Power supply threshold activation circuit
TECHNICAL FIELD 
     The present invention relates to a new power supply threshold activation circuit for sequencing the turn on and turn off of multiple circuit devices or subcircuits dependent on a common power supply. The invention is particularly applicable in tristate enable circuits for maintaining the high Z or high impedance third state of a common bus tristate output device during power down, power up, and power out transitions of the common power supply. More generally the invention is useful in any application where a portion of circuitry must be kept active or for introducing a backup power supply during power transitions or power outages of the conventional power supply. 
     BACKGROUND ART 
     In electronic circuit applications where more than one output is tied to a single connecting line or common bus, the use of tristate output devices is required. By way of example, a prior art Fairchild Advanced Schottky TTL three state or tristate output control device is illustrated in FIG. 1. Several elements or stages can be identified in such a TTL tristate output gate. The pullup element for sourcing current from the higher level voltage and power supply V cc  consists of transistors Q12 and Q13 forming a Darlington current source. The Darlington current source supplies current from power supply V cc  to the output V out . The &#34;pulldown&#34; element or stage for sinking current and voltage from the output V out  to ground consists of transistor Q14 with a conventional squaring network including transistor Q16 at its base. In the example of FIG. 1 the pulldown transistor Q14 also includes a turn off accelerating transistor Q15 and associated diodes for sinking Miller current and accelerating turn off of pull down transistor Q14 during transition from low to high at the output V out . Step up transistor Q10 is coupled in series with the phase splitter Q11 to increase the input threshold at the logic data input V in . 
     When a low level voltage, binary zero in positive logic, appears at the input V in , a high level voltage, binary one, appears at the output V out . In this state the pullup element Q12 and Q13 is conducting affording a low impedance path to the output from the high level voltage V cc . The pulldown element Q14 is nonconducting affording a high impedance path to the low level voltage or ground. 
     When a high level voltage binary logic one appears at the input V in , a low level voltage binary zero appears at the output V out . In this state the pullup element Q12 and Q13 is nonconducting affording a high impedance blocking path from the high level voltage V cc  to the output. The pulldown element Q14 is conducting affording a low impedance path from the output to ground. 
     The third state or high impedance state of the tristate device illustrated in FIG. 1 is afforded by the output enable terminal OE. When a low level potential or binary zero from the enable gate appears at terminal OE the active elements of the tristate device are deprived of base drive current. Thus, terminal OE provides a low impedance path to ground and all transistors become nonconducting. In this high impedance third state both the pullup element and pulldown element exhibit high impedance to signals appearing at the output V out . With all the elements deprived of base current the output effectively becomes a high impedance to any exterior circuitry, for example, a common bus to which the output V out  may be connected. In this condition the tristate output device will neither source nor sink current at the output V out  and will behave effectively as if nothing were there. Such a tristate device is therefore particularly applicable and suitable for applications in which a plurality of output gates are tied together or coupled to a common bus structure. In such common bus applications only one output, that is only one of many output devices of the type illustrated in FIG. 1 coupled to the bus structure, determines the voltage (high or low) of the bus while the other outputs for the remaining gates are in the high impedance third state. 
     When a high level voltage binary one appears at the output enable terminal OE the output device functions as a bistate output device sinking or sourcing current at the output V out  according to whether the pulldown or pullup element is conducting. It should be noted that the output device is inherently inverting as a binary zero at the input V in  generates a binary one at the output V out  and vice versa. As stated above only one such tristate output device of many coupled to the common bus is active at any one time. 
     The enable gate which delivers a high or low voltage signal, binary one or zero, to the output enable terminal OE is itself a bistate TTL gate. In order to relinquish the common bus, not shown, to another tristate output device which is actively passing logic data in a bistate mode, the enable gate must remain actively conducting through its own pulldown element. Thus, the enable gate pulldown element affords a low impedance path from the output enable terminal OE to ground, continuously sinking base current from the transistors of the tristate output device. It is essential that the enable gate remain continuously activated for sinking current in order to maintain the high impedance or high Z state and prevent interference with high and low signals on the bus. 
