JPH0451711A - Power-on clear circuit - Google Patents

Abstract

PURPOSE:To obtain a clear signal with an optional time length and to realize an inexpensive power-on clear circuit by setting a current at the time of turning on a power supply optionally and adjusting the rising time of an output voltage so as to output a clear signal for initial setting from a comparator. CONSTITUTION:A voltage VA being voltage-divided by resistors R1, R2 is supplied to the inverting input of a comparator 1 at the time of turning on a power supply. On the other hand, a current IC flowing to a constant current diode RC is set optionally by a veriable resistor RV. While a voltage VC given to a noninverting input of the comparator 1 is smaller than the voltage VA, an output of the comparator is at a low level, and when the voltage VC is higher than the voltage VA, the output goes to a high level. Thus, a power-on clear signal being at a low level to clear a circuit connecting to the power-on clear circuit is outputted. Since a time width of the power-on clear signal is changed linearly continuously by varying a variable resistor RV, the power-on clear signal having an optional time width is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a power-on clear circuit used to automatically set the inside of a digital processing circuit to an initial state when power is turned on in the digital processing circuit.

[Conventional technology]

Generally, in a digital processing circuit (hereinafter referred to as a circuit), in order to start operation from a predetermined initial state when the power supply is turned on, the circuit detects when the power is turned on, and then maintains the internal state of the circuit for a certain period of time. It has a circuit section that generates an initial setting signal (hereinafter referred to as a power-on clear signal) for initial setting.

The operation of the conventional power-on clear circuit will be explained below with reference to FIG. When the power is turned on, the "-" input of the comparator 1 is given the voltage VA divided by the resistors R1 and R2. In addition, when the input of "10" is turned on, the voltage of VB rises from 0 to VCC voltage after a certain period of time due to the operation of capacitor C and the time constant of resistor R3 and capacitor C. The output of the comparator 1 is at a low level while it is smaller than VA, and when it becomes larger than VA, the output is at a high level, and a low-level power-on clear signal for clearing the circuit is output. Conventionally,
Since this power-on clear signal time requires a long signal of several milliseconds to several tens of milliseconds, the capacitor C has a large capacitance on the order of μF.

[Problem to be solved by the invention]

Therefore, in conventional power-on clear circuits, a variable capacitor cannot be used as a capacitor due to the capacitance, so a fixed capacitor must be used. Therefore, the time length of the power-on clear signal is The selection had to be made step by step, which imposed a constraint on circuit design. Another drawback is that it cannot be used in circuits that require a power-on clear signal with an arbitrary time width.

An object of the present invention is to provide an inexpensive power-on clear circuit that can obtain a clear signal of any length of time.

[Means to solve the problem]

The power-on clear circuit of the present invention includes first and second resistors that divide the supply voltage, a variable resistor that arbitrarily sets the current when the supply power is turned on, and the power-on clear circuit that is set by the variable resistor. A constant current diode that allows the current to pass through and adjusts the rise time of the output voltage, and a voltage divided by the first and second resistors is compared with the output voltage of the constant current diode for initial setting. It is a refinement equipped with a comparator that outputs a clear signal.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

Referring to FIG. 1, which shows one embodiment of the present invention, the power-on clear circuit includes a resistor R1, which divides the supply voltage VCC;
R2, a variable resistor Rv that arbitrarily sets the current when the supply power VCC is turned on, a constant current diode RC that allows the current set by the variable resistor Rv to pass through and adjusts the rise time of the output voltage, and resistors R1 and R2. The comparator 1 compares the voltage divided by the constant current diode RC with the output voltage of the constant current diode RC and outputs a clear signal for initial setting.

The operation will be explained below. When the power is turned on, the “-” of comparator l
A voltage divided by resistors R1 and R2 is applied to the input. Since RC is a constant current diode, the current Ic flowing there when the power is turned on can be arbitrarily set by the variable resistor Rv. VC is O when the power is turned on
The time it takes for voltage to rise from v to VCC minus the voltage drop across constant current diode RC is determined by the current flowing through constant current diode Rc, and the smaller the IC, the slower it is. That is, the output of the comparator 1 provides a power-on clear signal with a long time width. At this time, while the voltage of VC input to "+" of comparator 1 is lower than VA, the output of comparator 1 is at a low level, and is vA.
When it becomes larger than , the output becomes high level, and a low level power-on clear signal is output to clear the circuit. Since the time width of this power-on clear signal varies linearly and continuously by changing the variable resistor Rv, it is possible to obtain a power-on clear signal having an arbitrary time width. Furthermore, since the constant current diode Rc and variable resistor RV according to the present invention are several times lower in price than a large capacity capacitor, they can be realized at a lower cost than conventional circuits.

〔Effect of the invention〕

As explained above, the present invention includes a variable resistor that arbitrarily sets the current when the power supply is turned on, and a constant current diode that allows the current set by the variable resistor to pass through and adjusts the rise time of the output voltage. By providing a clear signal for initial setting and outputting the clear signal from the comparator, it is possible to obtain a clear signal of any length of time and to provide it at low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional power-on clear circuit. 1-... Comparator, C... Capacitor, R
1 to R3...Resistance, Rc... Constant current diode, RV... Variable resistor.

Claims (1)

[Claims] first and second resistors that divide the supply voltage; a variable resistor that arbitrarily sets the current when the supply power is turned on; and a rise of the output voltage by passing the current set by the variable resistor. A constant current diode that adjusts the time, and a comparator that compares the voltage divided by the first and second resistors with the output voltage of the constant current diode and outputs a clear signal for initial setting. A power-on clear circuit featuring the following features: