the time stored in the storage unit 43 is Txb, and the voltage value at the time Txb is Vxb.

  Thereafter, the auxiliary power supply 15 continues to supply voltage to the control circuit 44 and the clock circuit 17. Then, after falling below the operation guarantee voltage of the timepiece circuit 17, the timepiece circuit eventually stops operating (S305).

  Even after the clock circuit stops operating, the auxiliary power supply 15 continues to discharge (S306).

  Next, processing after the user turns on the power switch 12 again will be described with reference to FIG. First, the user turns on the power switch 12 again (S401). Then, the supply of power from the main power supply 11 to the power supply circuit 40 is resumed. Accordingly, the power supply mixing circuit 41 switches the power supply output to the control circuit 44 and the clock circuit 17 from the auxiliary power supply 15 to the main power supply 11 (S402).

  Next, the control circuit 44 reads the voltage value measured by the voltage monitoring unit 18 (S403). In the second embodiment, this voltage value is Vyb.

  Subsequently, the control circuit 44 issues an instruction to switch the switching circuit 14 and starts charging the auxiliary power supply 15 (S404).

  Next, the control circuit 44 reads the error history of the clock circuit 17 and determines whether or not the time information generated by the clock circuit 17 is valid (S405). Specifically, the control circuit 44 reads the value of the error flag of the clock circuit 17. When the control circuit 44 determines that the time information is valid (S405: Yes), the control circuit 44 determines whether or not the time limit of the time-limited encryption key is valid (S406). When the control circuit 44 does not determine that the time information is valid (S405: No), the control circuit 44 determines whether or not the elapsed time can be calculated (S408).

  In determining whether the elapsed time can be calculated, the control circuit 44 refers to the time information Txb stored in step S304, the voltage value Vxb at that time, and the voltage value Vyb measured in step S403. When the voltage value Vyb is within the predetermined range, the control circuit 44 can calculate the elapsed time based on the principle described with reference to FIG. When the control circuit 44 can calculate the elapsed time (S408: Yes), the control circuit 44 updates the time information (S409), and proceeds to step S406. On the other hand, when the voltage monitoring unit 18 cannot measure the voltage value within the predetermined range in step S403, the control circuit 44 cannot calculate the elapsed time (S408: No). In that case, the control circuit 44 initializes the time information of the clock circuit 17. Then, the control circuit 44 proceeds to a step (S410) for allowing the user to input time information. For example, a display unit (not shown) displays a message that prompts input of time information. When the user inputs time information via a display unit and an input unit (not shown), the control circuit 44 proceeds to step S406.

  Subsequently, the control circuit 44 determines whether or not the time information is within the expiration date of the preset encryption key (S406). When the control circuit 44 determines that the time information is within the validity period of the encryption key (S406: Yes), the control circuit 21 validates the function of the electronic circuit unit 23, which is a wireless communication means (S407). That is, the electronic device 200 is in a state where communication using the encryption key is possible, and the control circuit 44 accepts an operation by the user. On the other hand, when the control circuit 44 does not determine that the time information is within the validity period of the encryption key (S406: No), the control circuit 21 invalidates the function of the electronic circuit unit 23 that is a wireless communication unit (S411). ). That is, communication using the encryption key is invalid in the electronic device 200, and the control circuit 44 does not accept an operation by the user.

  Note that step S304 in FIG. 7 may be performed once, or may be performed a plurality of times before the operation of the clock circuit 17 is stopped, and the data in the storage unit may be overwritten each time.

  As described above, according to the present embodiment, by monitoring the voltage of the auxiliary power supply even after the main power supply is turned off, the elapsed time can be accurately calculated, and the time limit function can be disabled. Can be prevented.

Embodiment 3
Next, a third embodiment according to the present invention will be described. The block configuration of the third embodiment is the same as that of the second embodiment (FIG. 6). The difference between the second embodiment and the third embodiment lies in the processing from when the main power source is turned off to before the main power source is turned on again. Therefore, description of the block diagram is omitted here.

  Next, processing of electronic device 200 according to Embodiment 3 of the present invention will be described with reference to the flowchart shown in FIG. FIG. 9 is a flowchart showing processing after the main power is turned off and before the main power is turned on again. Steps S301 and S302 are the same as those in the second embodiment, and a description thereof is omitted here.

  Next, the control circuit 44 reads the voltage value measured by the voltage monitoring unit 18 and determines whether or not it falls below a predetermined voltage set in advance (S503). Here, the predetermined voltage set in advance refers to a voltage at which the timepiece circuit 17 can operate by the auxiliary power supply 15. In other words, the predetermined voltage set in advance is a voltage that does not fall below the operation guarantee voltage, and is a voltage higher than V2 in FIG. When the voltage is not lower than the predetermined voltage (S503: No), the control circuit 44 further reads the voltage value. When the voltage falls below the predetermined voltage (S503: Yes), the control circuit 44 reads the time information and the voltage value of the auxiliary battery at that time, and stores them in the storage unit 43 (S504). Here, in the third embodiment, the time stored in the storage unit 43 is Txc, and the voltage value at the time Txc is Vxc.

  Since the subsequent processing is the same as that of the second embodiment, the description of FIG. 9 from here to the continuation of discharge (S306) will be omitted.

  Further, in the electronic device 200 according to the third embodiment, the processing after the user turns on the power switch 12 again is the same as in FIG. However, in the third embodiment, the voltage value read by the control circuit 44 in step S403 is Vyc. In determining whether the elapsed time can be calculated, the control circuit 44 refers to the time information Txb stored in step S304, the voltage value Vxb at that time, and the voltage value Vyb measured in step S403. The other processes in the third embodiment are the same as the contents described in the second embodiment with reference to FIG.

  Note that step S504 may be performed once, or may be performed a plurality of times before the operation of the clock circuit 17 is stopped, and the data in the storage unit 43 may be overwritten each time.

  Since the processing when the main power supply 11 is turned on again in the third embodiment is the same as that in the second embodiment, the description thereof is omitted here.

  As described above, according to the third embodiment, the elapsed time can be accurately calculated by monitoring the voltage of the auxiliary power supply 15 even after the main power supply 11 is turned off, and the time-limited function cannot be used. Can be prevented.

  Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention. For example, the supply of power from the main power supply 11 can be directly supplied to the switching circuit 14 without passing through all the power switches 12. In addition, in order to reduce power consumption when operating using the voltage from the auxiliary power supply 15, the control circuit 44 may include an auxiliary arithmetic processing unit in addition to the main arithmetic processing unit. In that case, when the power switch 12 is turned off, the control unit 42 stops its operation, and an auxiliary calculation processing unit (not shown) is operated by the voltage supplied from the power supply mixing circuit 41, and each calculation is performed via the communication bus 22. May be performed. The electronic circuit unit 23 is not limited to a wireless communication unit as long as it is an electronic device whose operation or operation is restricted by an encryption key with an expiration date set.

100, 200 Electronic device 11 Main power supply 12 Power switch 13, 40 Power supply circuit 14 Switching circuit 15 Auxiliary power supply 16, 41 Power supply mixing circuit 17 Clock circuit 18 Voltage monitoring unit 19, 42 Control unit 20, 43 Storage unit 21, 44 Control circuit 22 Communication bus 23 Electronic circuit section

Claims (5)

An electronic device having a time-limited function,
A main power supply,
An auxiliary power source charged by power supply from the main power source;
A voltage monitoring unit for measuring a voltage value of the auxiliary power source;
A timepiece circuit that generates time information and operates by power supply from a main power supply when the main power supply is on, and operates by power supply from the auxiliary power supply when the main power supply is off;
A storage unit for storing the time information and the voltage value;
The time information of the first time, the first voltage value at the first time, and when the main power source is switched from off to on after the first time, the main power source to the auxiliary power source A controller that determines whether or not the time-limited function is valid at a second time based on a second voltage value measured prior to starting charging to
Comprising
Electronics. The controller is
Based on the time information at the first time, the first voltage value of the auxiliary power source at the first time, and the second voltage value of the auxiliary power source at the second time, from the first time Calculating an elapsed time until the second time;
The electronic device according to claim 1. The first time is
It is a time determined based on the timing at which the main power source is switched from on to off.
The electronic device according to claim 1 or 2. The first time is
After the main power is switched from on to off, the voltage value falls below a predetermined value.
The electronic device according to claim 1 or 2. Adding the calculated elapsed time to the first time and displaying the added time information;
The electronic device according to claim 2.

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