JPH04417Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention uses a capacitor to monitor runaway of the CPU or an abnormality in the power supply voltage. This invention relates to a CPU reset signal sending circuit that sends out a reset signal to forcibly reset the CPU.

[Conventional technology]

Conventionally, in this type of reset signal sending circuit that sends a reset signal of a predetermined width to the CPU, a signal of a predetermined width is obtained by inserting a one-shot circuit at the next stage of the voltage judgment circuit that judges the terminal voltage of the capacitor. was.

[Problem that the invention attempts to solve]

However, the system using such a one-shot circuit has the disadvantage that there is a risk of malfunction due to noise and that the number of elements constituting the circuit increases.

The present invention has been made to solve these drawbacks.

[Means for solving problems]

To this end, the present invention generates a reset signal of a predetermined width based on the result of determining the terminal voltage of a capacitor that monitors CPU runaway or power supply voltage abnormality.
In the CPU reset signal sending circuit that sends out the CPU reset signal, there is a switching element that is turned on by periodic pulses from the CPU, and a charging/discharging device that charges the capacitor with the power supply when the switching element is turned on and discharges it when it is turned off. A circuit, a comparator that compares the terminal voltage of the capacitor with a reference voltage and sends a reset signal to the CPU when the terminal voltage falls below the reference voltage, and a comparator that compares the terminal voltage of the capacitor with a reference voltage and sends a reset signal to the CPU when the reset signal of this comparator is output. Voltage to higher second
The device is equipped with a reference voltage generation circuit that switches to a reference voltage of 1, and a rectifier that turns on when the power supply voltage drops below the terminal voltage of the capacitor to rapidly discharge the charge in the capacitor.

[Effect]

That is, by raising the reference voltage when outputting the reset signal, the reset signal is sent until the terminal voltage of the capacitor that has started charging exceeds the raised second reference voltage.
Further, the capacitor is charged by turning on the switching element controlled by periodic pulses from the CPU, and discharged by turning it off. As a result,
If the CPU goes out of control and the periodic pulses from the CPU are cut off, the CPU will be reset. Furthermore, if an abnormality occurs in the power supply and the power supply voltage drops below the terminal voltage of the capacitor, the charge in the capacitor is rapidly discharged. As a result, the CPU can be reset quickly.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the same figure, C ₁ and C ₂ are capacitors D ₁ and D ₂
is a diode, R ₀ to R ₉ are resistors, Tr is a transistor, and 1 is a CPU. While the CPU 1 continues its normal operation, a pulse signal P of a constant period is sent out. This pulse signal P
is inverted by the inverter 2, then differentiated by a differentiating circuit consisting of a capacitor _C1 and a resistor _R2 , and becomes the base input of the transistor Tr. A power supply voltage V is applied to the emitter terminal of the transistor Tr via a resistor _R4 , and the collector terminal is grounded via a capacitor _C2 . Therefore, the transistor Tr is turned on only for a period determined by the constant of the differentiating circuit, and a current of a magnitude determined by the resistor _R4 flows to charge the capacitor _C2 . While the above pulse signal P does not come, the transistor Tr
is turned off, and the charge stored in capacitor C ₂ is transferred to resistor R ₅ which has a relatively high resistance value during this time.
is discharged through the terminal, and its terminal voltage gradually decreases.

The terminal voltage of capacitor _C2 becomes the input of comparator 3. Comparator 3 divides this terminal voltage V ₊ into a first setting value of reference voltage V _- obtained by dividing power supply voltage V by a parallel connection circuit of resistor R ₆ , resistor R ₇ , and resistor 8.
Compare with V ₁ . While the CPU 1 is operating normally and sending out the pulse signal P at a constant cycle, the capacitor C ₂ is repeatedly charged, and its terminal voltage is always at a value well above the reference voltage.
When the CPU 1 goes out of control and the pulse signal P is no longer sent, the terminal voltage V ₊ gradually decreases as shown in FIG. 2a, and finally drops below the set value _V1 of the reference voltage _V- . At this time t ₁ , the comparator 3 sends an "L" level output to the CPU 1 as a reset signal RESET, as shown in FIG. 2b.

On the other hand, this reset signal is sent to analog switch 4.
is added as a control signal to analog switch 4.
is in the off state. As a result, the resistor R ₈ connected in parallel with the resistor R ₆ and the resistor R ₇ that divides the power supply voltage V is removed, so the reference voltage V _- of the comparator 3 is set to the first set value V ₁ to 2nd higher
increases to the set value V ₂ . Therefore, the reset signal RESET is not activated until the terminal voltage V ₊ of the capacitor _C2 , which has started charging again, reaches the second set value _V2 .
continues to be sent out, but at the time t ₂ when V ₊ exceeds V ₂
Then, the output of comparator 3 becomes "H" level again,
Reset is canceled. Since the analog switch 4 is also turned off at the same time, the reference voltage V _- returns to the first set value V ₁ at a low level. That is, from t ₁
This means that a reset signal with a width of _t2 has been sent.

