Abstract:
A tri-state driver circuit is disclosed which provides rail-to-rail output swings and does not consume a significant amount of d.c. power.

Description:
This application is a Continuation of application Ser. No. 07/989,929, filed Dec. 11, 1992 now abandoned which is a continuation of Ser. No. 07/758,434, filed Sep. 3, 1991, now abandoned which is a continuation of Ser. No. 07/540,542 filed Jun. 19, 1990 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Traditionally, a multiplexer is used to select a signal from among a number of inputs. However, as this number increases, the speed of the multiplexer decreases and the physical size (area) of the multiplexer increases, resulting in consumption of valuable space. A tri-state scheme of selection would be more desirable than multiplex selection, particularly, if multiplex selection must be made from greater than 8 inputs. However, tri-state schemes generally impart more loading on a bus connected to tri-state driver outputs than desired such that the output bus line switching speed is decreased. Furthermore, tri-state circuits which do not have rail-to-rail voltage output swings, generally consume a significant amount of d.c. power. Therefore, a need exists to increase the switching speed of a tri-state circuit. Additionally, a need exists to significantly decrease d.c. power consumption by tri-state circuits which do not have full voltage output swings. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a new and improved BiCMOS tri-state driver circuit. 
     It is another object of the invention to provide a new and improved selection system using Bi-CMOS tri-state driver circuits. 
     These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read together with the accompanying drawings in which applicable reference numerals have been carried forward. 
     SUMMARY OF THE INVENTION 
     The foregoing objects of the invention are provided by two switches, each of which are connected to associated bipolar transistors of a totem pole connected pair. The switches when turned on, are capable of turning their associated bipolar transistors off. A first bipolar transistor of the pair is controlled by the state of an enable signal. A switch is connected across the first bipolar transistor so as to be capable of driving the circuit&#39;s output to a predetermined voltage when the circuit is enabled and a predetermined input is received by the circuit. The switch connected across the first bipolar transistor and the switch connected to the first bipolar transistor operate in a complementary fashion (i.e. when one is on, the other is off). The invention eliminates current leakage from the output when the circuit is disabled and provides a full output voltage swing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic drawing of a first preferred embodiment of the BiCMOS tri-state circuit. 
     FIG. 2 is a chart showing the relative on and off states of transistors in response to EN, EN -- , and IN signals. 
     FIG. 3 illustrates a truth table of the output OUT in relation to the voltage, logic high or logic low, at signals EN, EN --  and IN. 
     FIG. 4 illustrates a schematic drawing of the tri-state circuit with functional logic. 
     FIG. 5a illustrates a functional equivalent of the invention implementing inverting tri-state logic. 
     FIG. 5b represents a voltage vs. time plot of a simulation of an embodiment of the invention carrying out inverting functionality as shown in FIG. 5a. 
     FIG. 6a illustrates a schematic drawing of an alternative embodiment of the invention implementing NAND logic. 
     FIG. 6b illustrates a corresponding operation table for the circuit of FIG. 7a showing the on/off states of transistors and both binary input/output and enable/disable signals. 
     FIG. 7a illustrates a schematic drawing of an alternative embodiment of the invention implementing NOR logic. 
     FIG. 7b illustrates a corresponding operation table for FIG. 7a showing the on/off states of transistors and both binary input/output and enable/disable signals. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a schematic drawing of a first preferred embodiment of the BiCMOS tri-state circuit. Tri-state control signals EN and EN --  the complement of signal EN, can be generated by an inverter. Assuming signal EN is at logic low and its complement, signal EN --  is at logic high, p-channel transistor 2 and n-channel transistor 4, whose gates are connected to EN --  and EN, respectively, are turned off. Therefore, no base current flows to bipolar pull-up transistor 6 or bipolar pull-down transistor 8, connected by the emitter and the collector, respectively, to output OUT. Base current fails to flow in transistor 6 due to the isolation of supply voltage Vcc by turned off transistor 2 from p-channel input transistor 10 which is connected at its drain to the base of transistor 6 and connected at its source to the drain of transistor 10. Base current does not flow in transistor 8 due to input n-channel transistor 12, whose drain is connected to the drain of turned off transistor 4 and whose source is connected to the base of transistor 8. P-channel transistor 14 which has one source/drain connected to n-channel transistor 16 prevents false triggering of transistor 6. N-channel transistor 18 has its gate connected to supply voltage Vcc, its drain connected to the base of transistor 8 and its source connected to circuit ground. Likewise, transistor 18 maintains the base of transistor 8 near ground and prevents false triggering of transistor 8. However, transistor 8 will turn on temporarily to discharge a high logic level previous output after the circuit&#39;s current input and EN signal are logic high. The foregoing described circuit conditions exist irrespective of the voltage at the common input signal IN to gates of transistors 10 and 12. 
     Assuming signal EN is at logic high, its complement, EN --  at logic low and input signal IN at logic low, transistors 2 and 10 turn on to provide base current to transistor 6. Transistor 6 pulls the voltage at output OUT up to one base emitter voltage drop shy of Vcc in response to the base current supplied by transistors 2 and 10. Transistor 14 gradually charges output OUT. Eventually transistor 14 will pull output OUT up to supply voltage Vcc, thus turning off transistor 6. Assuming that input signal IN changes to logic high and that the initial output voltage is at Vcc, transistors 4 and 12 turn on to provide base current to transistor 8. 
