Abstract:
A mixed voltage output driver includes an output sensing circuit that senses an output voltage at an output terminal and generates a voltage signal that corresponds to a voltage level at the output terminal. Next, an impedance selection circuit receives the voltage signal and generates a control signal in response to the output voltage having a higher logical uplevel than the mixed voltage output driver. The control signal is then received by an adjustable drive impedance circuit that is also coupled to an input terminal of the mixed voltage output driver and, in response thereto, the adjustable drive impedance circuit modifies an output drive impedance of the mixed voltage output driver. In another advantageous embodiment, the mixed voltage output driver only determines if the output voltage at the output terminal is at a logical uplevel before adjusting the output drive impedance.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention: 
     The present invention relates generally to interface circuitry for low voltage digital technologies and in particular to output drivers. Still more particularly, the present invention a mixed voltage output driver with automatic impedance adjustment. 
     2. Description of the Related Art: 
     Efforts to decrease the size, increase the speed and reduce the power consumption of electrical circuits have created the need for low voltage silicon construction. Lower voltage requirements result in lower power consumption which complements, e.g., battery powered, portable electronics. 
     With advances in semiconductor fabrication techniques, the size of electronic devices has been reduced to the sub-micron level and the voltage requirements of these devices have been reduced significantly. Nevertheless, when a new low-voltage integrated circuit (IC) technology is developed, it is often desirable for that new technology to be able to operate with existing relatively high-voltage circuitry. The voltage of a particular technology is typically defined by the gate-oxide breakdown voltage and/or the punch-through between the source and drain. 
     As a result of the differing IC technologies, there are occasions when interfaces between “chips” occur where different chips drive different uplevel, i.e., logical high, voltages. For example, one driver circuit may drive the wire, i.e., interconnection, to an uplevel voltage of 2.0V. After this driver has driven the wire, it could tristate, i.e., go into a high impedance mode, relinquishing its turn on the wire. At this time, a second driver circuit on the wire may drive the wire to a 1.5V uplevel. In this example, the receivers on the wire would probably have logic thresholds of about 0.9V or 1.0V. 
     A problem, however, may arise when driving the interconnection from a “higher high” to a “lower high” if the second driver&#39;s impedance is too low. If the impedance of the driver that is trying to establish the 1.5V uplevel (in the above example) is too low, the signal on the wire will significantly undershoot the 1.5V level enough to make a designated receiver erroneously switch, or at the minimum, reduce the signal margin at the receiver below safe limits. Simulations have shown that a driver, with an uplevel of 1.5V minus a supply tolerance, in its lower impedance mode, e.g., 20 ohms, would cause the signal received at a receiver to drop to about 1.0V. On the other hand, the same driver in a higher impedance mode, e.g., 40 ohms, would provide a 1.25V signal at the receiver. An even higher impedance driver will drive the net at an even higher voltage uplevel. 
     Accordingly, what is needed in the art is an improved driver circuit that overcomes or mitigates the above discussed limitations. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved output driver. 
     It is yet another object of the present invention to provide a mixed voltage output driver with automatic impedance adjustment. 
     To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein, a mixed voltage output driver is disclosed. The mixed voltage output driver includes an output sensing circuit that senses an output voltage at an output terminal and generates a voltage signal that corresponds to a voltage level at the output terminal. Next, an impedance selection circuit receives the voltage signal and generates a control signal in response to the output voltage having a higher logical uplevel than the mixed voltage output driver. The control signal is then received by an adjustable drive impedance circuit that is also coupled to an input terminal of the mixed voltage output driver and, in response thereto, the adjustable drive impedance circuit modifies an output drive impedance of the mixed voltage output driver. In another advantageous embodiment, the mixed voltage output driver only determines if the output voltage at the output terminal is at a logical uplevel before adjusting the output drive impedance. 
     The present invention discloses a novel output driver circuit that automatically adjusts its output drive impedance depending on the voltage level that is present on an interconnection that it is connected to. Consequently, the problems associated with multiple output drivers having different logical uplevel voltages on the same interconnection are substantially reduced or eliminated. 
