Abstract:
The logic gate has at least one input terminal in which a digital input signal is applied having two possible logical signal values and at least one output terminal to output an output signal having a logical signal values. Two different logical voltage levels are allocated to both possible logical signal values of the output signal and a logic circuit is provided between the input and the output terminals. The logic circuit has several switching elements, especially switching transistors, working or produced according to the logical voltage level. The logic circuit is supplied with a supply potential that exceeds the logic voltage level. The logic circuit has at least two switching elements, especially switching transistors, in the output path allocated to the output terminal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE99/01618, filed Jun. 1, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a logic gate with at least one input terminal, at which an input signal is present in two possible logic signal values, and with at least one output terminal for outputting an output signal with a logic signal value. The two possible logic signal values of the output signal are assigned two different logic voltage levels (HIGH, LOW). The configuration further has a logic circuit, which is provided between the input and output terminals and a plurality of switching elements, in particular switching transistors, which are produced or operate according to the logic voltage levels. The logic circuit is supplied by a supply potential that exceeds the logic voltage levels. 
     Logic gates are the elementary basic building blocks of digital circuits and systems. They control the signal flow through the entire system. The designation gate indicates that they can be opened and closed by the signals present at the input and, in this way, either pass the information on or prevent it from being passed on. Elements appertaining to two-value (binary) logic are considered below, the two logic signal values 0 and 1 being differentiated by the two different voltage levels H or HIGH (typically 3.3 V or 5 V) and L or LOW (typically 0.2 V up to about 1.4 V). 
     In digital CMOS circuitry, so-called transmission gates (T-gates for short) are widespread in addition to the usual basic gates such as inverters, NAND, NOR and complex gates. The term pass transistor logic also crops up occasionally in this context. This involves a transistor pair consisting of an N-MOS and a P-MOS transistor which are connected in parallel and driven inversely at their gates. As a rule, a T-gate thus comprises two paths, an N-channel transistor path, which can switch through low to medium potentials, and a P-channel transistor path, which can switch through medium to high potentials. In the case of the T-gates known heretofore, each of these paths consists of just one transistor. In one case, both transistors are turned off and constitute an extremely high resistance between their two terminals. In the other case, a conductive connection with a finite resistance is produced between the terminals. A T-gate thus operates like a switch but with the limitation that only voltage potentials lying within the operating voltage of the T-gate can be switched. 
     The customary operating voltage for the HIGH level has been 5 V for many years. That operating voltage can no longer be adhered to in modern CMOS processes since the transistors have become so small that the physical limit of the maximum field strength is exceeded at 5 V. This is the reason why nowadays there are already many integrated circuits which operate internally with an operating voltage for the HIGH level that has been reduced to 3.3 V. For reasons of compatibility, however, externally the circuits still operate predominantly with 5 V signals. Therefore, level converters are necessary at the interface between the digital circuit and the outside world (that is generally the pad driver circuit). In the case of analog circuits, that means that T-gates have to establish the connection to the outside world. To ensure that the full swing of the analog signals of 5 V can be utilized, circuitry measures must be implemented to construct a T-gate in such a way that no voltages of 5 V occur at the 3.3 V switching transistors of the digital circuit which are to be driven. Otherwise, the service life of the transistors produced using 3.3 V processes would be considerably reduced. 
     Up to now there have been known only implementations in purely digital circuits which operate with 3.3 V internally and with 5 V externally. Chandrakasan, Burstein, and Brodersen, in “A Low-Power Chipset for a Portable Multimedia I/O Terminal”, IEEE J. of Solid-State Circuits, Vol. 29, No. 12, pp. 1415-28, December 1994, describe a circuit which amplifies 1.1 V signals to the external 5 V swing. The technology thereby used is 5 V compliant. 
     Pelgrom and Dijkmans, in “A 3/5 volt compatible I/O buffer”, Proc. ESSCIRC, p. 140-43, Ulm 1994, describe a bi-directional circuit using 3.3 V technology which operates with 3.3 V internally and permits external signals of up to 5 V. 
     Both the aforementioned cases involve level converters which establish the connection between two circuit environments with different operating voltages. A 5 V compliant T-gate and the drive logic required therefor using 3.3 V technology have not been disclosed heretofore. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a logic gate circuit which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which can perform the function of a T-gate and is constructed in such a way that 5 V signals can be switched without a transistor being operated with impermissibly high voltages. 
