JPH09319468A - Reset circuit for microcomputer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a malfunction after the hit of power source of a microcomputer. SOLUTION: When a power source VDD of a microcomputer 1 is hit due to a voltage lower than the bottom voltage for normally operating the microcomputer 1, a threshold voltage VH of an inverter circuit 6 and an output *RES of an integration circuit cross at time T5 and T6, after the recovery of the power source VDD. Namely, just during this time T5 and T6, the microcomputer 1 is reset and the malfunction of the microcomputer 1 after hit is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reset circuit for a microcomputer, which can surely reset the microcomputer when the power supply voltage of the microcomputer is momentarily stopped until it falls below the minimum operable voltage of the microcomputer.

[0002]

2. Description of the Related Art There are the following methods for detecting the state of the power supply voltage of a microcomputer. (1) An IC for detecting the power supply voltage is provided outside the microcomputer. (2) A comparator is provided inside the microcomputer, the power supply voltage VDD is applied to one input terminal (+ terminal) of the comparator, and the minimum voltage at which the microcomputer can operate is applied to the other input terminal (− terminal). It is applied as a reference voltage VREF, and the microcomputer is reset only when the comparison output becomes low level. (3) An integrator circuit including a resistor and a capacitor is provided between the microcomputer power supply and ground, and a Schmitt inverter is connected to the output of the integrator circuit. The output of the Schmitt inverter resets and resets the microcomputer. Make an action. Note that this method will be described below with reference to the characteristic diagrams of FIGS. 4 and 5.

First, a power-on reset when the power supply VDD is turned on will be described with reference to the characteristic diagram of FIG.
The horizontal axis in FIG. 4 represents time, the vertical axis represents voltage, and * R
ES represents the output of the integrating circuit. At time T0, when the power supply VDD is turned on, the power supply VDD starts to rise, and along with this, the output * RES of the integration circuit gradually rises according to the time constant determined by the resistance value of the resistor and the capacitance of the capacitor that configure it. start. Of course, VDD> * RES.

The power supply VDD is also applied as the power supply of the Schmitt inverter, and when the power supply VDD rises to time T1, the Schmidt inverter is in a normal operation state. That is, at time T1, when the output * RES of the integration circuit exceeds the low side threshold voltage VIL set in the Schmitt inverter, * RES is lower than the high side threshold voltage VIH set in the Schmitt inverter. Then, the output of the Schmitt inverter becomes high level, and the microcomputer is reset based on this.

After that, when the power supply VDD rises and becomes stable and then * RES exceeds the threshold voltage VIH at time T2, the output of the Schmitt inverter becomes low level, which causes the reset release of the microcomputer. Shift to normal operation. Next, a case where an instantaneous power failure of the power supply VDD occurs during normal operation when the power supply VDD is stable will be described with reference to the characteristic diagram of FIG.
Note that V1 is the minimum voltage at which the microcomputer can operate normally.

At time T3, the power supply VDD momentarily stops due to some factor and starts to drop. Along with this, * RES, which is the output of the integrator circuit, causes the charge of the capacitor to move from the capacitor to the power supply VDD. Since it is discharged through the diode connected in the forward direction toward the power supply VDD
It begins to descend along the descending line of. And at time T4
At, when the power supply VDD is restored, the power supply VDD starts to rise and then stabilizes, and the output * RES of the integration circuit starts to rise according to its time constant. The threshold voltages VIL and VIH of the Schmitt inverter also change with the fluctuation of the power supply VDD.

[0007]

In the case of the prior art (1), since the two-chip structure is used, there is a problem that the mounting area on the board becomes large and the set price using this becomes high. . Further, in the case of the method (2) of the conventional technique, since the reference voltage source for the reference voltage VREF must be built in the microcomputer, the chip area becomes large and the desired reference voltage VREF is obtained. Therefore, it is necessary to match the characteristics of each element forming the reference voltage source, and it is very difficult to match the characteristics, and there is a problem that an appropriate reference voltage VREF cannot be easily obtained.