     The present invention further includes an input buffer, coupled to the input terminal of the output driver that comprises first and second inverters. First and second inverters, as is well known in the art, provide an buffer and high gain stage for an incoming Data signal. In a related embodiment, the output driver is embodied in an integrated circuit (IC). 
     In one embodiment of the present invention, the output sensing circuit is a noninverting receiver. Those skilled in the art should readily appreciate that if the implementation technology supports devices that can tolerate higher voltages, the noninverting receiver could be simplified or even eliminated. Thus, in other advantageous embodiments, the output terminal of the output driver may be directly coupled to the impedance selection circuit. 
     In another embodiment of the present invention, the impedance selection circuit includes a latch and a “set and reset” circuit. The set and reset circuit has an output coupled to an input of the latch and adjusts an output of the latch, in response to a voltage signal received at an input of the set and reset circuit. In a related embodiment, the output of the latch is provide to a NAND gate that performs a Boolean logic function. It should be readily apparent to those skilled in the art that the latch may also be set by other boolean equivalents, as would be obvious to those familiar with boolean implementations. 
     In yet another embodiment of the present invention, the adjustable drive impedance circuit raises a resistance value of the output drive impedance. In an advantageous embodiment, the resistance value of the output drive impedance is raised from twenty ohms to forty ohms. Of course these resistance values are arbitrary and the present invention does not contemplate limiting its practice to any particular set of resistances for the output drive impedance of the output driver. 
     The foregoing description has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a high-level simplified block diagram of an exemplary circuit utilizing a plurality of output drivers having different voltage levels to drive a plurality of receivers; 
     FIG. 2 illustrates a simplified schematic diagram of an embodiment of a mixed voltage output driver constructed according to the principles disclosed by the present invention; and 
     FIG. 3 illustrates a simplified schematic diagram of a second embodiment of a mixed voltage output driver constructed according to the principles disclosed by the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference now to the figures, and in particular, with reference to FIG. 1, there is depicted a high-level simplified block diagram of an exemplary circuit  100  utilizing a plurality of output drivers having different voltage levels to drive a plurality of receivers. Circuit  100  includes first, second and third output drivers  110 ,  120 ,  130  that are implemented with different integrated circuit (IC) technologies to drive a plurality of receivers, designated  150   a-   150   e.  For example, first output driver  110  may drive a transmission line of an interconnection  140  to an uplevel of 1.5V, whereas second and third output drivers  120 ,  130  may drive interconnection  140  to an uplevel of 2.0V. It should be noted that each node in circuit  100  may be a driver, receiver or both. 
     As discussed previously, problems may arise when driving interconnection  140  from a “higher high” to a “lower high,” if the second driver&#39;s impedance is too low. For example, after second output driver  120  has driven interconnection  140  to an 2.0V uplevel, if the impedance of first output driver  110  that is trying to establish the 1.5V uplevel is too low, the signal on interconnection  140  will significantly undershoot the 1.5V level enough to make a designated receiver erroneously switch, or at the minimum, reduce the signal margin at the designated receiver below safe limits. 
     Referring now to FIG. 2, there is illustrated a simplified schematic diagram of an embodiment of a mixed voltage output driver  200  constructed according to the principles disclosed by the present invention. Output driver  200  includes an input buffer  210 , an adjustable drive impedance circuit  220 , an impedance selection circuit  230  and an output sensing circuit  240 . The output driver  200 , in an advantageous embodiment, is embodied in an integrated circuit (IC). It is also assumed for the purposes of describing the operation of output driver  200  that the field effect transistors (FETs) utilized in the illustrated implementation can tolerate voltages only up to a supply voltage Vdd of 1.5V. 
     Input buffer  210  includes first and second inverters Inv 1 , Inv 2  that function as a high gain stage for a data signal received at an input terminal  250 . First and second inverters Inv 1 , Inv 2  are predrivers to adjustable drive impedance circuit  220 , specifically a first PFET P 1  and a second NFET N 2 , and also provide slew rate control (not shown) through conventional means well known in the art. 