     In a bus system with multiple tristate output devices coupled to the single connecting line, it is common to &#34;power down&#34; the unused devices in the high Z state in order to reduce power consumption. A problem frequently occurs during such a power down transition with the high Z state output devices interfacing at the bus. During the power down and reduction in voltage the enable gate coupled to the tristate output device tends to turn off or deactivate before the elements of the tristate output device itself. Once the enable gate is deactivated and becomes nonconducting there is no longer a route to ground for the base drive current at the phase splitter transistor Q11 through the terminal OE. As a result there is a time interval during which the tristate output device phase splitter transistor Q11 and pull down element Q14 become conducting. The pulldown element Q14 affords a low impedance path to ground at the tristate output V out  and therefore at the entire common bus affecting all other devices connected to the bus. The system reliability is therefore reduced during power down and power up voltage transitions at the power supply V cc . 
     A further discussion of tristate output devices can be found, for example, in U.S. Pat. No. 4,311,927, issued Jan. 19, 1982, assigned to the assignee of the present invention; in U.S. Pat. No. 4,287,433, also owned by the present assignee; and in the FAST DATA BOOK, published 1982 by Fairchild Camera and Instrument Corporation, Digital Products Division, 333 Western Avenue, South Portland, Me. 04106. The prior art known to applicants does not directly address the problem of loss of the high Z state and therefore loss of reliability in tristate output devices during power down, power up, and power out transitions in the common power supply. 
     In U.S. Pat. No. 4,104,734, assigned to the assignee of the present application, the problem of protecting volatile random access memory cells during drop in the power supply voltage is addressed. According to this patent, a threshold type circuit is provided for detecting a drop in the system power supply voltage and for turning off bias networks in a sequence making it impossible to write into or change the data in a memory cell. The memory cells are thus protected while the power supply voltage is reduced to the minimum cell current requirements for maintaining the cell contents. 
     Threshold activation and control circuits and concepts have never been applied however to the unique requirements of tristate output devices and their associated enable gate circuits. More generally, such threshold circuits and concepts have not been applied to interdependent logic gate output circuits and subcircuits. Nor would the threshold detecting circuit of U.S. Pat. No. 4,104,734 be of a type that could optimally be applied in such environments all as hereafter described. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide tristate output devices and associated enable gate circuits with high reliability in the high impedance or high Z third state even during power down, power up, and power out transitions of the common power supply. 
     Another object of the invention is to provide new enable gate circuits for sequencing the turn off or deactivation and turn on of circuit elements during power down and power up transitions of the common power supply so that spurious loss of high impedance coupling of the inactive devices at a common bus may not occur. 
     A further object of the invention is to provide a new and improved threshold activation and control circuit applicable for sequencing turn off and activation of interdependent circuits and subcircuits. Yet an additional object of the invention is to provide threshold activation and control circuits for introducing a backup power supply to maintain active circuit devices and subcircuits during power down transitions or power outages in the conventional power supply. 
     DISCLOSURE OF THE INVENTION 
     In order to accomplish these results the present invention contemplates providing an improved tristate enable circuit for activating a tristate enable gate to maintain the high impedance third state of a common bus tristate device during power down and power up transitions of the common power supply V cc . This is true even when the enable gate circuit elements normally turn off at a higher voltage level V cc2  than the lower voltage level V cc3  at which the circuit elements of the common bus tristate device turn off. Thus, spurious introduction of high and low voltage signals at the common bus is avoided. To accomplish this the invention provides a threshold activation circuit coupled to the enable gate for activating the enable gate when the threshold activation circuit senses a common power supply voltage level V cc1 . The threshold activation circuit is designed for operatively activating the enable gate over a voltage range from V cc1  to a lesser common power supply voltage level V cc4 . The threshold activation circuit component values are selected for operatively relating the voltage levels so that V cc1  &gt;V cc2  and V cc3  &gt;V cc4 . A feature and advantage or this circuit selection is that the turn off of circuit elements is sequenced by the threshold activation circuit in the desired order. 
