JPS6016981Y2 - memory protection circuit device - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a memory protection circuit device.

In order to protect the memory when the power is turned off, it is necessary to disable the memory chip select signal.

Conventionally, when a power outage is detected, the contents of the program counter and accumulator in the processor are saved to memory, and then the chip select signal of the memory is inhibited.

During this time, the power supply voltage is maintained at a predetermined value using battery backup.

Since such a protection circuit includes a transformer and the like, it takes up a considerable amount of space.

This point causes a problem in the manufacture of l-board CPUs.

The purpose of this invention is to reduce the volume of a memory protection circuit device so that it can be mounted within a printed circuit board of a single-board CPU.

In this invention, a resistor and a Zener diode are connected in series between a power supply terminal and a ground line, and the connection point between the resistor and the Zener diode is input as a reference potential to one of the input terminals of a comparator. , a variable resistor is connected between the power supply terminal and the ground line to divide the power supply voltage and lead it to the other input terminal of the comparator, and when the power supply voltage is at a predetermined value, the input of the comparator The voltage applied to the other end is set equal to the reference potential, and when the power supply voltage falls below the predetermined value, the output of the comparator is inverted and read-out of the memory is prohibited. There is.

Therefore, according to this invention, the Zener diode keeps the reference potential constant, the comparator accurately detects when the power supply voltage falls below a predetermined value, and prohibits memory readout. Can be done.

Therefore, a transformer or the like is not required, the volume can be reduced, and it can be mounted within the printed circuit board of a single-board CPU.

Further, in this invention, a transistor is provided which is cut off when the power supply voltage is lower than the predetermined value and falls below a predetermined value, and the comparator is connected to the power supply terminal via the transistor. It is configured to supply power to the

Therefore, according to this invention, before the operation of the comparator becomes unstable and the read-out inhibit signal of the memory becomes unstable, the transistor is cut off and the power supply to the comparator is cut off. Malfunctions such as content changes can be reliably prevented.

However, with this invention, the contents of the program counter and accumulator cannot be saved.

However, this problem can be resolved by performing initial processing at the time of restoration, so there is no problem.

This invention will be described in detail below with reference to the illustrated embodiments.

In FIG. 1, the terminal indicated as 1 is a terminal to which power supply voltage A is applied.

Terminal 1 is connected to the cathode of a Zener diode, designated 2, and to the collector of a transistor, designated 3.

The anode of the Zener diode 2 is connected to one end of the resistor 4.

The other end of the resistor 4 is grounded. A connection point between the resistor 4 and the Zener diode 2 is connected to the base of the transistor 3.

Normally, when the voltage at terminal 1 is maintained at the rated voltage EO, conduction is established between the collector and emitter of transistor 3, and a predetermined potential is applied to a comparator shown as block 5.

- This potential is approximately equal to the power supply voltage applied to terminal 1 minus the voltage drop across Zener diode 2, and is set to a voltage E2 at which comparator 5 can operate normally.

Terminal 1 is further connected to a resistor 6 and one end of a variable resistor 7.

The other end of resistor 6 is connected to the cathode of a Zener diode shown as 8.

The anode of Zener diode 8 is grounded.

The other end of variable resistor 7 is also grounded.

The output end of the variable resistor 7 is connected to one of the input ends of the comparator 5.

A hysteresis resistor 9 is connected to one of the input terminals and the output terminal, and hysteresis is provided to the input and output, and the comparator 5
stabilize the operation.

A connection point between a resistor 6 and a Zener diode 8 is led to the other input terminal of the comparator 5.

Now, when the voltage at terminal 1 is at El (4.75 volts), which is slightly lower than the rated voltage EO (5 volts), variable resistor 7
is adjusted to set the voltage at its output terminal equal to the cathode potential (reference potential) of the Zener diode 8.

In other words, the voltages at the two input terminals of the comparator 5 are set equal.

Therefore, if the voltage at terminal 1 exceeds El, a signal HIGH is present at the output of comparator 5.

The output terminal of the comparator 5 is given as a chip select signal of the C-MO3 circuit memory shown as 10.

When the chip select signal of the C-MO3 circuit 10 is HIGH, the data of the C-MO3 circuit can be read and written via the TTL circuit shown as 11, as is well known.

Although not shown, the C-MO5 circuit 10 and the T
The power supply voltage A applied to the terminal 1 is also supplied to the power supply terminal of the TL circuit.

When the power is cut off, the voltage A at terminal 1 drops as shown in Figure 2, and when it reaches a value slightly below El, the voltage at El becomes equal to the potential at the connection point between the Zener diode and resistor 6, as described above. Since the set voltage at the output of variable resistor 7 is now below the voltage at this connection point, comparator 5 produces a signal LOW at its output B.

As a result, the chip select signal of the C-MO5 circuit 10 becomes LOW, making it impossible to read or write data.

This El is set to the minimum power supply voltage within the operating range of the TTL circuit.

When the voltage A at the terminal 1 further falls below E2, the Zener diode 2 is cut off.

This causes the base potential of transistor 3 to become zero.

Transistor 3 is cut off, and no voltage is applied to power supply terminal C of comparator 5.

This configuration is because the power supply voltage drops and the comparator 5
This is to prevent the chip select signal from becoming unstable due to unstable operation.

As mentioned above, according to this embodiment, since the comparator 5 is used, the chip select signal of the C-MO3 circuit is
The potential to be applied to W can be determined extremely accurately.

Therefore, when the power supply voltage of the TrL circuit 11 falls below its operating range, it is no longer possible to read out data from the C-MO5 circuit.
It becomes possible to reliably protect the data of the C/MO3 circuit.

Note that this invention is not limited to protection of C-MO3 circuits, but can be applied to general RAM.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an embodiment of this invention, and FIG. 2 is an explanatory diagram of the operation of FIG. 1. 2.8: Zener diode, 3: transistor, 5: comparator, jQ: c-MO3 circuit, 11: TrL circuit.

Claims (1)

[Scope of utility model registration request] A resistor and a Zener diode are connected in series between a power supply terminal and a ground line, and a connection point between the resistor and the Zener diode is input as a reference potential to one of the input terminals of a comparator, and the connection point between the power supply terminal and the ground line is input as a reference potential to one of the input terminals of a comparator. A variable resistor is connected between the ground line and the power supply voltage to be divided and guided to the other input terminal of the comparator, and when the power supply voltage is at a predetermined value, it is applied to the other input terminal of the comparator. The voltage is set equal to the reference potential, and when the power supply voltage falls below the predetermined value, the output of the comparator is inverted to inhibit readout of the memory, and the power supply voltage is set to be equal to the reference potential. Protection of a memory configured to provide a transistor that is cut off when the value is lower than a predetermined value and is lower than a predetermined value, and to supply power to the comparator from the power supply terminal via the transistor. circuit device.