Further, in the case of the conventional method (3),
Although the Schmitt inverter can prevent the influence of external noise and the like, the threshold voltage VIL on the low side is the power supply V
Since it changes with the change of DD, for example, as shown in FIG. 5, when the instantaneous blackout of the power supply VDD drops below the minimum voltage V1 that guarantees the normal operation of the microcomputer, Since normal operation cannot be guaranteed, it is necessary to reset the microcomputer once. However, the output * R of the integration circuit
ES cannot cross the threshold voltage VIL that has dropped with a change in the power supply VDD. That is, the output of the Schmitt inverter cannot be changed at the low level before the power supply VDD is momentarily stopped, and the microcomputer cannot be reset. Therefore, the microcomputer has a problem that it continues to malfunction even after a momentary power failure.

Therefore, the present invention provides the above-mentioned (1) and (2).
It is an object of the present invention to provide a reset circuit for a microcomputer that can solve the problem (3).

[0010]

The present invention has been made to solve the above problems, and is characterized in that in a reset circuit of a microcomputer,
An integrating circuit that integrates the power supply voltage of the microcomputer with a predetermined time constant; and an inverter circuit that outputs the output of the integrating circuit as a high level or a low level with a predetermined threshold voltage as a boundary, and resets or resets the microcomputer. , And the threshold voltage set in the inverter circuit is the integration in the case where the power supply voltage of the microcomputer is momentarily stopped and the power supply voltage is below the minimum voltage that enables the microcomputer to operate. The point is set so as to change according to the fluctuation of the power supply voltage so as to intersect with the output of the circuit.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be specifically described with reference to the drawings. FIG. 1 is a circuit block diagram showing a reset circuit of a microcomputer of the present invention. In FIG. 1, (1) is a microcomputer, a power source VD
Power supply terminal (2) to which D is applied, and integrated voltage * RE
It has a reset terminal (3) to which S is applied. A resistor (4) is placed between the power supply terminal (2) and the reset terminal (3).
And a capacitor (5) is connected between the reset terminal (3) and ground. The resistor (4) and the capacitor (5) form an integrator circuit, and the power supply VDD
Output of the integration circuit, that is, the reset terminal (3)
The voltage * RES appearing at the voltage gradually rises due to the time constant determined by the resistance value of the resistor (4) and the capacitance of the capacitor (5). Reference numeral (6) is an inverter circuit, which has only the threshold voltage VH on the higher side than the normal inverter circuit. The inverter circuit (6) is configured by connecting a P-channel type MOS transistor and an N-channel type MOS transistor in series between the power supply VDD and the ground, that is,
The threshold voltage VH of the inverter circuit (6) changes as the power supply VDD changes. Inverter circuit (6)
The input terminal of is the reset terminal (3) which becomes the output of the integrating circuit
Is connected to Here, the level of the threshold voltage VH of the inverter circuit (6) is the minimum voltage V at which the power supply VDD guarantees the normal operation of the microcomputer (1).
The level is set so that the output * RES of the integrator circuit, which rises as a result of this recovery, crosses the threshold voltage VH when it recovers after a momentary power failure to less than one. Power supply V
Considering that the threshold voltage VH also changes with the change of DD, the P-channel type MOS transistor and the N-channel MOS transistor forming the inverter circuit (6) and N so as to intersect the rising line of * RES based on the time constant of the integrating circuit. It is necessary to set the size ratio of the channel type MOS transistor. In other words, the resistance value of the resistor (4) and the capacitance of the capacitor (5) that form the integrator circuit and the size ratio of the P-channel MOS transistor and the N-channel MOS transistor that form the inverter circuit are equal to the above threshold voltage VH. It has a specific relationship to realize. (7) is a discharging diode connected in the forward direction from the capacitor (5) toward the power supply VDD.

First, the operation at power-on reset when the power VDD is turned on will be described with reference to FIG. At time T0, when the power supply VDD (solid line) is turned on and starts to rise, the voltage * RES (broken line) appearing at the reset terminal (3) is accordingly changed to the resistance value of the resistor (4) and the capacitance of the capacitor (5). It starts to rise according to the time constant determined by. Of course, VDD> * RES.

Then, at time T1, the power supply VDD is V
When 2 (<V1), the voltage V2 is the lowest voltage at which the inverter circuit (6) can normally operate, that is, the inverter circuit (6) starts to work normally. Further, the threshold voltage VH (dashed line) also starts to rise as the power supply VDD rises. Here, the relationship between VDD, * RES, and VH is VDD>VH> * RES. Therefore, the output of the inverter circuit (6) becomes high level and the microcomputer is reset.

