JPH063454Y2 - Memory backup circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a computer system including a microprocessor and a memory, and a memory backup circuit adapted to supply backup power to the memory and retain the data in the memory in the event of a power failure. It is about.

A conventional circuit of this type is shown in FIG.
In the figure, (1) is a microprocessor (hereinafter, CPU
(2) is a random access memory (hereinafter referred to as RAM) forming a computer system with the CPU, and (3) is a power supply Vcc of the CPU (1) and the RAM (2).
Constant voltage diode (11) between cc (3) and ground GND
And the resistor (12) are connected in series, and the resistors (13) and (1
4) is connected in parallel with the series connection body, and the above resistance (12)
The voltage across both ends of the comparator (15) is applied as the reference voltage (+), while the divided voltage between the resistors (13) and (14) is applied as the comparison input (-) of the comparator (15). Has become. Then,
The output terminal of the comparator (15) is an open collector output terminal, which is connected to the power source Vcc (3) through a resistor (16), and the + terminal of the capacitor (17) and the reset terminal of the CPU (1). And is operated as follows.

That is, when the power supply voltage is equal to or lower than the predetermined voltage value Vcc ¹ when the power supply Vcc (3) rises, the comparison input of the comparator (15).
By selecting the circuit constant so that (-) becomes higher than the reference voltage (+), the output of the comparator (15) is in the L state, and therefore the CPU (1) is in the reset state at this time. , When the above power supply voltage exceeds the specified voltage value Vcc ¹ ,
By selecting the circuit constant so that the comparison input (-) becomes lower than the reference voltage (+), the output of the comparator (15) becomes H state, at which time the capacitor (17) starts charging and the resistor (16) ) And the capacitor (17) after a time determined by the CPU
The reset terminal (1) is in the H state and the reset is released.

On the other hand, on the RAM (2) side, the power supply Vcc (3) and the ground GND
A resistor (21), a constant voltage diode (22) and a resistor (23) are connected in series between them, and a transistor (24) is provided between the power supply Vcc (3) and the RAM (2). In the body, a series body of a transistor (25) whose base is connected to the connection point of the constant voltage diode (22) and the resistor (23), a resistor (26) and the transistor (24) is connected in parallel, and In addition to the above, the collector of the transistor (24) and the emitter of the transistor (25), that is, the power supply Vcc via the transistor (24).
A resistor (28) and a capacitor (27) that serves as a backup power supply are connected in series between (3) and ground GND, and RAM
Power is supplied to (2) as follows.

That is, when the power supply Vcc (3) becomes equal to or higher than a predetermined voltage value at the time of power supply startup, the constant voltage diode (22) becomes conductive,
As a result, the transistor (25) is turned on, and accordingly, a current flows through the base of the transistor (24), whereby the transistor (24) is turned on, and the power is supplied from the power supply Vcc (3) to the RAM (2). Similarly, power is supplied to the capacitor (27) serving as a backup power source via the resistor (28) to be charged.

In the configuration shown in FIG. 1, the transistor (24) is normally O.
The constant voltage diodes (11) and (22) are selected so that the CPU (1) is released from reset after N. Therefore, when the power supply voltage rises, the RAM (2) becomes the operating state. After that, the reset of the CPU (1) is released, whereby the system becomes operable.

On the other hand, at the fall of the power supply voltage, the RAM (2) is in the standby state after the CPU (1) is in the reset state, and the data can be held. Further, the capacitor (27) serves as a data holding power source in case of power failure.

Since the conventional device is configured as described above, the CPU
In order to reliably switch the power supply for switching the power supply of the RAM (2) from the power supply Vcc to the capacitor (27) after the reset state of (1), the variations of the constant voltage diodes (11) and (22) are required. It is necessary to select a constant so that it can be guaranteed even if it is taken into consideration. Normally, the voltage for resetting the CPU (1) Vcc ¹
The difference between the power supply switching voltage Vcc ² and the power supply switching voltage Vcc ² needs to be about 1 V, for example. Therefore, the capacitor (27) may be discharged to Vcc ² on the circuit, and it was necessary to increase the capacitor capacity in order to extend the backup time. Further, since the charging of the capacitor (27) is started after the power supply is switched, the charging current of the capacitor is generated as a drop of the power supply voltage, which may cause a malfunction.

