TECHNICAL FIELD The present invention concerns a novel circuit for providing a rapid reset action for a digital logic circuit which reset action is not dependent on a storage element to generate the reset signal.

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BACKGROUND ART Often in using digital logic circuitry there is a need to reset a circuit, such as a digital counter, that has been turned off. If the digital counter is turned off but then turned on quickly thereafter, it may happen that the reset circuitry has not functioned and that when the circuit is turned on it has not been completely reset. For example, assume that the digital counter has counted to 100. The counter is then turned off but turned back on promptly thereafter. Although it is desired that the counter now be at zero, because of the rapid turn-on the reset circuitry may not have recovered and the counter may be at an arbitrary number rather than zero. This problem is particularly found when a reset circuit is used requir- ing a capacitor discharge. In such circuits, if the power supply is turned off, the supply voltage will decrease rapidly to zero. However, such capacitor circuits utilize a capacitor which must be discharged to below a certain point before the circuit can properly be reset again. lit sterilization apparatus, it is particularly important that time or quantity measurements be made accurately and that the digital logic circuitry, such as the digital counters, which may be utilized in making such measurements, be reset properly. For example, if a counter measuring the amount of ultraviolet radiation which has been used to sterilize an ^* item has not been reset properly, the sterilization may be incomplete because the counter may not have started counting at its zero point.

It is, therefore, an object of the present invention to provide a circuit which resets a digital logic circuit, without being dependent on a storage element such as a capacitor to generate the reset signal.

Another object of the present invention is to pro¬ vide a rapid reset for digital logic circuitry and a self- test to determine whether this rapid reset has actually occurred.

Other objects and advantages of the invention will

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beco e apparent as the description proceeds.

DISCLOSURE OF THE INVENTION In accordance with the present invention, a circuit is provided which does not utilize a capacitor that has to 5 be discharged. Thus when the power supply is turned off and the voltage decreases, as soon as the power supply is turned on again the action of the present invention will occur to reset the digital logic circuit.

To this end, the circuit of the present invention 10 comprises a switch operable to provide a first signal when a predetermined condition has occurred. A bistable device, coupled to the output of the switch, is operable to receive the first signal and to provide an output reset signal when the bistable device is in a first state, in response 15 to the first signal. A digital logic circuit to be reset is coupled to the output of the bistable device for receiving the output reset signal. Feedback means are coupled from the digital logic circuit back to the bi¬ stable device for providing a signal to put the bistable 20 device into a second state whereby the output reset signal is terminated.

In the illustrative embodiment, means are provided for determining when the predetermined condition has occurred. The determining means comprise an LED and the 25. switch comprises a phototransistor optically coupled to the LED. The bistable device comprises a flip-flop with the phototransistor being coupled to the clock input of the flip-flop and with the feedback means being coupled to the reset input of the flip-flop. The feedback means 30 includes means for sensing that the digital logic circuit has been reset a predetermined amount.

In the illustrative embodiment, the digital logic circuit comprises a multi-stage counter and the feedback means is coupled to check the upper eight bits of the 5 counter. The feedback means includes a NOR gate with the

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upper eight bits' outputs connected to inputs of the NOR gate, and the output of the NOR gate is coupled to the flip-flop.

A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS . Figure 1 is a block diagram of a reset circuit constructed in accordance with the principles of the present invention.

Figure 2 is a schematic circuit diagram of one form of the invention used to reset a multi-stage digital counter.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Referring to Figure 1, resistor 10 is utilized to limit the maximum current that will flow through zener diode 12 and light emitting diode (LED) 14, all connected in series. Since zener diode 12 needs a particular voltage to conduct and LED 14 has an intrinsic offset voltage thereby requiring a particular voltage to conduct, once the sum of these two voltages is reached a photo- transistor 16, which is optically coupled to LED 14, will become conductive. As illustrated in Figure 1, the collector of transistor 16 is coupled via line 18 and line 20 to the power supply and the emitter of transistor 16 is coupled via resistor 22 to ground.

In effect, diodes 12 and 14 are utilized to program the level of the voltage at which a reset action will be initiated. Once transistor 16 is conductive, the current flowing through resistor 22 generates a positive going voltage across resistor 22 which is applied to the clock

input of a flip-flop 24 via line 26. Lines 18 and 20 are coupled to the data (D) input of flip-flop 24 via line 28.

Although no limitation is intended, flip-flop 24 is preferably a D-type edge triggered flip-flop such as a CMOS 4013 type.

