JPH08205424A - Backup power supply circuit - Google Patents

Abstract

PURPOSE: To prevent erroneous function of a microcomputer when the voltage of a backup battery in a backup power supply circuit drops. CONSTITUTION: The backup power supply circuit comprises circuits 4, 5 for detecting the battery voltage being fed to a microcomputer 8. When a data must be erased even if the battery voltage is higher than the lowest level for retaining the data in a memory, a backup battery 3 is discharged until the voltage thereof drops below the lowest data retaining voltage level thus erasing the data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backup power supply circuit for supplying power to a memory or a microcomputer by using a battery, and more particularly to a backup power supply circuit for a device having a clock function.

[0002]

2. Description of the Related Art Conventionally, in various devices having a clock function, backup power is supplied to operate the clock function even when the power is turned off. However, data or the like in the memory may be destroyed due to a battery failure or the like during backup power supply. Therefore, after turning on the power,
A technique for checking the data in the memory and initializing the memory and notifying the user when it is determined that the data has been destroyed is described in, for example, Japanese Patent Laid-Open No. 5-298195. Using this technique, the user can determine whether the data in the memory has been destroyed or not after the power is turned on. Further, in this technique, if the data in the memory is not destroyed, the data before the occurrence of the battery failure is used as the data when the power is turned on.

On the other hand, in wireless devices such as mobile phones, the operating voltage is becoming lower, the difference between the power supply voltage and the operating voltage of the microcomputer is narrowed, and the voltage of the backup battery and the microcomputer or memory (for example, SRAM). The difference from the operating voltage is also narrowing. Therefore, the power supply voltage from the backup battery to the microcomputer processing the clock data etc. tends to fall below the minimum operating voltage of the microcomputer. For example, the battery voltage may temporarily fall below the operating voltage of the microcomputer when the wireless device is inserted into or removed from the charger, or when the vehicle-mounted wireless telephone turns the ignition key of the car.

When the battery voltage drops due to such a disturbance or the like, using the above-described conventional technique, it is determined whether the contents of the memory are used as they are or the initial state is determined depending on whether the data in the memory is destroyed. There is no problem because it is done.

[0005]

However, when the power supply battery is kept removed for a long time and the backup battery capacity decreases, and the backup battery voltage becomes lower than the operating voltage of the microcomputer, the above-mentioned conventional technique is used. Since the memory retains the state before the backup battery voltage drops even after the power is turned on, there is a problem that the display time and the like are not accurate. This is due to the difference in the minimum voltage between the operating voltage of the microcomputer and the holding voltage of the memory.

In a portable wireless device, the minimum holding voltage of the memory is generally lower than the operating voltage of the microcomputer. For this reason, when the backup battery voltage drops due to the fact that the backup battery actually continues to supply power and the battery capacity decreases, the minimum operating voltage of the microcomputer is reached first. At this time,
In order to prevent the malfunction of the microcomputer, the microcomputer is forcibly put into the stopped state. When the operation of the microcomputer is stopped, the current consumption is extremely reduced, so that the backup battery voltage hardly drops. Therefore, if the backup battery voltage does not reach the minimum holding voltage of the memory, the memory normally stores data and the like.

That is, since the backup battery voltage is lower than the operating voltage of the microcomputer and higher than the minimum holding voltage of the memory, the data in the memory is not destroyed.
The abnormality cannot be detected after the power is turned on, and the data in the memory is retained. Then, even if the microcomputer is stopped for a long time, the time displayed after the power is turned on is the time when the data held in the memory, that is, the backup battery voltage becomes equal to or lower than the operating voltage of the microcomputer. It will shift. However, since the abnormality is not notified, the user cannot determine whether or not the correct time is displayed.

The object of the present invention is to solve the above-mentioned problems,
Another object of the present invention is to provide a backup power supply circuit that erases the contents of the memory when the battery voltage drops due to the battery capacity drop.

Another object of the present invention is to provide a backup power supply circuit capable of distinguishing a battery voltage drop due to disturbance or the like and a battery voltage drop due to battery capacity drop.

[0010]

In order to achieve the above-mentioned object, a backup power supply circuit according to the present invention includes a power supply battery, a backup battery, and a voltage detecting means for detecting the supplied battery voltage. And a determination unit that determines whether to discharge the backup battery according to the battery voltage.

The determining means may determine to discharge the backup battery when the battery voltage is the first voltage, for example, the minimum operating voltage of the microcomputer or less and the second voltage, for example, the minimum memory holding voltage or more. The backup power supply circuit may include a discharge circuit that discharges the backup battery to a voltage equal to or lower than the second voltage.

[0012]

By adopting the above-described structure, the backup power supply circuit according to the present invention can reduce the backup battery voltage to the minimum memory holding voltage by controlling the discharge of the backup battery when the battery voltage is within the predetermined voltage range. Therefore, the data in the memory can be erased. This prevents the old data in the memory from being used again when the power is turned on again.