     With the above and other objects in view there is provided, in accordance with the invention, a logic gate, comprising: 
     an input terminal for receiving an input signal with two possible logic signal values; 
     an output terminal for outputting an output signal with two possible logic signal values assigned two different logic voltage levels; 
     a logic circuit connected between the input terminal and the output terminal and supplied by a supply potential exceeding the logic voltage levels; 
     the logic circuit having a plurality of switching elements, such as switching transistors, configured to operate according to the logic voltage levels, the logic circuit having an output path connected to the output terminal, the output path having at least two switching elements configured to operate according to the logic voltage levels, connected in series, and acting as a voltage divider. 
     In other words, the invention provides a logic gate circuit having at least one input terminal, at which an input signal is present in two possible logic signal values, and having at least one output terminal for outputting an output signal with a logic signal value, the two possible logic signal values of the output signal being assigned two different logic voltage levels and having a logic circuit, which is provided between the input and output terminals and has a plurality of switching elements, in particular switching transistors, which are produced or operate according to the logic voltage levels, which logic circuit is supplied by a supply potential exceeding the logic voltage levels, in which case the logic circuit has, in its output path assigned to the output terminal, at least two switching elements, in particular switching transistors, which are connected in series and act as a voltage divider. 
     In accordance with an added feature of the invention, the logic circuit has output path has two reciprocally operating output paths (pull-up path and pull-down path) with switching elements constructed and operating complementarily with respect to one another. 
     The two output paths may expediently be constructed symmetrically with respect to one another with the output terminal as the point of symmetry. 
     An essential advantage of the arrangement according to the invention is that a T-gate constructed in such a way can switch higher voltages than a conventional T-gate with the same dielectric strength of the individual transistors. The invention makes it possible to use 3.3 V manufacturing technology to construct 5 V CMOS logic which is suitable for driving T-gates according to the invention which are able to switch (analog) signals with a voltage swing of 5 V. 
     In accordance with a preferred embodiment of the invention, a CMOS inverter may be provided for driving the T-gate, which inverter can process 5 V signals at the input and once again supplies the full 5 V swing at the output. This circuit is also designed in such a way that the voltage across the transistors always remains distinctly below 5 V. By means of suitable extension, a NAND or a NOR gate and the like may be constructed from the inverter. The combination of the these two extensions can, finally, lead to a complex gate. The invention thus encompasses a complete CMOS logic including T-gates which operates with 5 V signals and can be realized using a modern 3.3 V process. 
     In accordance with a further preferred embodiment of the invention, the logic circuit has, in each output path, a further, third switching element connected in series with the two first and second switching elements. 
     In accordance with an additional feature of the invention, the logic circuit has, in each output path, a protective diode device for discharging leakage currents from the switching elements. By way of example, two transistors connected as diodes can dissipate the leakage currents of the limiting transistors and prevent an impermissible voltage rise across the turned-off switching transistors when the T-gate is switched off. The use of such diodes makes it possible to ensure that no impermissible gate voltages occur at the switching transistors due to leakage currents. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a logic gate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a logic gate formed as an inverter according to a first exemplary embodiment; 
     FIG. 2 is a circuit diagram of a logic gate formed as a NAND gate according to a secondary exemplary embodiment; 
     FIG. 3 is a circuit diagram of a logic gate formed as a transmission gate according to a third exemplary embodiment; and 
     FIG. 4 is a circuit diagram of a control circuit having a transmission gate according to a fourth exemplary embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a logic gate  1  that is configured as an inverter (NOT gate), which operates with 5 V signals and can be manufactured using 3.3 V technology. The logic gate  1  has an input terminal IN, at which an input signal is present in two possible logic signal values HIGH=5 V and LOW=0 V, and an output terminal OUT for outputting an output signal with a signal value, LOW or HIGH, which is the inverse of that of the input signal, and a logic circuit  2 , which is provided between the input and output terminals and comprises two alternatingly operating output paths  3  and  4 , the pull-up and pull-down paths. The two paths  3  and  4  have switching elements which are constructed and operate complementarily with respect to one another. The logic circuit  2  is supplied by a supply potential PUP exceeding the logic voltage levels HIGH and LOW. Each output path  3  and  4  of the logic circuit  2  has two P-channel MOS switching transistors MP 21  and MP 22  and N-channel MOS switching transistors MN 21  and MN 22 , respectively, which are connected in series and are connected as voltage dividers. The operating voltage is fed to the terminals PUP and PDN. As a rule, PDN is at 0 V, the reference-ground potential, and PUP is at 5 V, the maximum operating voltage. Signals smaller than 5 V are required for driving the T-gate which is explained below. That is achieved by connecting PUP or PDN to an auxiliary voltage. The auxiliary voltages are fed to the terminals  5  (LOW) and  6  (HIGH). They have values of 1.4 V for LOW and 3.6 V for HIGH relative to the reference-ground potential. The realization is based on an N-well process, the wells (not illustrated) being put at 5 V and the substrate (not illustrated) being put at 0 V. The pull-up path  3  and the pull-down path  4  are symmetrical with respect to one another, respective P-channel transistors corresponding to the N-channel transistors, and vice versa. 