     In a preferred embodiment the threshold activation circuit comprises a threshold transistor and a threshold ladder circuit coupled to the terminals of the threshold transistor. The output of the threshold activation circuit is coupled through an inverting transistor to the enable gate pulldown element in parallel with the enable gate phase splitter. As a result, during a power down or power out transition with the operative voltage levels sequenced by the threshold activation circuit as set forth above, the threshold activation circuit begins driving the enable gate pulldown transistor in parallel with the phase splitter at a relatively higher voltage level V cc1 . During further loss of power a voltage level V cc2  is reached at which the enable gate phase splitter element turns off. The threshold activation circuit however continues to drive the pulldown element of the enable gate providing a low impedance path to ground which continues to deprive the tristate output device of base drive current. The tristate output device seeing a low level potential or binary zero at the output enable terminal OE remains in the high Z state. Without the threshold activation circuit in accordance with the invention a high impedance would appear at terminal OE upon deactivation or turn off of the enable gate. As a result, the tristate output device may interfere with data passing on the bus from another device in the active bistate mode. 
     As the power down or power loss continues a voltage level V cc3  is reached at which the tristate output device itself becomes deactivated and its active element transistors are no longer conducting. Below this voltage level spurious results on the common bus can no longer occur. Therefore, at a final lower voltage level V cc4  the threshold activation circuit itself turns off. This sequence takes place in reverse during power up transitions of the power supply so that spurious voltage levels on the common bus are eliminated during transitions in either direction according to the present invention. 
     The threshold ladder circuit may take a number of forms but includes as basic elements at least a collector resistor coupled between the collector of the threshold transistor and the power supply; clamping devices such as diodes, and a base supply resistor coupled in series between the base of the threshold transistor and power supply; and a base to ground resistor coupled between the base of the threshold resistor and ground or low level potential. The output from the threshold activation circuit is taken from the collector of the threshold transistor through a coupling gate such as a diode or active element transistor according to the application. For use in enable gate circuit applications for controlling the high Z state of a tristate output device, a pass transistor inverter is used for coupling the threshold transistor to the enable gate pulldown element transistor while inverting the voltage signal. 
     More generally, the threshold activation circuit is applicable for sequencing the activation or turn off of interdependent multiple circuit devices or subcircuits during power down, power up, and power out transitions of the common power supply. 
     According to the another aspect of the invention the threshold activation circuit is used to control introduction of a separate or independent backup power supply, for example, into a memory board or other application where active subcircuits must be maintained during power down transitions or unintentional power losses. 
     Other objects, features, and advantage of the invention will become apparent in the following specification and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a circuit diagram or circuit schematic of a prior art tristate output device for the control of which the present invention is particularly applicable. 
     FIG. 2 is a circuit diagram or schematic diagram of an enable gate circuit and threshold activation circuit in accordance with the invention. 
     FIG. 3 is a circuit diagram or schematic diagram of a threshold activation circuit for use in introducing an independent backup power supply and showing variations in the voltage clamping devices. 
     FIGS. 4 and 5 are additional circuit diagrams showing further variations and alternatives in the threshold activation circuit according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE OF THE INVENTION 
     A tristate enable circuit modified in accordance with the present invention is illustrated in FIG. 2. Such an enable circuit is suitable for controlling tristate output devices of the type described above with reference to FIG. 1. The enable circuit also referred to as an enable gate is itself a TTL device. The primary operative element of the enable gate is the active element pulldown transistor Q7 which sinks current from the output OE of the enable gate when conducting. In this state of the enable gate the pullup element transistors Q5 and Q6 forming a Darlington are nonconducting and a binary zero or low potential appears at the output OE affording a low impedance path to ground from the collector to emitter of pulldown transistor Q7. In this condition the enable gate controls or holds an associated tristate output device in the high impedance or high Z third state. 
     When the pulldown transistor Q7 is nonconducting interposing a high impedance in the route to ground and the Darlington pullup element Q5 and Q6 is conducting a high potential or binary one appears at the output OE. A binary one or high voltage level at the terminal OE enables the associated tristate device to function in the bistate mode sourcing and sinking current at the common bus according to the logic data at the input V in  as for example shown in FIG. 1. 