The present invention has been made to eliminate the above-mentioned drawbacks, and the power supply can be surely switched after the CPU is reset, and the difference between Vcc ₁ and Vcc ² can be 0.1 V, for example. Therefore, it is easy to increase the backup time, and the capacitor capacity can be reduced,
Moreover, it is an object of the present invention to provide a memory backup circuit that does not cause a malfunction.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 shows an embodiment of the present invention, and those having the same symbols as those in the conventional example have the same or equivalent functions. And its description is omitted. Then, (19) is the comparator
(15) is the first comparator for resetting the CPU (1), while having a common reference voltage with this comparator,
It receives a comparison input having a lower potential than that of the comparison input of the comparator (15), and the RAM according to the voltage value of the power supply Vcc (3).
A second comparator adapted to switch the power supply to (2), a resistor (18) and a diode (20).

The above-mentioned configuration of FIG. 2 operates as follows. That is, when the power supply Vcc (3) of the CPU (1) is lower than a certain voltage Vcc ¹ , the comparison input (-) of the comparator (15) is higher than the reference voltage (+), so that the comparator (15) The output is in the L state, so C
PU (1) is in a reset state. Next, when the Vcc (3) becomes higher than the certain voltage Vcc ^{1, the} comparator input (-) becomes lower than the reference voltage (+), so that the output of the comparator (15) becomes H state, The capacitor (17) starts charging and resets the CPU (1) in the same manner as in the conventional example.

Therefore, when the comparison input (-) of the comparator (19) is lower than the certain voltage Vcc ^{2 of the} power source Vcc, the comparison input (-) of the comparator (19) is lower than the reference voltage (+). Since it is high, the comparator (1
The output of 9) becomes L state. On the contrary, when the power supply Vcc is higher than the certain voltage Vcc ² , the comparison input of the comparator (19)
(-) Is lower than the reference voltage (+), so the comparator
The output of (19) becomes H state. That is, since the output of the comparator (19) is an open collector, the HL signal is output as a power supply switching signal. This power supply switching signal is input to the base of the transistor (25) via the constant voltage diode (22). When the power supply switching signal is in the H state, the transistor (25) is in the ON state and the transistor (24) is in the ON state. A base current is passed to turn on the transistor (24).
That is, when the power supply Vcc is higher than the voltage value Vcc ² , the transistor (24) is turned on and the RAM (2) is turned on.
Supply power to. The series circuit of the resistor (28) and the capacitor (27) has a function as a backup power source at the time of power failure, and the capacitor (27) stores the electric charge accumulated when the transistor (24) is ON. Can be used as a backup power source.

When the power supply Vcc is lower than the power supply switching voltage Vcc ² , the transistor (24) is turned off and the supply from the power supply Vcc to the RAM (2) is stopped.
Since it is backed up by the capacitor (27),
The data in the RAM (2) is retained.

From the above, since the comparison input (-) of the comparator (19) is set to a lower potential than the comparison input (-) of the comparator (15), at the rise of the power supply Vcc, the power supply When Vcc is lower than the power supply switching voltage Vcc ² , the CPU
(1) is in the reset state, and the RAM (2) is also in the standby state. When the power supply Vcc is slightly higher than the power supply switching voltage Vcc ^{2 and} lower than the reset voltage Vcc ¹ , the RAM (2) is connected to the power supply Vcc and the capacitor (17) is in a charged state. Become. Further the power source
When Vcc becomes higher and becomes higher than the reset voltage Vcc ^{1, the} CPU (1) is also released from the reset, and the system becomes operable.