     In order to explain the method of operation of the circuit according to FIG. 1, it suffices, for symmetry reasons, to consider just the pull-down path  4 . 
     If the inverter input IN is at 5 V, the transistor MN 1  pulls the node N 1  high to 3.6 V (HIGH). Above this voltage value, the transistor MN 1  is pinched off (UGS=0) and prevents a further voltage rise at the node N 1 . This ensures that the gate-source voltage at the switching transistor MN 21  does not exceed 3.6 V. The transistor MN 21  holds the node DN down, as a result of which the transistor MN 22  also starts to conduct and pulls the inverter output OUT to 0 V. Without the transistor MP 11 , leakage currents in the steady-state case would charge the gate of the transistor MN 21  further, since the gate of a MOS transistor provides better insulation than a switched-off transistor. The diode MP 11  starts to conduct when the potential at the node N 1  exceeds the potential value HIGH (3.6 V), and thus prevents a further potential rise at the node N 1 . 
     If the inverter input IN is at 0 V, the transistor MN 1  conducts and discharges the node N 1 . As a result, the switching transistor MN 21  is turned off. The pull-up path  3  now operates in an analogous manner to that described above. The node P 1  goes from 5 V down to 1.4 V (LOW) and drives the switching transistor MP 21 . The inverter output OUT is subsequently pulled high to a full 5 V via the transistor MP 22 . However, the node DN follows only up to 3.6 V, because the transistor MN 22  is pinched off above this value. Therefore, this transistor limits the voltage rise at the node DN in the same way as the transistor MN 1  limits the voltage rise at the node N 1 . Any leakage currents present are unproblematic here since they do not act on an insulated gate but on a turned-off transistor MN 21 , which is likewise affected by leakage current. Should the node DN rise to higher voltage values than HIGH, the gate-source voltage of the transistor MN 22  becomes negative, so that this transistor conducts even less than the transistor MN 21 , whose gate-source voltage is 0 V. A further voltage rise at the node DN is thus precluded. 
     The method of operation of the inverter circuit according to FIG. 1 can be summarized as follows. The transistors MN 21  and MP 21  form the switching transistors for the pull-down path  4  and pull-up path  3 , respectively, and are connected by their source terminals to the operating voltages PUP and PDN, respectively. The PDN and PUP transistors MN 1  and MP 1  limit the gate voltage, and the transistors MN 22  and MP 22  limit the drain voltage of the switching transistors. The diodes MN 11  and MP 11  discharge the leakage currents from the transistors MN 1  and MP 2 , respectively. As a result of the series circuit of three N-channel transistors MN 1 , MN 21 , MN 22  in one path and three P-channel transistors MP 1 , MP 21 , MP 22  in the other path, the effect achieved according to the embodiment as shown in FIG. 1 is that the high supply voltage of 5 V, for example, is split between the individual transistors. The four outer transistors MN 1 , MN 22 ; MP 1 , MP 22  limit the voltage across the inner switching transistors MN 21  and MP 21 , respectively. Such a circuit makes it possible to switch 5 V signals even though the individual transistors MN 1 , MN 21 , MN 22 , MP 1 , MP 21 , MP 22  are designed only for 3.3 V. 
     By means of corresponding extension, a NAND gate according to FIG. 2 is produced from the inverter according to FIG. 1 by two N-channel pull-down transistors being connected in series and two P-channel pull-up transistors being connected in parallel. The same reference numerals designate analogous circuit parts. The transistors MN 21  and MN 22  are connected in series and are protected against excessively high voltages by the transistor MN 23 . The transistors MP 21  and MP 22  are connected in parallel and are protected against excessively low voltage potentials at the node UP by the transistor MP 23 . Each switching transistor MP 21  or MN 21 , respectively, and MP 22  or MN 22 , respectively, is provided with its own protective circuit in the form of the transistors P 1  or N 1 , respectively, and P 2  or N 2 , respectively, at its gate. 