     The state or condition of the enable gate is controlled by phase splitter transistor Q4 and step-up transistor Q3 which increases the threshold voltage at the enable gate input OE for switching the enable gate. 
     The logic circuits illustrated in FIGS. 1 and 2 are typically incorporated in integrated circuits using any of a number of technologies, for example, MOS, CMOS, ECL, and Fairchild Advanced Schottky TTL (FAST) technologies. However, the invention is also applicable to discrete component circuits. During power down or power out transitions of the common power supply V cc  there is a tendency in TTL integrated circuits for the enable gate to turn off or deactivate before the tristate output device. With enable gate pulldown transistor Q7 nonconducting, an interval of time therefore exists before turn off of the tristate output device when the output device is no longer held in the high Z state. As a result spurious signals may appear on the common bus interfering in the transfer of data from another output device, then in the active bistate mode. In particular the pulldown element of the tristate output device may become conducting pulling the common bus down to ground or low potential. 
     In order to avert this undesirable result, the invention incorporates a novel threshold activation circuit 20 as an integral part of the enable circuit. The threshold activation circuit includes an active element threshold transistor Q1 with circuit components coupled at its terminals between the high level power voltage V cc  and low level ground potential or ground, in the form of a &#34;threshold ladder&#34; circuit. In particular, collector resistor R3 connects the collector of transistor Q1 to power supply V cc . Clamping devices in this instant diodes D1 and D2 in series with base supply resistor R1 connect the base of transistor Q1 to the common power supply V cc . The base to ground resistor R2 connects the base of transistor Q1 to ground or low level potential and the emitter of transistor Q1 is also connected to ground potential. The purpose of threshold activation circuit 20 is to sense a transition in the power supply voltage V cc  and take corrective or preventive action to prevent the enable gate pulldown transistor Q7 from turning off before the tristate output device. 
     During normal static supply operation V cc  is high enough to keep threshold transistor Q1 conducting. With threshold transistor Q1 conducting the coupling gate transistor Q2 according to the invention is deprived of base drive current and remains off. 
     Transistor Q2 is a pass transistor inverter used to interface or couple the threshold activation circuit 20 to the enable gate and is particularly suited for that purpose. In other applications a coupling gate other than a transistor, for example, a diode may be used. 
     In this example an inverting transistor Q2 is required for controlling the pulldown transistor Q7 of the enable gate. Transistor inverter Q2 is coupled with its collector and emitter in parallel with the enable gate phase splitter transistor Q4 and in this position is able to control and drive the pulldown transistor Q7 in the event the phase splitter transistor Q4 turns off. 
     As stated above during normal static power supply operations transistor Q1 is conducting and inverter transistor Q2 is off so that the threshold activation circuit does not affect the enable gate. The supply current for transistor Q1 and the power consumption in this standby condition is determined by resistors R1 and R3. 
     As power supply V cc  is reduced a threshold V cc1  is reached where Q1 will no longer conduct. This voltage threshold is given approximately by the following equation: ##EQU1## where V D1  and V D2  are the voltage drops across diodes D1 and D2 respectively and VBE Q1  is the voltage drop across the base to emitter junction of threshold transistor Q1. This voltage threshold V cc1  may be adjusted by appropriate values for R1 and R2, and the number of clamping devices, in this instance diodes. At voltage levels below the threshold V cc1 , threshold transistor Q1 turns off and inverter transistor Q2 turns on as a result of base drive current through resistor R3. The coupling transistor Q2 in turn supplies base drive current to the enable gate pulldown transistor Q7 holding it on in the conducting state. At the same time the emitter to collector low impedance conducting path of transistor Q2 holds the Darlington consisting of transistors Q5 and Q6 off and nonconducting. 