In response to the power supply voltage on the D input of flip-flop 24 and the rising voltage on its clock input, the Q input of flip-flop 24 goes high. The Q output on line 30 of flip-flop 24 is the output reset signal which will not go low unless a high signal is applied to the reset (R) input of the flip-flop 24. Thus line 30 is coupled to the reset input of the logic to be reset 32. Logic 32 may be a clock, with some input function, output function and reset input. It is assumed that the logical function, such as that of a counter, must be reinitialized to some state every time the unit is turned on. When the logical function is operational, the logic will be reset in response to the signal developed by flip-flop 24.

A reset condition decoder 34 is provided and forms part of a feedback circuit including line 36, reset condi¬ tion decoder 34 and line 38. This feedback circuit is coupled from the logic 32 to the reset input of flip-flop 24. Reset condition decoder 34 determines if in fact the logic to be reset 32 has actually been reset. If it has been reset, a high signal is developed and applied to the reset input of flip-flop 24, causing line 30 to go low and removing the reset signal applied to the logic 32.

Referring now to Figure 2, a reset circuit is provided for use in a germicidal chamber in which sterilization occurs. In this, embodiment, a disposable object is placed in a drawer and ultraviolet sterilization is applied. A counter is utilized as a measuring device but the counter should not begin counting until the drawer is closed and the disposable is in its proper place within the drawer. In the Figure 2 embodiment, resistor 10 is in series with a diode 40 which is a standard silicon rectifier

having a predetermined intrinsic offset voltage. LED 14a_ and LED 14b are optically coupled to phototransistors 16a and 16b, respectively. LED 14sι and phototransistor 16a_ are positioned to indicate that the drawer is closed and LED 14b and phototransistor 16b are positioned to indicate that the disposable is in its proper place within the drawer. When the power is turned on, the diodes will conduct once the voltage reaches a certain level. However, phototransistor 16a_ will not be turned on unless the drawer is closed and phototransistor 16b will not be turned on unless the disposable is in its proper location within the drawer. The drawer blocks the actual paths when the drawer is open. The optical paths are cleared when the drawer is closed. Thus phototransistors 16a and 16b_ are turned on when the drawer is closed, initiating a reset action. In this manner, the optical coupler provides both a "power- on" reset and a process (brings counter to zero) reset.

There is a "door open alarm" circuit coupled to the emitter of transistor 16a_ which is operative under certain conditions. The emitters of transistors 16a and 16b are coupled to the inputs of an AND gate 42 via lines 43 and 44, respectively. AND gate 42 provides a high output signal only if both transistors 16a and 16b are conducting. The high output signal on output 46 of AND gate 42 is fed to the clock input of flip-flop 24 and the data (D) input of flip-flop 24 is coupled to the voltage supply. Like¬ wise, the collectors of transistors 16a and 16b are con¬ nected to the power supply voltage.

Once flip-flop 24 is provided with a positive going clock signal, the Q output of flip-flop 24 will be high thus providing an output reset signal via line 48 to the reset input of up-counter 50. In response to the reset input of up-counter 50 going high, the Q outputs of counter 50 should go to zero. If all of the Q outputs of counter 50 go to zero, NOR gate 52 will provide a high output, via line 54, to the reset input of flip-flop 24. Thereupon,

the Q output of flip-flop 24 will go low and allow counter 50 to count normally. Should any of the outputs of counte 50 not be zero, NOR gate 52 will have a low output contin¬ uously thereby preventing flip-flop 24 from becoming reset and the Q output of flip-flop 24 will always remain high, preventing counter 50 from counting.

Although no limitation is intended, in the illustra¬ tive embodiment counter 50 is a 14 stage ripple counter, type 4020. NOR gate 52 acts as a decoder to check that the upper eight bits of counter 50 have in fact been reset. If the upper eight bits of counter 52 have been reset, as stated above NOR gate 52 generates a high output via line 54 to the reset input of flip-flop 24, causing the Q output of flip-flop 24 to go low thereby removing the reset signal from counter 50. By checking the upper eight bits the circuit effectively determines whether the reset has been successful at least to a predetermined degree.

Should the drawer of the germicidal chamber become opened and then quickly closed, or should the power supply voltage be removed and then turned on again quickly, a reset signal will be generated that is not dependent on a time constant for recovery in the reset signal generator. Such disruptive actions, such as the opening of the drawer or the removal of the voltage supply, could happen with relatively great frequency without the problems concomitant with the use of a capacitor in the reset circuit.

In using a ripple counter, such as counter 50, the outputs may be slightly offset and in this manner the transition of the counter outputs are not always simul¬ taneous. Thus a transient might appear at the output of NOR gate 52. To avoid this problem, a resistor 56 and a capacitor 58 are provided as a delay network in order to shift the Q7 output, later in time, to the degree necessary so that spurious outputs are provided by NOR gate 52.

Although an illustrative embodiment of the invention

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has been shown and described, it is to be understood that various modifications and substitutions may be made without departing from the novel spirit and scope of the present invention.