By the way, in this embodiment, the diode _D1 is connected between the connection point of the capacitor _C1 and the resistor _R2 and the output terminal of the comparator 3, with the former being on the anode side. As shown in Figure 3b, while the "L" level output of the comparator 3, that is, the reset signal output is being sent, the transistor Tr is forcibly turned on, and the capacitor
_C2 is charged. As shown in Figure 3a, the terminal voltage V ₊ of capacitor C ₂ is the reference voltage.
When V _- exceeds the set value V ₂ , the output of the comparator 3 becomes "H" level and the reset is released. At this time, the reference voltage is switched to the first set value _V1 .
Usually, there is a watchdog circuit that detects periodic pulses as described above and resets when an abnormality occurs;
It is provided separately from the power-on reset circuit when the power is turned on, and the OR output of both circuits is used as the reset signal RESET. In this embodiment, the diode _D1 is provided as described above, and the By forcibly turning on the transistor Tr by outputting a reset signal from the watchdog circuit, the watchdog circuit can also be used as a power-on reset circuit.

Note that when there is an abnormality in the power supply and its voltage drops below the terminal voltage of capacitor C ₂ , capacitor C ₂ naturally starts discharging, and the terminal voltage V ₊ When the reference voltage falls below the first set value _V1 of the reference voltage, the reset signal RESET is output and at the same time the reference voltage becomes the second set value V1.
is switched to the set value _V2 , and when the terminal voltage V ₊ of the capacitor _C2 exceeds _V2 , the output of the reset signal is stopped. In this case, the diode D ₂
Otherwise, the discharge of capacitor C ₂ would take place gradually through the relatively high resistance resistor R ₅ as before. However, in the case of such a drop in the power supply voltage, the capacitor _C2 is gradually discharged and its terminal voltage V ₊ is increased to the comparator 3 as described above.
Until the first setting value V ₁ is reached, CPU1
may malfunction. Therefore, in this embodiment, in order to prevent this, a diode _D2 is connected between the capacitor _C2 and the power supply, with the former serving as the anode side. As a result, when the power supply voltage V falls below the terminal voltage V ₊ of the capacitor C ₂ , discharge occurs rapidly through the diode D ₂ and reset is quickly performed. In this embodiment, the capacitor _C2 is charged by turning on the transistor Tr, but other switching elements may be used to charge the capacitor _C2 by turning on this element.

[Effect of idea]

As explained above, according to the present invention, the comparator of the voltage judgment circuit that compares the terminal voltage of the capacitor with the reference voltage and sends a reset signal to the CPU when the former falls below the latter, By providing a reference voltage generation circuit that switches the reference voltage to a higher value when outputting a reset signal, it is possible to obtain a stable reset signal output that is resistant to noise with a simple circuit configuration without using a one-shot circuit.

In addition, the capacitor is charged when the switching element is turned on controlled by periodic pulses from the CPU, and discharged when the switching element is turned off.
If the CPU goes out of control and the periodic pulses from the CPU are cut off, the CPU will be reset. Furthermore, if an abnormality occurs in the power supply and the power supply voltage drops below the terminal voltage of the capacitor, the charge in the capacitor is rapidly discharged, allowing the CPU to be reset quickly.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
3 and 3 are timing charts showing the operation. 1...CPU, 3...Comparator, 4...Analog switch for switching reference voltage, _C2 ...Capacitor, _R6 to _R8 ...Resistance for setting reference voltage.

Claims (1)

[Scope of utility model registration request] In the CPU reset signal sending circuit that sends a reset signal of a predetermined width to the CPU based on the result of determining the terminal voltage of a capacitor that monitors CPU runaway or power supply voltage abnormality, the reset signal is turned on by periodic pulses from the CPU. a charging/discharging circuit that charges the capacitor with the power source when the switching element is turned on and discharges it when the switching element is turned off; a comparator that sends a reset signal to the CPU when the voltage drops below the voltage level; a reference voltage generation circuit that switches the reference voltage from the first reference voltage to a higher second reference voltage when the comparator outputs the reset signal; and a power supply. and a rectifying element that turns on when the voltage drops below the terminal voltage of the capacitor to quickly discharge the charge in the capacitor, so that the power supply voltage can be adjusted even when the power supply voltage is abnormal.
A CPU reset signal sending circuit characterized in that the CPU is reset.