Further, the microcomputer can be controlled by distinguishing the battery voltage drop due to the battery capacity drop from the voltage drop due to other factors.

[0014]

The present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of a backup power supply circuit showing an embodiment of the present invention. The backup power supply circuit includes a power supply battery 1, a charging circuit 2, a backup battery 3,
It is composed of a power supply battery voltage detection circuit 4, a backup battery voltage detection circuit 5, diodes 6 and 11, a discharge determination circuit 7, a microcomputer 8, a switch 9 and a discharge circuit 10.

Battery voltage of the battery 1, for example 4.5 to 5.0
V is a diode 1 in the microcomputer 8 that controls the clock function, etc.
It is supplied via 1. The microcomputer 8 is composed of, for example, a one-chip microcomputer, and has a memory (for example, SRAM) for storing time information and the like therein. This microcomputer 8
Is connected to a watch not shown. The minimum operating voltage of the microcomputer 8 is, for example, about 2.0V, and the minimum holding voltage of the internal memory is, for example, about 1.8V. The battery voltage is also supplied to the backup battery 3 via the charging circuit 2 composed of a resistor, for example. This is to prevent the battery current from the battery 1 from suddenly flowing to the backup battery 3 and destroying the battery 3. However, the charging circuit 2 is not always necessary, and the battery voltage from the battery 1 may not be supplied to the backup battery 3.

The battery voltage detection circuit 4 detects whether or not the battery voltage of the battery 1 or the backup battery 3 is equal to or higher than the minimum operating voltage (2.0V) of the microcomputer, and the detection output is detected by the discharge determination circuit 7 and the microcomputer 8. Supplied to the reset pin. Then, when the detected battery voltage is equal to or lower than the minimum operating voltage of the microcomputer, the microcomputer is reset.

The backup battery voltage detection circuit 5 detects whether the backup battery voltage is equal to or higher than the minimum holding voltage (1.8 V) of the memory, and the detected voltage is supplied to the discharge determination circuit 7.

The discharge determination circuit 7 determines whether or not the backup battery 3 is discharged, based on the outputs of the battery voltage detection circuit 4 and the backup battery voltage detection circuit 5, and the O of the switch 9 is turned on.
Control N / OFF. When the battery voltage is above the minimum operating voltage of the microcomputer or below the minimum holding voltage of the memory, the switch 9 is turned off. When the battery voltage is below the minimum operating voltage of the microcomputer and above the minimum holding voltage of the memory, the switch 9 is turned on. Become. When the switch 9 is ON, the backup battery 3 is connected to the discharge circuit 10, so that the discharge is started and the battery voltage supplied to the microcomputer 8 becomes low. The discharge circuit 10 is composed of a resistor, and one end thereof is grounded.

When the switch 9 is off, the battery voltage of the backup battery 3 is supplied to the microcomputer 8 via the diode 6. That is, when the battery voltage is equal to or higher than the minimum operating voltage of the microcomputer, the microcomputer is operating normally, and it is not necessary to discharge the backup battery 3. Further, when the battery voltage is equal to or lower than the minimum holding voltage (1.8 V) of the memory, the contents of the memory are erased and it is not necessary to discharge the backup battery any more. On the other hand, when the battery voltage is below the minimum operating voltage of the microcomputer and above the minimum holding voltage of the memory, the backup battery 3 is discharged so that the backup battery voltage falls below the minimum holding voltage of the memory. At this time, the contents of the memory are destroyed.

The microcomputer 8 is reset to prevent malfunction when the supplied battery voltage is equal to or lower than the minimum operating voltage of the microcomputer. When the supplied battery voltage exceeds the minimum operating voltage of the microcomputer, the memory contents are checked. If the battery voltage is destroyed, the memory contents are erased and an initial start is performed. On the other hand, if the contents of the memory are not destroyed, the contents of the memory are not erased, and the contents are restarted from the stored contents. For checking the memory contents of the memory in the microcomputer 8, for example, as shown in the above-mentioned prior art, predetermined data is stored in the memory in advance, and it is checked whether or not this data is destroyed. May be used.

Here, when the battery voltage supplied to the microcomputer 8 becomes lower than the minimum holding voltage of the memory, the contents of the memory are destroyed. Therefore, in the present invention, when the battery voltage supplied to the microcomputer becomes equal to or lower than the minimum operating voltage of the microcomputer, the backup battery 3 is discharged, and the battery voltage is forced to be equal to or lower than the minimum holding voltage of the memory. As a result, the contents of the memory are destroyed and an initial start is performed when the microcomputer restarts. On the other hand, if the voltage drops instantaneously due to disturbance or the like, the backup battery is discharged, but the battery voltage immediately recovers and exceeds the minimum operating voltage of the microcomputer, so the discharge is stopped and the contents of the memory are not destroyed.