     Adjustable drive impedance circuit  220  also includes a second PFET P 2  and a first NFET N 1 . In the illustrative embodiment, first and second NFETs N 1 , N 2  are conventional NFET pulldown driver devices. They are stacked, i.e., connected in series, with the top device&#39;s, i.e., first NFET N 1 , gate tied to a 1.5V supply voltage to reduce a drain to source voltage (Vds) stress. This arrangement is required because the output will be pulled above the implementation technology&#39;s normal supply voltage Vdd. It should be noted that the 1.5V supply voltage utilized in output driver  200  is an exemplary value and that other supply voltages may also be employed. Those skilled in the art should readily recognize that the supply voltage is dependent on the technology used to implement the output driver. Since the field effect transistor (FET) devices can only tolerate 1.5V, first NFET N 1  is used to divide the voltage stress across first and second NFETs N 1 , N 2 . Second NFET N 2  cannot tolerate a 2.0V drain to source voltage (Vds) without suffering voltage breakdown or hot carrier problems. With first NFET N 1  configured as a “source follower” with its gate tied to 1.5V, its source terminal will not get higher than a NFET threshold voltage (Vtn) of about 0.5V below 1.5V, or about 1.0V maximum. Thus, if the voltage at output terminal  260  is at 2.0V, there will be approximately 1.0V across first NFET&#39;s N 1  drain and source terminals and also approximately 1.0V across second NFET&#39;s N 2  drain and source terminals. It should also be noted first and second PFETs P 1 , P 2  do not have this problem. Since their source terminals are tied to 1.5V and when the output is at 2.0V, there will only be 0.5V across their drain and source terminals. Furthermore, if second NFET N 2  is implemented with a device that can tolerate higher voltages, first NFET N 1  would not be required. 
     In the illustrated embodiment, the impedance of first and second NFETs N 1 , N 2 , along with any series resistance between them and an output terminal  260 , is assumed to be 20 ohms. Of course, sizing first and second NFETs N 1 , N 2  and adding additional resistors allows for any impedance value to be realized. First and second PFETs P 1 , P 2  are coupled in parallel and, for the purposes of the present discussion, the impedance of each (along with any associated resistances) will be 40 ohms. Thus, if first PFET P 1  is driving an interconnection, or net, at output terminal  260  by itself, the resulting drive impedance of output driver  200  will be 40 ohms. If both first and second PFETs P 1 , P 2  are actively pulling the output up, the resulting drive impedance of output driver  200  will be 20 ohms. It should also be emphasized again that the value of the drive impedance is selectable and can be set at any value by selecting the appropriate devices and resistances. 
     Output sensing circuit  240  includes a noninverting receiver comprising of third, fourth and fifth PFETs P 3 , P 4 , P 5  along with third, fourth and fifth NFETs N 3 , N 4 , N 5 . Third NFET N 3  is coupled to output terminal  260  and is employed to limit the voltage at a node D. Specifically, the voltage signal generated by the output sensing circuit  240  is provided to the gates of a sixth PFET P 6  and a sixth NFET N 6  that along with a seventh NFET N 7  make up a set and reset circuit. It should be noted that the design implementation shown in the illustrated embodiment assumes that all the FET devices can only tolerate a supply voltage of 1.5V. If the technology supports devices that can tolerate higher voltages, the noninverting receiver circuitry could be simplified or even eliminated. In this case, output terminal  260  can be directly connected to the gates of sixth PFET P 6  and sixth NFET N 6  directly. 
     Impedance selection circuit  230  also includes a latch comprising seventh and eighth PFETs P 7 , P 8  and eighth and ninth NFETs N 8 , N 9 . It should be readily apparent to those skilled in the art that the latch may also be set by other boolean equivalents, as would be obvious to those familiar with boolean implementations. The input, or DATA, signal received at input terminal  250 , along with the output of the latch, are provided as input signals to a gate NAND 1  that performs a Boolean logic function, i.e., NAND, to generate a control signal. The control signal is provided to impedance selection circuit  220 , specifically, to the gate of second PFET P 2 . The control signal is utilized to control the switching of second PFET P 2  that, in turn, changes the drive impedance of output driver  200 . 