     The enable gate phase splitter transistor Q4 and the threshold activation circuit coupling transistor Q2 thus cooperate in parallel to maintain a high Z or high impedance third state in the associated tristate output device connected to terminal OE. From another perspective, the enable gate phase splitter transistor Q4 and the threshold activation circuit coupling gate transistor Q2 effectively comprise a two input logic OR gate. One input Q4 of the logic OR gate receives the tristate enable signal from OE while the other input Q2 of the logic OR gate receives the threshold signal from the threshold activation circuit 20. The output of the effective logic OR gate formed by transistors Q4 and Q2 is coupled to drive the base of enable gate pulldown transistor Q7 when either a logic high level tristate enable signal at OE, or a logic high level threshold signal from the threshold activation circuit appears at an input of the logic OR gate as illustrated in FIG. 2. 
     As the power supply V cc  continues in a downward or power down transition, there is a point V cc2  where phase splitter transistor Q4 shuts off. This is given approximately by the following equation: ##EQU2## where VBE is the voltage drop across the base to emitter junction and VSAT is the voltage drop across the collector to emitter junction of the transistor in saturation. 
     In TTL integrated circuit technologies it is frequently the case that the enable gate circuit elements and in particular the phase splitter transistor Q4 turns off during a downward transition of the power supply V cc  before the elements of the tristate output device. In accordance with the invention therefore the circuit component values are selected so that V cc1  &gt;V cc2 . Thus, when the voltage level V cc2  is reached and phase splitter transistor Q4 turns off, the coupling gate transistor Q2 alone carries current to the enable gate pulldown transistor Q7 keeping it on. In other words, the high level voltage or logic high level threshold signal from threshold activation circuit 20 through coupling transistor Q2 maintains activation of the enable gate pulldown transistor Q7 and keeps transistor Q7 conducting regardless of the level of the tristate enable signal input at OE. With Q7 conducting the output OE of the enable gate remains low, binary zero, and hence the tristate output device with which it is associated remains in the high Z state and does not perturb the system bus. 
     As the power down continues the voltage level of power supply V cc  reaches a lower level V cc3  at which the tristate device itself or other external circuit turns off. Referring to FIG. 1 the turn off level for tristate output device pulldown transistor Q14 is a function of the voltage drop, VBE, across the base emitter junctions of transistors Q10, Q11 and Q14 and is also a function of the various resistors R10, R11, R12, R14 and R15. The turn off of the Darlington is a function of VBE across transistors Q12 and Q13 along with other circuit components including resistors R11 and R17. While the turn off voltage of the pullup and pulldown transistors of the tristate output device of FIG. 1 may differ slightly, the turn off voltage range is referred to collectively as V cc3 . V cc3  is also a function of the potentials at the common bus presented by another tristate output device in the active bistate mode. In any event the voltage range over which complete turn off of the tristate output device results, and in particular the lower level of such voltage range is characterized herein by the designation V cc3 . 
     Finally, there will be a point V cc4  where the threshold activation circuit 20 will turn off given approximately by the following equation: ##EQU3## This is the voltage level at which transistor inverter Q2 turns off. The component values are selected to assure that V cc3  &gt;V cc4 . Since the threshold activation circuit is the last subcircuit or circuit portion to shut down and turn off, it is assured that the associated enable gate maintains the tristate output device in the high Z state until the high Z output device itself turns off. In other words, the threshold activation circuit 20 provides a logic high level threshold signal through coupling transistor Q2 whenever the common power supply voltage is in the range of V cc1  to V cc4  inclusive to ensure that the enable gate pulldown transistor element Q7 remains conducting. The result according to the invention is that the tristate output device never loads the system bus during power down transitions. That is, it never forces the bus either to high or low potential during power transitions or power loss. Any device active on the system bus during power down is free to control the system bus without interference from other devices on the bus. All the tristate output devices except the one active device relinquish the system bus for data transmission by the active device. 