     A primary advantage of the invention is its ability to eliminate current leakage from the circuit&#39;s output to the rest of the circuit by providing a first switch, i.e. transistor 14, across the first transistor of a bipolar pair of transistors connected in a totem pole fashion (i.e. transistor 6 of totem pole pair comprising transistors 6 and 8) together with a second switch, (i.e. transistor 16) which operates in complementary fashion with the first switch, i.e. when one transistor is on, the other is off. Both first and second switches are controlled by the state of the circuit&#39;s enable signal. When the first switch is on, it aids in bringing the output up to substantially the circuit supply voltage. A third switch (i.e. transistor 18) connected across the second transistor of the bipolar pair, which is always on, functions as a resistor. Thus, when the circuit is disabled, the second transistor of the bipolar pair is turned off. The first and third switches provide full output voltage swing to drive other circuits. The first transistor provides the upper swing limit (i.e. Vcc, the circuit supply voltage) and the third transistor provides the lower swing limit (i.e. circuit ground). Thus d.c. power consumption is minimized. Therefore any other circuity can be driven without substantial loss of d.c. power from stage to stage. The circuit of FIG. 1 was implemented in a 0.8 micro meter (um) BiCMOS technology experimental prototype having the following transistor sizes: transistors 2 and 10=15.6 (um), transistor 14=2.7 um, transistors 16 and 22=3.2 um, transistors 4 and 12=13 um transistor 18=1.5 um and transistors 6 and 8=4.48 um2. 
     FIG. 2 is a chart showing the relative on and off states of transistors in response to EN, EN -- , and IN signals. On-means on initially and off eventually at steady state. OFF+ means off initially and on eventually at steady state. 
     FIG. 3 illustrates a truth table of the output OUT in relation to the voltage, logic high or logic low, at signals EN, EN --  and IN. 
     The tri-state function of the circuit of FIG. 1 can be implemented independent of the logic implemented by transistors 10, 12, and 22. FIG. 4, a schematic drawing of a second preferred embodiment of the invention, illustrates such a scheme with transistors 10, 12 and 22 of FIG. 1, replaced by functional logic circuits 30, 32 and 34 respectively. Functional logic circuits 30, 32 and 34 each comprise the appropriate network including transistors, which are required to implement a desired logic function in a circuit with tri-state capability. This means logic functions including AOI (AND-OR-INVERT), NOR, OAI (OR-AND-INVERT), and NAND can be implemented with the invention. For example, a NAND gate tri-state driver can be implemented with the following: circuit 32 and 34 each comprising two n-channel transistors having the source of a first n-channel transistor connected to the drain of a second n-channel transistor; and circuit 30 comprising two p-channel transistors having each source connected together and each drain connected together. The foregoing combines logic functionality and tri-state functionality in a single gate, the advantage of which is shorter delay in comparison with multiple levels of logic using a stand alone tri-state driver. 
     FIG. 5a illustrates a functional equivalent of the invention implementing inverting tri-state logic with inverters 40 and 42. Signal IN1 is input into inverter 40 and signal IN2 is input into inverter 42. Enabling signal EN and its complement, signal EN -- , control the output at node OUT among a logic high state, a logic low state, and a high impedance state as discussed previously. In accordance with common logic symbol representation, a small circle preceding a line signifies complementation of an input signal. 
     FIG. 5b represents a voltage vs. time plot of a simulation of an embodiment of the invention carrying out inverting functionality as shown in FIG. 5a . Time is shown in nanoseconds (nsecs) starting from time, t, equal 0 nsecs to t=50 nsecs. High and low points along the graphs are marked as shown in accordance with their time of occurrence. The voltage levels of signal EN and input voltages to inverters 40 and 42, VIN1 and VIN2, respectively, range from 0.0 volts to 5.0 volts. The voltage of signal EN --  ranges from 0.0 volts to 5.1 volts. The input voltages into inverters 40 and 42, VIN1 and VIN2 respectively, range from 0.0 volts to 5.0 volts respectively. The output voltage at node OUT ranges from 0.5 volts to 5.0 volts. 
     FIG. 6a illustrates a schematic drawing of an alternative embodiment of the invention implementing NAND logic. Its corresponding operation table showing the on/off states of transistors and both binary input/output and enable/disable signals is illustrated in FIG. 6b. 
     FIG. 7a illustrates a schematic drawing of an alternative embodiment of the invention implementing NOR logic. Its corresponding operation table showing the on/off states of transistors and both binary input/output and enable/disable signals is illustrated in FIG. 7b. 
     Although the invention has been described in detailed herein with reference to its preferred embodiment, it is to be understood that this description is by way of example only and is not to be construed in a limiting sense. It is to be further understood that numerous changes in the details of the embodiments of the invention, and additional embodiments of the invention, will be apparent to, and may be made by persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the true scope and spirit of the invention as claimed below.