JPH0217836A - memory backup circuit - Google Patents

Abstract

PURPOSE:To prevent the outflow of a back-up current to a ground and to stand long use by providing an excess charge prevention element for preventing the excess charge of a back-up battery between a plus electrode of the back-up battery and a switching circuit. CONSTITUTION:When the external power is supplied, transistors TR2 and TR1 make continuity to supply the external power supply to a RAM 7, and, at the same time, a lithium battery (E) is charged. When the voltage of the lithium battery (E) reaches 3V, a Zener diode D2 makes continuity, so that an excess charge over 3V is prevented. When the external power supply is not supplied, the power is supplied to the RAM 7 from the lithium battery (E), but the transistor TR2 is turned off, so that the current supplied to a ground from the lithium battery (E) through the transistor TR1 is prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a memory bank amplifier circuit equipped with a backup battery, and in particular to a memory bank amplifier circuit that prevents current from leaking from the backup battery through an overcharge prevention element. Concerning what has been configured.

[Prior art]

Generally, in electronic devices such as electronic typewriters and word processors, input data is stored in volatile memory such as static RAM, and when the power switch of the electronic device is turned off, it is stored in volatile memory by a memory backup circuit. Data is continuously stored and retained.

This memory backup circuit is equipped with a rechargeable backup battery, which charges the bank-up battery with the voltage of the power supply when the electronic device is powered on, and backs up the backup battery when the power switch is turned off. It is configured to store and hold data in a volatile memory as a power source.

Further, when the bank amplifier battery is a rechargeable secondary battery, it is common to provide an overcharge prevention element such as a Zener diode to prevent overcharging of the hack-up battery.

For example, the memory backup circuit 18 is configured as shown in FIG. 4, and when the power switch is turned on, the Zener diode D3 becomes conductive when the voltage at the terminal 20 rises and reaches 3V, and a voltage is generated across the resistor R5. The voltage causes the base current of the transistor TR4 to flow, making the transistor TR4 conductive. Along with this, the transistor TR
3's base current flows, transistor TR3 also becomes conductive,
+5V is applied to RAM22 (7) terminal ■CC, RA
The data of M22 is stored and held. At this time, the lithium battery E1 is charged by the +5V power supply, but when the voltage of the lithium battery E1 reaches 3■, the Zener diode D4 becomes conductive, and the excess current is transferred to the terminal 21 via the Zener diode D4. Because it flows, lithium battery El
is designed to prevent overcharging.

Further, when the power switch is cut off, the voltage at the terminal 20 becomes lower than 3cm, and the Zener diode D3 is turned off, and the transistors TR4 and TR3 are also turned off. At this time, the backup voltage of the lithium battery El is applied to the terminal Vcc of the RAM 22, so that the data in the RAM 22 is continuously stored and held.

[Problem to be solved by the invention]

As shown in FIG. 8, the characteristics of the Zener diode D4 with a Zener voltage of 3.5 V and a Zener voltage of 3.5 V are applied to the Zener diode D4. A relatively large Zener current flows to the ground terminal 21 until it reaches 5■. As a result, as shown in FIG. 5, the voltage of the lithium battery E1 decreases by a voltage drop corresponding to the load when backing up the RAM 22 from the time t2 when the power switch is cut off to the time t3 after a predetermined time. , Zener diode D4
Due to the characteristics of the Zener diode D4, there is a large voltage drop including the voltage drop corresponding to the Zener current flowing out through the Zener diode D4, and there is a problem that the retention time for memorizing and retaining data in the RAM 22 with the lithium battery E1 is significantly shortened. .

Therefore, as shown in FIG. 6, a memory backup circuit 19 can be considered in which the memory bank amplifier circuit 18 is improved to suppress the power consumption of the lithium battery E1 by the Zener diode D4. That is, since the Zener diode D4 is connected to the positive terminal of the lithium battery E1 via the diode D6, the diode D6 prevents current from flowing from the lithium battery E1 to the Zener diode D4 when the power switch is cut off. Then, the power of the lithium battery El is consumed only for backing up the data in the RAM 22. As shown in FIG. 7, the voltage of the lithium battery E1 at this time is equal to the voltage drop corresponding to the load when backing up the RAM 22 from time t4 when the power is cut off to time t5 after a predetermined time. Therefore, the retention time for storing and retaining data in the RAM 22 using the lithium battery E1 can be extended.

However, this memory bank amplifier circuit 19 becomes complicated and requires two extra diodes D5 and D6, and a process such as wiring change is required to mount these two diodes D5 and D6 on a printed circuit board. There is a problem in that the cost is high due to the need for separately.

SUMMARY OF THE INVENTION An object of the present invention is to provide a memory backup circuit that can increase backup time by minimizing leakage of current from a backup battery with a simple circuit configuration.