     Thus, in common bus tristate output integrated circuit applications where the turn off voltage level V cc2  of the enable gate may be higher than the turn off level V cc3  of the tristate device, the invention provides a corrective measure. The invention provides a threshold activation circuit activated at the higher voltage level V cc1  to maintain the high impedance Z state through a voltage range to a lower voltage level V cc4 . According to the conditions of the invention, the component values are selected so that V cc1  &gt;V cc2  and V cc3  &gt;V cc4 . Typical values of the voltage levels in Fairchild Advanced Schottky TTL integrated circuit technology are as follows. The power supply V cc  is typically at 5 volts. V cc1  is typically about 2.8 volts while V cc2  is approximately 2.6 volts. The turn off voltage V cc3  of the tristate output device is typically in the range of 1.9 to 2 volts. The threshold activation circuit itself remains operative through the lower voltage level V cc4  of approximately  1.75 volts. By this threshold voltage level relationship achieved by the present invention the threshold activation circuit which drives the tristate enable circuitry will be active longer than the tristate output device. Furthermore, during a power up transition the threshold activation circuit and enable gate will come on first so that no interference can occur at the system bus. Thus, the invention functions as well for power up transitions as power down transitions. 
     A variation of the threshold activation circuit in accordance with the present invention is illustrated in FIG. 3. Components analogous or similar to those shown in FIG. 2 are given the same symbol designations. In this example the collector of threshold transistor Q1 is coupled through collector resistor R3 to an independent backup power supply V BB  such as a backup battery and battery trickle charger. The coupling gate transistor or inverter transistor Q2 is similarly coupled to the backup power supply V BB  through a collector resistor R32. The variation of the invention illustrated in FIG. 3 is suitable for applications where active circuit must be maintained despite unintentional power outages. Thus, the circuit of FIG. 3 is applicable not only for sequencing the shutdown of multiple circuit devices or subcircuits but also for maintaining circuits in an active state by introducing a backup power supply V BB  in the event of complete loss of the conventional power supply V cc . 
     Another variation according to the invention shown in FIG. 3 is the modification of the threshold ladder circuit to include the four clamping devices, D1, D2, D21, and D22. In this instance, the clamping devices are diodes, for example, Schottky diodes, positioned in series with resistor R1 in the base power supply circuit line. According to the invention variation in the threshold of activation of threshold transistor Q1 may be accomplished by changing the values of the resistors R1 and R2 or by changing the clamping structures, in this instance, diodes. The use of clamping devices in the threshold ladder circuit constitutes a feature and advantage of the present invention. By the use of one way passive elements such as diodes the desired threshold activation voltage can be selected using a smaller ratio of resistance values for resistors R1 and R2, and larger values for R1 and R2 which reduce power consumption. This also affords better tracking of the threshold voltage V cc1  relative to changes in V cc2  with variations in temperature. The number of diodes may be varied to change the threshold in the threshold ladder in a more efficient manner. The typical voltage drop V SD  across the Schottky diode is approximately 0.5 volts and the number can be varied as shown in FIG. 3 so that V cc1  &gt;V cc2 . 
     In applications where an inverting transistor Q2 is not required as the coupling gate at the output of the threshold activation circuit 20, or variations thereof, a diode D3 may be used as the coupling gate as illustrated in FIG. 4. Components the same as or analogous to those of FIGS. 2 and 3 are designated by the same symbols. The circuit of FIG. 4 provides introduction of an independent backup power supply V BB  through the coupling gate D3 at a desired threshold voltage level. The circuit may be used in applications where a subcircuit or circuit device is to be maintained in an active state despite power outage, for example, as a backup for active memories. 
     Another variation of the threshold activation circuit of the type illustrated in FIG. 3 is shown in FIG. 5. In this example the coupling gate at the output of the threshold activation circuit is a field effect transistor FET and may be useful, for example, in applications with memory boards. 
     More generally, the threshold activation circuit in accordance with the present invention is useful and applicable for sequencing the turn on or turn off of multiple circuit devices, subcircuits or circuit portions dependent upon a common power supply. However, the invention is also applicable for introducing a backup power supply at the desired threshold voltage level of a conventional power supply, not only for sequencing multiple circuits, but also for maintaining external circuits, circuit devices, subcircuits, etc. in an active state during power outage. 
     While the invention has been described with reference to particular example embodiments, it will be appreciated that it is intended to cover all variations and equivalents within the scope of the following claims.