The diode 6 is provided to prevent the current supplied from the battery 1 to the microcomputer 8 from flowing back to the backup battery 3. In addition, the diode 11
Is provided to prevent the current from the backup battery 3 from flowing back to the battery 1.

FIG. 2 is a diagram showing changes in the backup battery voltage. In FIG. 2, the solid line indicates the change in the backup battery voltage in the prior art, and the alternate long and short dash line indicates the change in the backup battery voltage in the present invention. Also, the two-dot chain line of indicates a temporary change of the backup battery voltage due to disturbance or the like.

When the backup battery voltage becomes equal to or lower than the microcomputer minimum operating voltage at time T1 when the power battery is not connected, the operation of the microcomputer is stopped. Therefore, the backup battery voltage does not drop to the memory minimum holding voltage (see the solid line), and the data in the memory is held. When the power battery is connected at time T2, in the conventional technique,
Since the data in the memory is not destroyed, (T2-T
Despite the passage of time between 1), at time T2, the time display is restarted based on the information of time T1.

On the other hand, in the present invention, as indicated by the alternate long and short dash line, the discharge determination circuit 7 turns on the switch 9 at the time T1 when the backup battery voltage falls below the minimum operating voltage of the microcomputer, so that the backup battery 3 is stored in the memory. The contents of the memory are destroyed by discharging to the minimum holding voltage. Therefore, when the battery is connected at time T2, the microcomputer 8
After the power is turned on, the memory contents check function erases the contents of the memory and performs an initial start.

The backup battery is discharged for a short time due to a temporary voltage drop (refer to the chain double-dashed line) due to disturbance such as when the power supply battery 1 is connected to the charger or when the ignition key of the vehicle is turned. However, since the backup voltage is immediately restored before it falls below the memory minimum holding voltage, the microcomputer minimum operating voltage is exceeded. Therefore, the switch 9 is turned off, the microcomputer checks the contents of the memory, and since the contents are not destroyed, the microcomputer restarts from the state held before the voltage drop.

Thus, the resistance value of the discharge circuit 10 is set so that the backup battery voltage does not fall below the memory minimum holding voltage in a short time so that it can be distinguished from the voltage drop due to disturbance.

Although the embodiment described above has two independent battery voltage detection circuits, one battery voltage detection circuit may be used for comparison with the minimum operating voltage of the microcomputer and the minimum holding voltage of the memory. In this case, one battery voltage detection circuit 4 or 5 in FIG. 1 is unnecessary.

[0030]

As described above, in the backup power supply circuit according to the present invention, when the battery voltage supplied to the microcomputer is less than the minimum operating voltage of the microcomputer and more than the minimum holding voltage of the memory, the backup battery is the minimum holding voltage of the memory. By discharging to the following and erasing the stored contents of the memory, the microcomputer starts the memory from the initial state after connecting the power battery.

Further, the discharge characteristic makes it possible to control the microcomputer by distinguishing battery voltage drop due to backup battery capacity drop from voltage drop due to other factors. The microcomputer does not malfunction due to a temporary voltage drop due to disturbance.

[Brief description of drawings]

FIG. 1 is a block diagram of a backup power supply circuit showing an embodiment of the present invention.

FIG. 2 is a diagram showing changes in backup battery voltage.

[Explanation of symbols]

 1 ・ ・ ・ Battery 2 ・ ・ ・ Charging circuit 3 ・ ・ ・ Backup battery 4 ・ ・ ・ Battery voltage detection circuit 5 ・ ・ ・ Battery voltage detection circuit 6 ・ ・ ・ Diode 7 ・ ・ ・ Discharge determination circuit 8 ・ ・ ・ Microcomputer 9 ・ ・ ・ Switch 10 ・ ・ ・ Discharge circuit 11 ・ ・ ・ Diode

Claims (7)

[Claims] 1. A backup power supply circuit, a power supply battery, a backup battery, voltage detection means for comparing a battery voltage supplied from the power supply battery or the backup battery with a predetermined voltage, and the backup according to the battery voltage. A backup power supply circuit, comprising: a determination unit that determines whether or not to discharge the battery. 2. A microcomputer having a built-in memory having a clock function or the like to which the battery voltage is supplied, wherein the minimum holding voltage of the memory and the minimum operating voltage of the microcomputer are different from each other. Backup power supply circuit. 3. The backup power supply circuit according to claim 2, wherein the determination means discharges the backup battery when the battery voltage is equal to or lower than the first voltage and equal to or higher than the second voltage. 4. The backup power supply circuit according to claim 3, further comprising discharging means for discharging the backup battery voltage to the second voltage. 5. The discharging means discharges the backup battery over a predetermined period of time or more.
The backup power supply circuit described. 6. The backup power supply circuit according to claim 3, wherein the first voltage is a minimum operating voltage of the microcomputer, and the second voltage is a minimum holding voltage of the memory. 7. The backup power supply circuit according to claim 1, wherein the backup battery is charged from the power supply battery.