[Means to solve the problem]

The memory backup circuit according to the present invention includes a switching circuit including a plurality of switching elements connected to a connection line from a power source to a volatile memory, which is closed when the power switch is turned off at 08, and opened when the power switch is turned off, and a switching circuit and a volatile memory. It is connected to the connection line that connects the power switch and is charged when the power switch is turned 08.
Sometimes a bank amplifier battery is provided to supply current to the volatile memory, and an overcharge prevention element of the backup battery is interposed between the positive terminal of the backup battery and the common connection point between the switching circuit and the switching circuit. However, it is configured to prevent current from leaking from the bank-up battery via the overcharge prevention element.

[Effect]

In the memory backup circuit according to the present invention, when the power switch is turned on, the plurality of switching elements are turned on to close the switching circuit, and the volatile memory is supplied with current from the power supply via the switching circuit. Therefore, data stored in volatile memory is retained. At this time, since the backup battery is connected to the connection line that connects the switching circuit and the volatile memory, it is charged with the current supplied from the power source via the switching circuit.

An overcharge prevention element is interposed between the positive terminal of this bank-up battery and the common connection point between the switching circuit's switching elements, so that after the backup battery is fully charged, the backup battery The current flowing through the battery is bypassed to ground via the overcharge prevention element and the switching element, thereby preventing overcharging of the backup battery.

When the power switch is turned off, the plurality of switching elements are turned off and the switching circuit is opened, so that the backup battery is no longer charged. However, since the backup battery supplies current to the volatile memory at this time, the data stored in the volatile memory is not erased and is continuously stored and retained.

In addition, the leakage current that leaks from the bank-up battery to the ground via the overcharge prevention element and switching element is
It is blocked by the OFF state of the switching element, so
The power of the bank-up battery is consumed only for backup purposes in volatile memory.

〔Effect of the invention〕

According to the memory backup circuit of the present invention, an overcharge prevention element for preventing overcharging of the backup battery is interposed between the positive electrode of the backup battery and the common connection point between the switching elements of the switching circuit. Therefore, when the power switch is turned off and the switching circuit is opened to back up the volatile memory with the bank-up battery, current leaks from the backup battery to the ground via the overcharge prevention element and the switching element. Since the power of the backup battery is consumed only for backing up the volatile memory, the backup time of the backup battery can be significantly increased.

Furthermore, since the circuit configuration is simple without providing any extra diodes, there is no need to increase the number of parts, and costs can be reduced.

〔Example〕

An embodiment in which the present invention is applied to an electronic typewriter will be described below with reference to the drawings.

As shown in the block diagram of the control system in FIG. 1, the typewriter 1 stores and retains various data stored in the RAM 7 when the printing mechanism 2, keyboard 3, control bag MC, and power switch are turned off. It is equipped with a backup circuit 4, etc.

The printing mechanism 2 includes a carriage drive mechanism that moves the carriage on which the print head is placed along the length of the platen, its drive motor, and its drive circuit, a paper feed mechanism that feeds the printing paper, its drive motor, and its drive. It consists of a circuit, a print head drive circuit, etc.

The keyboard 3 includes alphabet keys, numeric keys,
A large number of keys are provided, including character keys including a space key and function keys.

The control device C is an l-chip CPU (central processing unit).
5 and R connected to the CPU 5 via a data bus etc.
It consists of OM (read-only memory) 6 and RAM (random access memory) 7, etc.
The printing mechanism 2 and the keyboard 3 are each connected to the CPU 5 via a data bus or the like.

The ROM 6 has a printing mechanism 2 corresponding to code data input from each character key and various function keys of the keyboard 3.
Control programs and the like for controlling are stored. R.A.
M7 is a memory that stores data input from the keyboard 3 and temporarily stores the results of arithmetic processing by the CPU 5.
Various types of memory and the like are provided for Q. Furthermore, RAM
Reference numeral 7 is composed of a static RAM, which is a volatile memory.

Next, when the power switch of the typewriter L is turned off, a backup circuit 4 supplies power from the lithium battery E (bank-up battery) to the RAM 7, which is a volatile memory.
will be explained based on FIG.

Terminal IO connected to +5v line (constant voltage line) supplied from a power source not shown and terminal Vcc of RAM7
In the middle of the connection line L1 that connects the
A PNP type transistor TRI (switching element) is provided to cut off or conduct the RAM 7, and a terminal II connected to the ground and a terminal GND of the RAM 7 are connected by a ground line L2. Also, the connection line L1 between the terminal 10 and the emitter of the transistor TRI and the ground "f"
A Zener diode D1 with a Zener voltage of 3.5 cm and a resistor R1 are connected in series to the contact Vt line L3 that connects rML2.

A connection line L4 connecting the base of the transistor TRI and the ground line L2 includes a resistor R3 and an NPN transistor TR2 for flowing the base current of the transistor TRI.
(switching element) is connected in series with the base of the transistor TR2 and the Zener diode D3 connected to the base of the transistor TR2.
and the connection point between the resistor R1 and the resistor R1 are connected via the resistor R2.

Collector of transistor TRI and terminal Vcc of RAM7
A resistor R4 and a lithium battery E (secondary battery) for backing up data stored in the RAM 7 are connected in series to a connecting line L5 that connects the connecting line L1 and the ground line L2.

Furthermore, the positive electrode of the lithium battery E and the transistor TR
A Zener diode D2 (overcurrent prevention element) with a Zener voltage of 3V is interposed between the collector of the resistor R2 and the common contact P of the resistor R3.

Note that the resistance R1 has a relatively large resistance value, but the resistance R2
, resistor R3, and resistor R4 are set to small resistance values. Note that a switching circuit is configured by the transistor TRI, the transistor TR2, and the resistors R1 to R3.

Next, the operation of storing and holding data in each memory of the RAM 7 when the power switch of the typewriter I is turned on will be explained based on FIG.

When the power switch of the typewriter 1 is turned on and the voltage at the terminal IO rises to approximately 3■, the Zener diode DI becomes conductive and a Zener current flows through the connection line L3, and the voltage generated across the resistor R1 causes the transistor to The base current of TR2 flows and transistor TR2 becomes conductive. As a result, the base current of the transistor TRI flows through the connection line L4, and the transistor TR2 also becomes conductive, and +5V is applied to the terminal Vcc of the RAM7 via the connection line L1, and the RA
The data of M7 is stored and held.

At this time, the lithium battery E is charged by the +5■ power supply, but when the voltage (charging voltage) of the lithium battery E reaches approximately 3V and is fully charged, the excess current is transferred to the Zener diode D2, the connection line L4, and the transistor. Since the current flows to the ground line L2 via TR2, the lithium battery E will not be overcharged.

Next, the operation when backing up data in the RAM 7 using the lithium battery E when the power switch of the typewriter 1 is turned off will be explained.

Typewriter 1 is powered off and the +5 ■ line is about 3
When the voltage becomes lower than (2), the Zener diode DI is turned off, the transistor TR2 is turned off, and the transistor TRI is also turned off. Therefore, the connection line L
1 is cut off, and the +5V voltage is no longer supplied to the terminal Vcc, but at this time, the bank-up voltage of the lithium battery E is applied to the terminal Vcc of the RAM7, and current is supplied to the RAM7 from the lithium battery E. Data is continuously stored and retained without being erased.

At this time, the only current flowing from the positive terminal of the lithium battery E is the current 1) flowing to the terminal Vcc of the RAM 7.

This is because the transistor TRL is turned off, so the connection line L1 is cut off, and the transistor TR2
Since the connection line L4 is turned off, the connection line L4 is also cut off. Therefore, leakage of current from the lithium battery E to the +5■ line and leakage of Zener current based on the Zener characteristics of the Zener diode D2 (see FIG. 8) are prevented.

As can be seen from the voltage characteristics of the lithium battery E shown in FIG. Therefore, the life of the lithium battery E, that is, the time during which data stored in the RAM 7 can be stored and retained can be greatly increased.

[Brief explanation of the drawing]

1 to 3 of the drawings show embodiments of the present invention. FIG. 1 is a block diagram of a control system of a typewriter equipped with a memory backup circuit, FIG. 2 is a block diagram of a control system of a typewriter equipped with a memory backup circuit, and FIG. Figure 3 is a diagram of voltage characteristics of lithium batteries, Figure 4
The figure shows a memory backup circuit according to the prior art, Figure 5 is a diagram corresponding to Figure 3 of the memory pack amplifier circuit of Figure 4, and Figure 6
The figure shows a memory backup circuit according to the prior art, FIG. 7 is a diagram corresponding to FIG. 3 of the memory bank amplifier circuit of FIG. 6, and FIG.
The figure is a diagram of Zener diode characteristics. 7... Static RAM, D2... Zener diode, E... Lithium battery, TRI/TR2...
Transistor, R1-R3...resistance. Patent applicant: Brother Industries, Ltd. Figure 3 Figure Figure 6 Figure 7 Figure ii1

Claims (1)

[Claims] (1) Interposed in the connection line from the power supply to the volatile memory,
Closes when the power switch is turned on and the power switch is turned on.
It is connected to a switching circuit consisting of a plurality of switching elements that is opened during FF and a connection line that connects the switching circuit and the volatile memory, and is charged when the power switch is ON, and supplies current to the volatile memory when the power switch is OFF. a backup battery to be supplied, an overcharge prevention element of the backup battery is interposed between the positive terminal of the backup battery and a common connection point between the switching elements of the switching circuit, and the A memory backup circuit characterized in that it is configured to prevent current from leaking through an overcharge prevention element.