Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
1997-11-10
2001-01-30
Santamauro, Jon (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S086000, C326S033000
Reexamination Certificate
active
06181157
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to terminating electrical transmission lines. In particular, the present invention relates to terminating transmission lines with a matched resistance.
In communication systems, and within complex electronics systems, electrical signals are transmitted from one location to another over transmission lines. Every transmission line has an inherent impedance that affects the electrical signal. To reduce reflectance of the signal, it is known to match the impedance of the transmission line with a resistance at both the signal source and the signal destination. This resistance is formed by a termination circuit that is connected between the transmission line and a fixed voltage, such as ground or a power supply.
The location of the termination circuit, relative to the transmitter that creates the signal, or the receiver that receives the signal, is critical to properly preventing reflected signals. In particular, the termination circuit should be placed as close as possible to the transmitter or receiver.
When transmitters and receivers are located in an integrated circuit, locating the termination circuit near the transmitter or receiver becomes difficult. It is difficult to place the termination circuit in the integrated circuit itself because resistive materials deposited on an integrated circuit tend to have a large temperature coefficient that causes the resistance of the material to vary widely as the temperature of the circuit changes. Because of these variations, the termination circuit will not always match the impedance of the transmission line.
Alternatively, termination circuits can be placed in a package in which the integrated circuit is seated for connection to the circuit board. However, placing the resistance in the package makes the package “integrated circuit specific”, and thus increases the cost of manufacturing.
A termination circuit that can provide a stable resistance and is contained within the integrated circuit is desirable.
SUMMARY OF THE INVENTION
A circuit that provides a termination resistance to a transmission line includes a controllable termination resistor coupled between the transmission line and a termination voltage node. The circuit also includes a control circuit coupled to the controllable termination resistor and to a reference resistor. The control circuit matches the controllable termination resistor to the reference resistor.
In several embodiments of the present invention, the control circuit includes at least one controllable reference resistor. Preferably, the controllable reference resistor and the controllable termination resistor each include transistors operating in the triode region such that a change in the voltage at their gates causes a change in their resistance.
To control the resistance of the controllable termination resistor and the controllable reference resistor, the control circuit includes a feedback loop capable of controlling a voltage at a control node connected to the controllable reference resistor and the controllable termination resistor. In preferred embodiments, the control node is connected to the gates of the transistors in the controllable reference resistor and the controllable termination resistor. The voltage at the control node controls the resistance of the controllable reference resistor and the controllable termination resistor. Additional controllable termination resistors may be connected to the control node to act as termination resistances for any number of transmission lines.
Preferably, the control circuit draws current through both the reference resistor and the controllable reference resistor to develop two voltages that are applied to a differential amplifier. In some embodiments, the reference resistor and the controllable reference resistor are in series with respective additional resistances, and currents are drawn through the respective series of elements to develop the voltages at the inputs of the differential amplifier. The output of the differential amplifier is used to control the voltage at the control node and thus to control the resistance of the controllable reference resistor and any controllable termination resistors.
Operation of the control loop is as follows. If the resistance of the controllable reference resistor is too large, the differential amplifier creates an output causing the voltage at the control node to increase thereby decreasing the resistance of the controllable reference resistor. If the resistance of the controllable reference resistor is too small, the differential amplifier creates an output causing the voltage at the control node to decrease, thereby increasing the resistance of the controllable reference resistor. In this manner, the resistance of the controllable reference resistor, as well as of any other controllable termination resistors connected to the control node, are changed until their respective resistance, plus the resistance of any devices in series with the resistors, are together equal to the resistance of the reference resistor.
Thus, the present invention provides a stable resistance within an integrated circuit based on a stable reference resistance that can be maintained external to the integrated circuit.
REFERENCES:
patent: 5506528 (1996-04-01), Cao et al.
patent: 5793223 (1998-08-01), Frankeny
patent: 5821768 (1998-08-01), Rau
Le Don Phu
LSI Logic Corporation
Santamauro Jon
Westman Champlin & Kelly P.A.
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