     A NOR gate (not specifically illustrated in the figures for reasons of simplicty) can be realized in an analogous manner by suitably connecting the P-type transistors in series and the N-type transistors in parallel. 
     FIG. 3 illustrates a transmission gate or T-gate according to a third exemplary embodiment of the invention. Since a T-gate operates bidirectionally, a unique assignment of drain and source is not possible. For this reason, these transistor terminals and also the input and output, IN and OUT respectively, of the T-gate are designated as terminals. The T-gate shown in FIG. 3 can switch 5 V signals and is constructed using 3.3 V transistors. The transistors MN 2  and MP 2  constitute the switching transistors of the T-gate, and the nodes IN and OUT constitute the two terminals. To ensure that the T-gate turns off, the control terminal NG must be at 0 V and the control terminal PG must be at 5 V. In this case, the transistors MN 1 , MP 11  and the transistors MN 3 , MP 12  prevent the nodes N 1  and N 2  respectively (terminals of the switching transistor MN 2 ) from rising above the potential HIGH, exactly as in the case of the inverter according to FIG.  1 . The transistors MP 1 , MN 11  and MP 3 , MN 12  prevent the nodes P 1  and P 2  from falling below the potential LOW. What is thus achieved is that the voltage between the gate and the terminals of the switching transistors does not exceed the value of 3.6 V. 
     To ensure that the T-gate turns on, the node NG must be brought to high potential and the node PG to low potential. Low to medium potentials pass via the N-type paths of the transistors MN 1 , MN 2  and MN 3 , and medium to high potentials pass via the P-type path of the transistors MP 1 , MP 2  and MP 3  from one terminal of the T-gate to the other. Accordingly, the node NG must not exceed the maximum permissible gate-source voltage (approximately 3.6 V). The node PG is permitted to be at most approximately 3.6 V below the value VDD=5 V (1.4 V). The situation becomes particularly critical with regard to the smallest possible on resistance in the case of medium potentials (2.5 V). In this case, both paths conduct, the gate-source voltage of the switching transistors being only 3.6 V−2.5 V and, respectively, 2.5 V−1.4 V=1.1 V. In order to obtain the smallest possible on resistance, it is important that the gate-source voltage become as large as possible. In other words, the potential value HIGH should be as high as possible and the potential value LOW should be as low as possible. On the other hand, however, the maximum permissible gate-source voltage must not be exceeded. In the case of a 3.3 V process, it is perfectly possible to choose 3.6 V, taking account of ±10% tolerance. That leads to the chosen voltages of 3.6 V for HIGH and 5 V−3.6 V=1.4 V for LOW. 
     In all the exemplary circuits illustrated (T-gate, inverter, NAND, NOR) the auxiliary voltages are passed only to the gates of MOS transistors. As a result, the requirements with regard to the power for the auxiliary voltages are very modest. In the steady state, these voltages are practically unloaded, so that they can be generated without a great outlay on the chip. Voltage dips due to dynamic loading can be absorbed by means of a suitably dimensioned capacitance with respect to the operating voltages. 
     FIG. 4 illustrates the interconnection of a T-gate I 0  according to FIG. 3 with three inverters I 1 , I 2 , I 3  according to FIG.  1 . The inverter I 1  controls the node NG and is connected by its pull-up path PUP not to 5 V but only to 3.6 V. As a result, it supplies output voltages of 0 V (logic 0) and 3.6 V (logic 1), as are required for driving the node NG. By contrast, the inverter I 2  supplies output voltages of 1.4 V (logic 0) and 5 V (logic 1) for driving the node PG. The inverter I 3  accepts 5 V signals at its input and, at the output, supplies 5 V signals again and provides for the inverse driving of the inverters of I 1  and I 2 . If 5 V are present at the control input ON, the T-gate turns on and a finite resistance forms between the two terminals T 1  and T 2 . If ON is at 0 V, the T-gate turns off and a very high resistance forms between the two terminals T 1  and T 2 . 
     Although in the overall circuit according to FIG. 4, the auxiliary voltages are also passed to the inverter terminals PUP and PDN, the inverters again drive only the gates of MOS transistors, so that in this circuit, too, no steady-state currents have to be supplied from the auxiliary voltages.