Thereafter, the power supply VDD rises and stabilizes at a constant value, and when the time T2 is reached, the output * RES of the integrating circuit exceeds the threshold voltage VH of the inverter circuit (6),
The output of the inverter circuit (6) becomes low level, the microcomputer is released from reset, and normal operation is started according to the program. Although the reset release timing of the microcomputer (1) is delayed compared to the conventional one, the reset release time difference between the conventional technology and the embodiment of the present invention is such that the actual operation of the microcomputer is affected. is not.

Next, a case where the power supply VDD is momentarily stopped will be described with reference to the characteristic diagram of FIG. When the microcomputer (1) receives some factor from the stable state of the power supply VDD and the power supply VDD is momentarily stopped at time T3,
The power supply VDD starts to drop, and accordingly, the output * RES of the integrating circuit is discharged through the diode (7). That is, the output * RES of the integration circuit drops along the falling line of the power supply VDD. Similarly, the threshold voltage VH of the inverter circuit (6) also drops.

After that, when the power supply VDD drops to less than the minimum voltage V1 for normally operating the microcomputer (1) and returns at time T4, the power supply VDD starts to rise and the threshold voltage VH also starts to rise.
RES gradually increases according to the time constant. here,
Even if the power supply VDD is a momentary blackout, it is below the minimum voltage for normal operation of the microcomputer (1), so normal operation of the microcomputer (1) cannot be guaranteed after a momentary blackout of the power supply VDD. . That is, it is necessary to reset the microcomputer (1) after the momentary power failure of the power supply VDD.

The threshold voltage VH of the inverter circuit (6) set in the embodiment of the present invention crosses the output * RES of the integrating circuit at time T5, and the power source V
After DD stabilizes at a constant value, it crosses * RES again at time T6. That is, the output of the inverter circuit (6) becomes high level and the microcomputer (1) is reset only during the period between times T5 and T6, and malfunction of the microcomputer (1) is prevented.

Although the conventional Schmitt inverter is effective in preventing malfunction due to noise, since this Schmitt inverter is used only for resetting, compared to this effect, malfunction of the microcomputer after a momentary power failure of the power supply VDD is detected. The effect obtained by the inverter circuit (6) of the embodiment of the present invention that can be prevented is greater. From the above, in order to appropriately set the relationship between the characteristics of the inverter circuit (6) inside the microcomputer (1) and the time constant of the integrating circuit, it is necessary to use the two-chip configuration described in the prior art (1). It is possible to reduce the mounting area of the board and lower the set price.

Further, since it is not necessary to provide the comparator described in the prior art (2), the reference voltage source becomes unnecessary,
There is no need for complicated matching of characteristics for setting a desired reference voltage. Furthermore, even if the power supply voltage drops below the minimum voltage that guarantees normal operation of the microcomputer, as described in the related art (3), the embodiment of the present invention can surely reset the microcomputer. It is possible to reliably prevent malfunction of the microcomputer after a momentary power failure.

[0020]

According to the present invention, by appropriately setting the size ratio of the transistors forming the inverter circuit and the values of the resistors and capacitors forming the integrating circuit, the power supply of the microcomputer can be controlled by the microcomputer. In the case of a momentary power failure to the level below the minimum voltage at which the computer can operate normally, the microcomputer can be reliably reset, and the malfunction after the momentary power failure of the microcomputer can be reliably prevented.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing a reset circuit of a microcomputer of the present invention.

FIG. 2 is a characteristic diagram showing characteristics at the time of power-on reset of the present invention.

FIG. 3 is a characteristic diagram showing characteristics at the time of instantaneous blackout of the present invention.

FIG. 4 is a characteristic diagram showing a characteristic at the time of conventional power-on reset.

FIG. 5 is a characteristic diagram showing a characteristic at the time of a conventional instantaneous blackout.

[Explanation of symbols]

 (1) Microcomputer (4) Resistor (5) Capacitor (6) Inverter circuit

Claims (1)

[Claims] 1. In a reset circuit of a microcomputer, an integrating circuit for integrating a power supply voltage of the microcomputer with a predetermined time constant, and an output of the integrating circuit is output as a high level or a low level with a predetermined threshold voltage as a boundary, An inverter circuit for resetting or canceling the reset of the microcomputer, wherein the threshold voltage set in the inverter circuit is such that the power supply voltage of the microcomputer is momentarily stopped and the power supply voltage can operate the microcomputer. A reset circuit for a microcomputer, which is set so as to intersect with the output of the integration circuit when it falls below the minimum voltage.