Pulse or digital communications – Cable systems and components
Reexamination Certificate
1997-09-22
2001-06-12
Pham, Chi (Department: 2631)
Pulse or digital communications
Cable systems and components
C327S247000, C327S067000
Reexamination Certificate
active
06246721
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to electronic circuits, and more particularly to circuits for the transmission of digital signals. Even more particularly, this invention relates to a scheme for terminating a clock network.
2. Background of the Invention
Modern digital electronic devices, such as computers, often have many different components that receive the same timing reference or clock signal. This makes communication between these components much simpler. One common clock distribution topology, used to reduce clock skew, and reduce cost, involves driving multiple transmission lines of equal length with a single device. At the end of each of these transmission lines is a receiving device, and a termination impedance. The termination impedance is matched to the intrinsic impedance of the transmission line to prevent, or reduce, reflections off the end of the transmission lines. This, in turn, reduces the amount of noise on a signal allowing the designer to specify smaller noise margins. Smaller noise margins allow signals to switch faster, improving the overall speed of the electronic device.
Unfortunately, this termination scheme suffers from several problems. First, each termination impedance at the end of a transmission line increases the DC current the driver must supply. This increases the power drawn by the electronic device. For example, if there are five 50 &OHgr; terminations connected between ground and the end of five transmission lines, the driver must supply 100 milliamps to maintain the signal at 1 volt above ground. For a clock signal with a 50% duty cycle, the driver would draw an average of 50 milliwatts. In contrast, if each of the transmission lines has a 100 picosecond delay, and is unterminated, the driver would only draw an average of 330 microwatt for a 33 megahertz clock signal.
Another problem is the termination impedance. For each receiving device, a termination impedance is needed. To ensure reflections are minimized, this impedance must be calibrated against process variations to closely match the intrinsic impedance of the transmission line. That means that for five receiving devices, five calibration circuits must be built, or five passive impedances must be laser trimmed. This increases the cost of building the electronic device.
Accordingly, there is a need in the art for an improved termination scheme for clock networks that does not draw significant DC current. This scheme should minimize the number of devices that need to be matched, or calibrated. Furthermore, this scheme should be tolerant of moderate variations in the length of the transmission lines delivering the clock signal. These needs, and others, are met by the present invention.
SUMMARY OF THE INVENTION
An electrical network according to the present invention consists of multiple components each connected to a single signal source in a star arrangement. The signal source is connected at the central hub of the star arrangement. Each component is connected to the central hub of the star arrangement via a transmission line. Each of the transmission lines connecting the components to the central hub has the same delay between the component and the hub. The signal source is supplied by a driver device in series with a source termination resistor. The combined impedance of the source termination resistor, and the driver is matched to the parallel combination of the intrinsic impedances of the transmission lines. This causes the waves reflected off the end of each transmission line to meet at the hub at approximately the same instant in time, and have approximately the same magnitude. Because the reflected waves arrive at the same time, and have the same magnitude, but are all traveling towards the hub, they cancel each other out as they arrive at the hub instead of continuing to propagate down, or reflect off of, the other transmission lines connected to the hub. Each component receives the signal from a transmission line through a receiver. The output of the receiver is fed back to the input of the receiver to clamp minor reflections, and to help hold the signal at a constant voltage in the presence of noise.
REFERENCES:
patent: 5276415 (1994-01-01), Lewandowski et al.
patent: 5495186 (1996-02-01), Kanazawa et al.
patent: 5784126 (1998-07-01), Boccacio
patent: 5828241 (1998-10-01), Sukegawa
Hollenbeck David B.
Zhang Johnny Q.
Hewlett--Packard Company
Neudeck Alexander J.
Pham Chi
Tran Khai
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