     Also shown in the illustrated embodiment is a Tristate signal coupled to the gate of seventh NFET N 7 . As discussed earlier, when output driver  200  relinquishes its control of the bus, it asserts a tristate signal. Asserting the tristate signal also places output driver  200  is in a high impedance mode. For ease of explanation, the tristate control to first and second inverters Inv 1 , Inv 2  and gate NAND 1  are not depicted in the simplified schematic of the illustrated embodiment. However, it should be readily apparent to those skilled in the art that boolean implementations to implement the tristate condition are well known in the art. For example, in a more complete output driver  200  schematic, first inverter Inv 1  is a NAND gate, second inverter Inv 2  is a NOR gate and gate NAND 1  becomes a 3-way NAND. Thus, when a tristate signal is asserted, the outputs of first and second inverters Inv 1 , Inv 2  and gate NAND 1  are high, low and high, respectively, independent of the Data signal at input terminal  250 . Consequently, first and second PFETs P 1 , P 2  and first and second NFETs N 1 , N 2  are nonconducting and output driver  200  appears as an open circuit. 
     The operation of output driver  200  will hereinafter be described in greater detail with respect to the following conditions: (1) the bus is “low” and output driver  200  is enabled to drive “high,” (2) output driver  200  has driven the bus “high” and is still enabled, (3) output driver  200  gives up the bus to another driver, (4) output driver  200  regains bus ownership and drives the bus “high” and (5) output driver  200  drives the bus low. 
     1. With the voltage at output terminal  260  at logical low, i.e., at ground voltage, node D is low that, in turn, drives node E high. The high signal at the gates of fourth PFET P 4  and fourth NFET N 4  turns on turns on both devices driving node X low. The low signal, i.e., voltage signal, turns on sixth PFET P 6 , driving the output of the latch to a logical high at node H. A logical high “data” input at input terminal  250  switches node A low through first inverter Inv 1  and also enables the output of gate NAND 1 , i.e., node B, low. With their gates pulled low, first and second PFETs P 1 , P 2  are turned on. The paralleled impedance of first and second PFETs P 1 , P 2  is significantly lower than the impedance of the bus coupled to output terminal  260 . This is necessary if there are several loads, e.g., receivers, on the bus. First and second PFETs P 1 , P 2  will pull the voltage on output terminal  260  (along with the rest of the bus) to Vdd, i.e., 1.5V. It should be noted that the Tristate signal at the gate of seventh NFET N 7  is low, i.e., ground, since output driver  200  is enabled when it is driving the net. This signal keeps the stack of sixth and seventh NFETs N 6 , N 7  from driving node H low as soon as the voltage at output terminal  260  goes above the logical switch point of noninverting receiver  240 . Output driver  200  is required to have a lower drive impedance until the net becomes fully charged. 
     2. Output driver  200  has just driven the bus high with a lower drive impedance, i.e., 20 ohms. Additionally, since the Tristate signal has not gone high, the stack of sixth and seventh NFETs N 6 , N 7  has yet to conduct. Node H remains high and both first inverter Inv 1  and gate NAND 1  pass the inverted data signal at input terminal  250  to the gates of first and second PFETs P 1 , P 2 . Consequently, both first and second PFETs P 1 , P 2  are turned on and the bus is held high with a 20 ohms drive impedance. 
     3. Output driver  200  is now tristated, relinquishing control of the bus to another driver. With the voltage at output terminal  260  high due to another driver on the bus, node X is high. Along with the Tristate signal being high, the stack of sixth and seventh NFETs N 6 , N 7  can now conduct pulling node H low and setting the latch. 
     4. Output driver  200  regains ownership of the bus and drives its output terminal  260  high. Since node H is now low, the output of gate NAND 1  is driven low that, in turn, keeps second PFET P 2  turned off. Node A is pulled low by the Data signal at input terminal  250 , turning on first PFET P 1 . Output driver  200  drives output terminal to a 1.5V high, but only with a drive impedance of 40 ohms. 