When the power supply Vcc falls, when the power supply Vcc is lower than the reset voltage Vcc ^{1 and} higher than the power supply switching voltage Vcc ² , the CPU (1) is reset and the capacitor
Potential of (27) there is a possibility that discharged to Vcc ^2, it is possible to lengthen the backup time for Vcc ² has been set high. Further, when the power source Vcc becomes lower than the power source switching voltage Vcc ² , the discharge of the capacitor (27) is stopped and the power source for backup is used.

Here, the constant voltage diode (22) connected to the output of the comparator (19) surely cuts an undetermined output signal below the operation guarantee voltage of the comparator (19) and the transistor (25) does not turn on. To do so. Further, the transistor (2
Diode (20) connected between the emitter and collector of 4)
Is for charging the capacitor (27), and the charging of the capacitor (27) is started before the power source is switched, so that the drop of the power source voltage due to the charging current does not cause a malfunction.

It should be noted that in the above embodiment, the comparator is described as an open collector type, but it goes without saying that the same effect can be obtained by applying a normal comparator that is not an open collector. Also, the constant voltage diode was used to cut off the undetermined output signal below the operation guarantee voltage of the comparator (19), but it goes without saying that the same effect can be obtained by appropriately connecting the diode. Nor.

As described above, according to the present invention, when the voltage of the power supply falls, the reset signal is output to the microprocessor driven by the voltage application of the power supply, and the OFF signal is output to the switching means inserted between the power supply and the memory. And a power supply voltage detection means for outputting an ON signal to the switching means and then outputting a reset release signal to the microprocessor when the power supply rises. Therefore, in both rising and falling modes of the power supply voltage, A reliable switching timing between the reset signal and the backup of the memory can be ensured, the voltage of the power supply at the time of switching can be set relatively high without considering the backup of the components of the switching means, and the capacitor of the capacitor that varies the memory can be set. The effect that can be obtained with a relatively small capacity A.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a conventional example, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. (1): Microprocessor (CPU) (2): Random access memory (RAM) (11), (22): Constant voltage diode (15), (19): Comparator, (17), (27): Capacitor (20): Diode, (24), (25): Transistor Vcc: Power supply

Claims (1)

[Scope of utility model registration request] 1. A switching means inserted between a power source and a memory, and a memory connected in parallel to the memory via a resistor,
A capacitor that is charged by the power supply when the switching means is ON and discharges to back up the memory when the switching means is OFF, and a voltage change of the power supply is detected when the power supply falls / rises and is driven by applying the voltage of the power supply. Reset signal to microprocessor
A first voltage output for outputting a reset release signal and a power supply voltage detection means for outputting an OFF / ON signal to the switching means, wherein the power supply voltage detection means outputs a voltage lower than the power supply voltage by a predetermined value. And a second means for outputting two different voltages obtained by dividing the power supply voltage.
Voltage output means and the output voltage of the first voltage output means are input as a reference signal, and the higher voltage of the two different output voltages of the second comparison voltage output means is input as a comparison signal. A first comparison means for outputting a reset signal / reset release signal to the microprocessor when the power supply voltage drops below a first predetermined voltage value / when the power supply voltage exceeds a first predetermined voltage value , The same reference signal as that of the first comparison means is input, and the lower one of the two different output voltages of the second voltage output means is input as the comparison signal, and the power supply voltage is the first An OFF / ON signal is output to the switching means when the voltage drops below a second predetermined voltage value lower than the predetermined voltage value / when the power supply voltage exceeds a second predetermined voltage value lower than the first predetermined voltage value. With the second comparison means And a PNP transistor the switching means which is inserted into the power source positive side, the PN
For a memory characterized by comprising a second switching means inserted between the base of the P-transistor and the negative electrode side of the power supply, and turning the PNP transistor OFF / ON by inputting an OFF / ON signal from the second comparing means. Backup circuit.