Apparatus for Generating Pulses Upon Decreases in Supply Voltage

The present invention relates to electrical circuits for resetting computing equipment such as microprocessors for defined changes in supply voltage.

In a well-known apparatus, a reset pulse is applied to a microprocessor when electrical power is first applied. However, there is no provision made for resetting the microprocessor also when it may be in some undefined condition due to decreases in the supply voltage. If the computation is then continued without taking account of such decreases, large errors in the result can occur.

It is an object of the present invention to provide equipment which will generate a reset pulse for amicroprocessor not only when the equipment is first turned on, but also if there are supply voltage decreases which could lead to anomalies in the condition of the microprocessor.

The apparatus is to have a wide range of application, that is both the magnitude of supply voltage decrease which results in the generation of a reset pulse and the duration of the pulse should be adjustable. Additionally, the pulse which is generated when the power is first applied should have a predetermined pulse width, so that it can be differentiated from pulses resulting from a supply voltage decrease.

In a preferred embodiment of the invention, the actual output from the microprocessor is monitored so that a reset pulse is only furnished when the operation of the microprocessor has been affected. For example, a counter in the microprocessor may not have reached a predetermined count within a predetermined time or a voltage may not have reached a certain value within a given time. Such malfunction would prevent the generation of pulses which otherwise appear at regular intervals at an output of the microprocessor. Failure to receive such pulses from the microprocessor causes the generation of the reset pulse.

Preferably, a switch connected to the circuit of the present invention receives the pulses from the microprocessor. Operation of the switch prevents a capacitor in the circuit from reaching a predetermined voltage, while lack of such pulses allows it to reach the voltage. When the capacitor reaches the predetermined voltage, the reset pulse is furnished.

The switch may be a transistor switch, either a standard transistor or a field effect transistor being suitable. When any such transistor switch is used, the apparatus of the present invention can readily be constructed in integrated circuit form.

FIG. 1 is a first embodiment of a circuit according to the present invention;

FIG. 2 is a second embodiment of the circuit of the present invention;

FIG. 3 shows the variation with respect to time of the voltages in different parts of the circuit of FIG. 2.

In FIG. 1, the supply voltage is applied between a line 1 and a reference potential which may be ground potential. A capacitor 2 is connected to line 1 and a resistor 3 is connected from the other terminal of capacitor 2 to ground potential. A diode 4 is connected in parallel with resistor 3 with the anode of diode 4 being connected to ground potential. A protective resistor 5 is connected between the common point of capacitor 2 and resistor 3 and the inverting input of a difference amplifier 6. The direct input of amplifier 6 is connected to a voltage divider consisting of resistors 7 and 8. Resistor 7 is connected to line 1, while resistor 8 is connected to ground potential. The output of difference amplifier 6 is connected through to a resistor 9 to supply line 1 and to its direct input through a resistor 10. The reset pulse which is the subject of the present invention is derived from the output of difference amplifier 6 and applied to a microprocessor M at reset input terminal M-R.

After the supply voltage is first applied, the voltage at the inverting input of difference amplifier 6 is higher than that at its direct input. Therefore a "0" signal appears at the output of difference amplifier 6. The signal may be used to reset the microcomputer M. Capacitor 2 now starts to charge through resistor 3 so that, after a predetermined time, the output of difference amplifier 6 switches to a "1" signal. If the value of resistors 7 and 8 are given, the time at which the circuit switches, and therefore the duration of the "0" pulse, is determined in the main by the time constant of the RC circuit consisting of resistor 3 and capacitor 2.

If the supply voltage decreases by an amount which exceeds the forward voltage drop across the diode, capacitor 2 starts to discharge through diode 4. If, thereafter, the supply voltage is raised, then, depending upon the threshold voltage which is determined by resistors 7, 8 and 10, a pulse is generated whose pulse width depends on the value of resistor 10. The magnitude of voltage drops for which a pulse is generated thus can be defined exactly by fixing the values of resistors 7, 8 and 10. If a sensitive operational amplifier, such as, for example, type LM 139 of the firm National Semiconductor is utilized then voltage drops of as little as 0.5 V can be detected since the threshold can be fixed to be near zero.

Embodiment of FIG. 2: The power supply line is denoted by reference numeral 11. A capacitor 12 is connected to supply line 11, the other terminal of capacitor 12 being connected through a resistor 13 and the collector-emitter circuit of a transistor T having a base 14 to ground potential. The inverting input of an operational amplifier 15 is connected to the common point of capacitor 12 and resistor 13. The direct input of operational amplifier 15 is connected to the tap of the voltage divider constituted by resistor 16 and 17. Specifically, one terminal of resistor 16 is connected to supply line 11 and one terminal of resistor 17 is connected to ground potential. A resistor 18 is connected between the output of operational amplifier 15 and supply line 11, while a resistor 19 is connected between the output and the inverting input. A resistor 20 is connected from the output of operational amplifier 15 to its direct input. The reset pulse is derived from the output of operational amplifier 15.

An input terminal 21 is connected through a resistor 22 and a capacitor 23 to the base 14 of transistor. Additionally, a diode 24 and a resistor 25 are connected between the base of transistor 14 and ground potential. The latter components constituted a protective circuit for the transistor.

The operation of the circuit of FIG. 2 will be explained with reference to FIG. 3. Operational amplifier 15 is connected as a comparator whose thresholds are determined by resistors 16, 17 and 20. In particular, the hysteresis of the comparator is determined by resistor 20 which thus determines pulse width of the output pulse. Because of resistor 19 and capacitor 12, the circuit acts as an RC oscillator whose frequency is determined by the value of these two elements. If the polarity of the output symbol is to be reversed, capacitor 12 may be connected to ground potential. The specific connection depends on the desired potential at the output of operational amplifier 15 after the supply voltage has been switched in.

The variation with respect to time at the inverting input of operational amplifier 15 is shown in FIG. 3a, while FIG. 3b illustrates the variation with respect to time at the direct input. FIG. 3c shows, in full line, the pulses P on line 21 and, in broken line, the switch-over time of comparator 15. When the supply voltage is first switched on, the potential at the inverting input exceeds that at the direct input. The output of the operational amplifier therefore is an "0" signal, which is used to reset the apparatus connected to the output of the operational amplifier e.g. microprocessor M' . Capacitor 12 starts to charge through resistor 19, so that comparator 15 switches after a predetermined time interval (broken line, FIG. 3c . The signal at its output is now a "1" signal which discharges the capacitor. If, after the microprocessor M' has been reset, a program begins to run which, after it is completed or, for a longer program, at predetermined time intervals, causes a pulse P to be generated at the output on line 21 of the microprocessor M' in accordance with a program which has been pretested, then an omission of the pulse at line 21 is indicative of a voltage drop. Thus if such a voltage drop occurs, a pulse will not appear at input 21 to the transistor T in time to prevent capacitor 12 from discharging to such an extent that the upper threshold voltage which is shown as a dot-dash line in FIG. 3a is reached. The comparator switches so that the reset pulse again appears at the output M-R. Because of positive feedback resistor 20, the voltage at the direct input of operational amplifier 15 also drops. Capacitor 12 discharges through resistor 19 until the lower threshold is reached. Thereafter, the output of the comparator is again a "1" signal. The computer is again ready for operation so that a new program can run.

The above circuits are easy to manufacture as integrated circuits. It is particularly advantageous to incorporate them in the microcomputer.

Many variations and changes of the above described circuits will be readily apparent to one skilled in the art and are intended to be encompassed by the following claims: