JPH0926835A - Reset circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reset circuit which can surely produce a reset signal for a microcomputer in a simple constitution when a power supply has a momentary power failure and drops down to the lowest operating voltage level or less. SOLUTION: At least a resistance ratio between the divided resistors 12 and 13 or the constants of the time constant circuits 11 and 14 are set so as to secure a period when the reference voltage varying in response to the change of a power supply VDD is higher than the voltage or both circuits 11 and 14 when the VDD drops down to the lowest operating voltage level of a microcomputer 8 or less from its steady state and then rises again up to the steady state. In such a constitution, a comparator 15 can surely produce a reset signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, in a microcomputer, generates a reset signal when the power supply voltage of the microcomputer is momentarily cut from a steady state to a voltage below the minimum operating voltage of the microcomputer. And a reset circuit configured as described above.

[0002]

2. Description of the Related Art FIG. 5 is a diagram showing a conventional reset circuit for generating a reset signal for a microcomputer. In FIG. 5, (1) is a microcomputer, which has terminals (2) and (3). The terminal (2) is connected to the power supply VDD (for example, 5 V), and the capacitor (4) is connected between the terminal (3) and the ground. The microcomputer (1) operates normally when the power supply VDD is 3 V or higher. Inside the microcomputer (1), (5) is a resistor, which is connected between the terminals (2) and (3) to form a time constant circuit together with the capacitor (4). Also, (6)
Is a diode, and its cathode and anode are respectively connected to terminals (2) and (3). Further, (7) is a Schmitt inverter, which has two different threshold voltages V
th (0.7VDD) and Vthl (0.3VDD)
And outputs a reset signal for the microcomputer (1). Of course, the threshold voltage of the Schmitt inverter (7) changes according to the magnitude of the power supply voltage VDD. Although not shown, the Schmitt inverter (7) is connected to the power supply VDD to operate, and is assumed to be configured to operate normally when the power supply VDD becomes 2 V or higher.

The operation of the conventional reset circuit of FIG. 5 configured as described above will be described with reference to the operation waveform diagrams of FIGS. 6 and 7. First, when the power supply VDD is turned on, the power supply VDD reaches a steady state (5 volts) after a transition period of rising (FIG. 6 solid line). The time constant circuit charges with a time constant determined by the resistance value of the resistor (5) and the capacitance of the capacitor (4), and the terminal voltage of the capacitor (4) appearing at the terminal (3) rises according to the time constant. (FIG. 6 dashed line). Then, at time T1, when the power supply VDD exceeds 2 volts, the Schmitt inverter (7) starts normal operation, and the Schmitt inverter (7) outputs a high-level reset signal. Since the period from time T1 to time T2 includes the transition period of the power supply VDD, the high level of the reset signal output from the Schmitt inverter (7) has a value that rises from 2 volts to 5 volts. After that, when the power supply VDD reaches a steady state and the terminal voltage of the capacitor (4) exceeds the high-side threshold voltage Vthh of the Schmitt inverter (7) at time T2, the output of the Schmitt inverter (7) becomes low level ( 0 volt). That is, the microcomputer (1) is reset between the times T1 and T2, released from the reset after the time T2, and starts executing the logical operation processing based on the program.

Now, in the steady state of the power supply VDD, consider a case where the power supply VDD is momentarily cut down to less than the minimum operating voltage (3 volts) of the microcomputer (1) for some reason (solid line in FIG. 7). . In this case, as a matter of course, the threshold voltages Vthh and Vthl of the Schmitt inverter (7) also change in response to the change in the power supply VDD (two-dot chain line and one-dot chain line in FIG. 7). In the steady state before the power supply VDD is interrupted, the terminal voltage of the capacitor (4) is fully charged to 5 V. After that, in the locus of the power supply VDD falling, the voltage of the terminal (2) is reduced. When the voltage becomes lower than the voltage of the terminal (3) by the forward voltage of the diode (6), the diode (6) becomes conductive and the terminal voltage of the capacitor (4) is discharged with the same slope as the downward trajectory of the power supply VDD. Then descends (dashed line). After that, when the power supply VDD starts to rise, the diode (6) is turned off, the capacitor (4) starts charging according to the time constant, and the terminal voltage thereof rises (broken line). That is, the terminal voltage of the capacitor (4) does not drop below the high side threshold voltage Vthh of the Schmitt inverter (7) when the power supply VDD is interrupted.

[0005]

As described above, the power source V
When the DD is momentarily cut to a voltage lower than the minimum operating voltage of the microcomputer (1), normal operation of the microcomputer (1) cannot be guaranteed. Therefore, in this case, it is necessary to generate the reset signal. However, the output of the Schmitt inverter (7) remains low level, that is, the reset signal is not generated. Therefore, there is a problem that the microcomputer (1) continues to malfunction without being reset even after the power supply VDD is restored from the momentary interruption, while being in the malfunction state due to the momentary interruption.

Further, in order to reliably generate the reset signal when the power supply VDD is momentarily cut below the minimum operating voltage of the microcomputer, a voltage level detection circuit separate from the microcomputer is provided and the circuit is provided. There is also a method of applying a reset signal from the microcomputer to the microcomputer. However, this method has a problem that the number of chips increases and the cost increases.

Therefore, the present invention has a simple configuration and is capable of reliably generating a reset signal for the microcomputer when the power supply is momentarily cut to below the minimum operating voltage of the microcomputer. The purpose is to provide.

[0008]

The present invention has been made to solve the above-mentioned problems, and is characterized in that in a microcomputer, the power supply voltage of the microcomputer is set to a predetermined ratio. A dividing resistor for dividing into a reference voltage, a time constant circuit having a predetermined time constant, to which the power supply voltage is applied, and a time constant circuit to which the reference voltage is applied to one input and the other input. And a comparator for generating a reset signal for the microcomputer, to which a voltage responsive to the power supply voltage is applied, the power supply voltage from a steady state to a voltage lower than a minimum operating voltage of the microcomputer. When the power supply voltage decreases and then rises to a steady state, between the voltage of the reference voltage and the voltage of the time constant circuit that changes according to the change of the power supply voltage, As can period towards the reference voltage becomes higher than the voltage of the time constant circuit, at least, is that setting either one of a constant resistance ratio or the time constant circuit of the dividing resistor.

[0009]

According to the present invention, when the power supply voltage decreases from the steady state to a voltage lower than the minimum operating voltage of the microcomputer and then the power supply voltage rises to the steady state, it changes according to the change of the power supply voltage. Between the reference voltage and the voltage of the time constant circuit, at least one of the resistance ratio of the dividing resistors or the constant of the time constant circuit is provided so that there is a period in which the reference voltage is higher than the voltage of the time constant circuit. By setting either one, it is possible to reliably generate the reset signal.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a diagram showing a reset circuit of the present invention. FIG.
The reset signal output at is used for initializing the microcomputer. In FIG.
(8) is a microcomputer, terminals (9) (1
0). The microcomputer (1) operates normally when the power supply VDD is 3 V or higher. Terminal (9)
Is connected to a power supply VDD (5 V in a steady state), and a capacitor (11) is connected between the terminal (10) and ground. Inside the microcomputer (8), reference numerals (12) and (13) are dividing resistors connected in series between the terminal (9) and the ground, and divide the power supply VDD into a predetermined ratio to obtain the resistors (12) ( The reference voltage is generated from the connection point 13). In this embodiment, the resistance (12)
The resistance value of (13) is set to 1: 4. That is, the reference voltage is 4 volts in the steady state. Reference numeral (14) is a resistor, which is connected between the terminals (9) and (10) and constitutes a time constant circuit together with the capacitor (11). (15)
Is a comparator, and the + terminal has a dividing resistor (12) (13)
Is applied to the negative terminal, and the output of the time constant circuit, that is, the terminal voltage of the capacitor (11) is applied to the-terminal, and
The reset signal is output according to the magnitude relationship between the input voltage of the terminal and the-terminal. Although not shown, the comparator (15) is operated by applying the power supply VDD,
The normal comparison operation is started when the power supply VDD becomes 2 volts or more. Further, (16) is a diode, the cathode and anode of which are connected to the terminals (9) and (10), respectively, and performs the same operation as the diode (6) of the conventional circuit of FIG.

Hereinafter, the operation of FIG. 1 will be described with reference to FIGS. 2, 3, and 4.
A description will be given based on the operation waveform diagram of FIG. First, when the power supply VDD is turned on, the power supply VDD reaches a steady state (5 volts) after a transition period of rising (FIG. 2 solid line). Further, the time constant circuit charges with a time constant determined by the resistance value of the resistor (14) and the capacitance of the capacitor (11), and the terminal voltage of the capacitor (11) appearing at the terminal (10) rises according to the time constant. (FIG. 2 two-dot chain line). Also, the dividing resistor (12)
The connection point voltage (reference voltage) of (13) rises according to the resistance ratio of both resistors (12) and (13) as the power supply VDD rises (dashed line in FIG. 2). When the power supply VDD exceeds 2 volts in the transition period of the power supply VDD, the comparator (15) outputs a high level, that is, a reset signal for initializing the microcomputer (8). Incidentally, time T
During the period from 1 to T2, the comparator (15) holds the high level output, but this high level changes in the range of 2 to 5 volts because it depends on the power supply VDD.
After that, at time T2 after the power supply VDD becomes in the steady state, the terminal voltage of the capacitor (11) becomes higher than the reference voltage, so that the output of the comparator (15) becomes low level (0 volt). That is, the reset signal becomes high level from time T1 to T2, and the microcomputer (8) is initialized only during this period.