     5. Output driver  200  drives the bus low. Since the stack of first and second NFETs N 1 , N 2  is designed to drive the net with a 20 ohms, output terminal  260  and the bus is driven low with a 20 ohms drive impedance. Consequently, with output terminal  260  low, noninverting receiver  240  drives node X low, turning off sixth PFET P 6  and sixth NFET N 6 . This results in node H being driven high. It should be noted that it did not really matter if output driver  200  or another driver on the net drove the bus low, node H will be driven high in either case. 
     Referring now to FIG. 3, there is depicted a simplified schematic diagram of a second embodiment of a mixed voltage output driver  300  constructed according to the principles disclosed by the present invention. Output driver  300  includes an input buffer  310  that is coupled to an input terminal  350  and an adjustable drive impedance circuit  320 . Input buffer  310  and adjustable drive impedance circuit  320  are analogous in function and construction to input buffer  210  and adjustable drive impedance circuit  220  illustrated in FIG.  2 . Output driver  300  also includes an output voltage sensing circuit  340  coupled to an output terminal  360  and an selection circuit  330 . 
     A third PFET P 3  in output sensing circuit  340  is coupled to output terminal  360  and is preferably, in an advantageous embodiment, a low-threshold-voltage device. Additionally, if third PFET P 3  is implemented in a silicon on insulator (SOI) technology, in a preferred embodiment, its body is connected to a supply voltage Vdd, i.e., 1.5V in the illustrated embodiment, in order to have its threshold values be most predictable. Third PFET P 3  is utilized to turn on when an output voltage at output terminal  360  rises above Vdd. In the event that the output voltage at output terminal  360  is greater than Vdd, a voltage level at node G, i.e., a voltage signal, will be pulled sufficiently high enough to cause an inverter created by fifth NFET N 5  and fifth PFET P 5  in impedance selection circuit  330  to go low. This will, in turn, enable a sixth PFET P 6 , a weak half-latch device in impedance selection circuit  330 , to pull the voltage at node G to the Vdd rail and hold it there, even if the output voltage falls to 1.5V and third PFET P 3  stops conducting. When the voltage at node H is low, a gate NAND 1  will produce a high enough signal at its output, i.e., node B, turning off second PFET P 2 . The output of gate NAND 1  is a control signal provided to adjustable drive impedance circuit  320  to modify the drive impedance of output driver  300 . 
     Node H will continue to remain low until such time as when output driver  300  or another driver on the net drives the voltage at output terminal  360  low enough to reset the half-latch sixth PFET P 6 . Third, fourth and seventh NFETs N 3 , N 4 , N 7  in the output voltage sensing circuit  340  ensure that the voltage at node G is at a low  35  level when the voltage at output terminal  360  is low. Third NFET N 3  is sized and selected to be strong enough to reset the half-latch defined by sixth PFET P 6 . It should be noted that seventh NFET N 7  operates to limit the voltage on the gates of fourth PFET P 4  and fourth NFET N 4  devices. Seventh PFET P 7  is also configured to operate as a half-latch that pulls a node D to the 1.5V rail and is a “weak” device relative to seventh NFET N 7 . 
     Third PFET P 3 , which is utilized to sense the voltage at output terminal  360 , conducts, i.e., turns on, when the output voltage is sufficiently higher than Vdd to turn it on. PFET devices&#39; threshold voltages are typically in the order of 200 mV and low-voltage PFET devices&#39; threshold voltages are generally about 60 mV less, or about 140 mV. Alternatively, in other advantageous embodiments, alternative means may be used instead of third PFET P 3  to accomplish generating the current needed to drive node G high enough to set the half-latch defined by sixth PFET P 6 . Although not shown or described, it will be necessary to utilize conventional means to prevent first and second PFETs P 1 , P 2  from conducting backwards into supply voltage Vdd when the voltage on output terminal  360  rises above Vdd. As shown in the illustrated embodiment, first and second PFETs P 1 , P 2  will conduct whenever the voltage on output terminal  360  rises above their threshold voltage above Vdd. Conventional techniques, e.g., a technique disclosed in U.S. Pat. No. 5,151,619, issued to Austin, et al., which is herein incorporated in its entirety by reference, may be advantageously employed to eliminate the problem described above. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.