Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1999-03-18
2001-07-17
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C375S287000
Reexamination Certificate
active
06262602
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to logic analyzer probes and more particularly to a circuit and method for detecting incident edges on a source terminated transmission line.
BACKGROUND OF THE INVENTION
Several techniques exist for interfacing integrated circuits and other digital electronic devices with one another in high-performance, transmission line environments. Some of these use load termination to eliminate unwanted signal reflections that occur because of impedance mismatches along the transmission line. Standard interfaces such as GTL (Gunning Transceiver Logic) and PECL commonly use this kind of transmission. Another technique that offers some advantages over this accepted method of termination is the source-terminated driver method. A source-terminated line does not use load termination. Instead, a source-terminated line relies upon reflections off an unterminated end-of-line to reach a valid voltage level and then terminates the reflected waves back at the source.
Source-terminated lines and busses may be difficult to probe because the electrical signals can appear distorted if viewed or probed at a location other than the receiving end of the bus. This distortion is caused by the separation between the incident and reflected waves of logic transition traveling on the line. This distortion can also cause glitches and timing uncertainty in the detected version of the probed signal's logic transitions.
Accordingly, there is a need in the art for a probe circuit that can reliably detect the incident wave of a logic transition on a source-terminated bus. By detecting the incident wave and ignoring any reflections, the timing of the logic transition can be measured with greater certainty than detecting both the incident wave and possibly some reflected waves. Furthermore, detecting just the incident wave prevents reflected waves from possibly being detected as glitches on the line.
SUMMARY OF THE INVENTION
A preferred embodiment of the invention provides a means of detecting the state of a source terminated line when not probing the line at the receiving end. The invention may be tuned to different logic levels, propagation delays, and is capable of separating minor reflections from true line glitches.
The present invention includes two comparators. One comparator detects rising transitions of the input waveform and one comparator detects falling transitions. Each comparator detects their respective transition with a different threshold voltage. The outputs of these comparators are multiplexed into the clock input of a flip-flop. The flip-flop's inverted output is connected through a time delay to the input of the flip-flop to form a toggling configuration. The output of the time delay is also connected to the select input of a multiplexer to control the multiplexer to multiplex the outputs of the two comparators into the clock input of the flip-flop. In a preferred embodiment the threshold voltages chosen for the two comparators are chosen to be in the center of the incident edges of the distorted signal. The time delay is chosen to be longer than the difference between the arrival of the incident wave and the arrival of the first reflected wave.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
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patent: 5336947 (1994-08-01), Lehning
patent: 5465059 (1995-11-01), Pan et al.
patent: 5544175 (1996-08-01), Posse
patent: 5933459 (1999-08-01), Saunders et al.
Agilent Technologie,s Inc.
Lam Tuan T.
Neudeck Alexander J.
Nguyen Hai L.
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