Now, the power supply VDD is in a steady state (5 volts)
Consider a case in which the microcomputer (8) is momentarily disconnected within a stable operating voltage range (3 to 5 volts) from a stable state (see FIG. 3, solid line). In this case, as a matter of course, the reference voltage of the comparator (15) also changes in response to the change in the power supply VDD (dotted line in FIG. 3). In the steady state before the power supply VDD is interrupted, the terminal voltage of the capacitor (11) is fully charged to 5 V. After that, in the locus in which the power supply VDD drops, the voltage of the terminal (9) is reduced. When the voltage becomes lower than the voltage of the terminal (10) by the forward voltage of the diode (16), the diode (16) becomes conductive and the terminal voltage of the capacitor (11) is also discharged with the same slope as the downward trajectory of the power supply VDD. And then descends (broken line in FIG. 3). After that, when the power supply VDD starts to rise, the diode (16) turns off and the capacitor (11)
Starts charging according to the time constant, and its terminal voltage rises (broken line in FIG. 3). At this time, since the voltage of the time constant circuit continues to be higher than the reference voltage, the output of the comparator (15) remains at the low level and no reset signal is generated. That is, when the power supply VDD is momentarily cut off within the stable operation voltage range of the microcomputer (8),
Since the microcomputer (8) can continue normal operation, generation of the reset signal is originally unnecessary and the reset signal is not generated.

On the other hand, the power supply VDD is in a steady state (5 volts)
Consider a case where the microcomputer (8) is momentarily disconnected from the stable state at a voltage lower than the stable operating voltage (less than 3 volts) by some reason (solid line in FIG. 4). In this case, as a matter of course, the reference voltage of the comparator (15) also changes in response to the change in the power supply VDD (dotted line in FIG. 4).
In the steady state before the power supply VDD is interrupted, the terminal voltage of the capacitor (11) is fully charged to 5 V. After that, in the locus of the power supply VDD dropping, the voltage of the terminal (9) is reduced. When the voltage becomes lower than the voltage of the terminal (10) by the forward voltage of the diode (16), the diode (16) becomes conductive and the terminal voltage of the capacitor (11) is also discharged with the same slope as the downward trajectory of the power supply VDD. And then descends (broken line in FIG. 4). After that, when the power supply VDD starts to rise,
The diode (16) is turned off, the capacitor (11) starts charging according to the time constant, and its terminal voltage rises (broken line in FIG. 4). Here, in the period from the transitional period when the power supply VDD recovers from the momentary interruption and rises until it stabilizes in a steady state, at least there is a period when the reference voltage is higher than the output voltage of the time constant circuit. Dividing resistor (12)
Either the resistance ratio of (13) or the time constant determined by the resistance value of the resistor (14) and the capacitance of the capacitor (11) may be set. As a result, the output of the comparator (15) becomes a high level between time T3 and T4, that is, a reset signal is generated, the microcomputer (8) is surely initialized, and the malfunction after the power supply VDD is interrupted. Can be prevented.

From the above, even if the power supply VDD of the microcomputer (8) is momentarily cut to less than the minimum operating voltage of the microcomputer (8), the reset signal for the microcomputer (8) can be surely made with a simple configuration. Can occur. Therefore, a reset signal cannot be generated in a situation where a reset signal is required, which has occurred conventionally,
Alternatively, it is possible to solve the problem that the cost is high due to the configuration for surely generating the reset signal.

[0015]

According to the present invention, the reset signal for the microcomputer can be reliably generated with a simple structure even when the power source of the microcomputer is momentarily cut to a voltage lower than the minimum operating voltage of the microcomputer. Therefore,
Advantages that can solve the problem that cost was forced because the reset signal could not be generated in the situation where the reset signal was required, or the reset signal was generated surely as in the past. Is obtained.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram showing an operation when the power source of FIG. 1 is turned on.

FIG. 3 is a waveform diagram showing an operation when a momentary interruption occurs at a voltage equal to or higher than the minimum operating voltage in FIG.

FIG. 4 is a waveform diagram showing an operation when a momentary interruption occurs below the minimum operating voltage in FIG.

FIG. 5 is a diagram showing a conventional reset circuit.

FIG. 6 is a waveform diagram showing an operation when the power is turned on in FIG.

FIG. 7 is a waveform diagram showing an operation below the minimum operating voltage of FIG.

[Explanation of symbols]

 (8) Microcomputer (11) Capacitor (12) (13) Dividing resistor (14) Resistor (15) Comparator

Claims (2)

[Claims] 1. In a microcomputer, a dividing resistor for dividing a power supply voltage of the microcomputer into a predetermined ratio to obtain a reference voltage, a time constant circuit having a predetermined time constant and to which the power supply voltage is applied, A comparator that applies the reference voltage to one input and a voltage corresponding to the power supply voltage obtained from the time constant circuit to the other input, and that generates a reset signal for the microcomputer; Comprising, when the power supply voltage drops from a steady state to a voltage lower than the minimum operating voltage of the microcomputer, and then the power supply voltage rises to a steady state, the reference voltage and the reference voltage that changes according to the change of the power supply voltage. Between the voltages of the time constant circuit, at least the dividing resistor is provided so that there is a period in which the reference voltage is higher than the voltage of the time constant circuit. Reset circuit and sets either one of the resistance ratio or constant of the time constant circuit. 2. The voltage of the time constant circuit is lower than the reference voltage when the power supply voltage drops from a steady state in a voltage range higher than the minimum operating voltage of the microcomputer and then the power supply voltage rises to a steady state. 2. The reset circuit according to claim 1, wherein at least one of the resistance ratio of the division resistor and the constant of the time constant circuit is set so that the reset circuit always remains in a high state.