Electronic digital logic circuitry – Interface – Supply voltage level shifting
Reexamination Certificate
1999-11-04
2001-05-29
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Supply voltage level shifting
C326S030000, C326S087000, C326S086000
Reexamination Certificate
active
06239617
ABSTRACT:
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'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.
REFERENCES:
patent: 5151619 (1992-09-01), Austin et al.
patent: 5306965 (1994-04-01), Asprey
patent: 5559447 (1996-09-01), Rees
patent: 5565749 (1996-10-01), Porter
patent: 5825206 (1998-10-01), Krishnamurthy et al.
patent: 6130556 (2000-10-01), Schmitt et al.
Cahill Joseph James
Guertin David LeRoy
Williams Robert Russell
Young Daniel Guy
Bracewell & Patterson LLP
International Business Machines - Corporation
Le Don Phu
Tokar Michael
LandOfFree
Mixed voltage output driver with automatic impedance adjustment does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mixed voltage output driver with automatic impedance adjustment, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mixed voltage output driver with automatic impedance adjustment will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2481219