Electronic digital logic circuitry – Tri-state – With field-effect transistor
Reexamination Certificate
1998-04-30
2001-01-23
Santamauro, Jon (Department: 2819)
Electronic digital logic circuitry
Tri-state
With field-effect transistor
C326S086000, C326S037000
Reexamination Certificate
active
06177808
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the integration of bidirectional switches with programmable logic to solve electronic signaling problems.
2. Description of the Related Art
Design defects are often encountered during development of electronic systems. When production schedules do not permit fixing the defects in the errant devices, or when the problem source cannot be isolated, programmable logic can be employed to both work around and identify the problems caused by the defective devices.
A programmable logic device (PLD) is a circuit which can be configured by the customer, to perform needed logic functions. Most standard PLDs consist of an AND array followed by an OR array, both of which may be programmable. The inputs are coupled to the AND array, which performs the AND functions and generates product terms. The product terms are coupled to the OR array. The OR array combines (sums) the outputs of the various product terms to produce the desired outputs. Programmable array logic (PAL) is a PLD that has a programmable AND array followed by a fixed OR array. Programmable logic and the basics of programming it are well known to those with ordinary skill in the art. The term programmable logic will be used for PLDs, PALs, and related logic such as gate arrays and programmable state machines. Programmable logic can be used for tracking bus states, configuring dynamically alterable state machines and a variety of other tasks.
Programmable logic can be used to decode errant signals from digital devices. Programmable logic has been used to work around defects in devices in some systems by using programmable logic that supports bidirectional signals. The signal from the errant device would be passed through transceivers in the programmable logic and on to the remainder of the system. In normal operation, the programmable logic would control the direction of the transceivers as appropriate, based on other signals in the system. To isolate the errant signal from the rest of the system, the transceivers could be disabled, and then the programmable logic would directly drive the signal to the system.
This technique, however, required the transceiver direction to be controlled by the programmable logic. This could become quite complicated for some signals.
Further, as clock speeds increased, timing problems also arose. Using multiple discrete devices could result in stub length and termination issues that in turn caused signal degradation and timing problems. Typical device isolation and debugging solutions often did not avoid these issues.
If an errant device was disconnected from the signal path problems associated with signal reflection could occur. Reflections from improperly terminated stubs could cause abnormal conditions within a system. As signal frequencies increased the possibility of reflections increased and the need for proper termination became important.
SUMMARY OF THE PRESENT INVENTION
A system according to the invention provides a bidirectional switch, such as a FET switch or a transmission gate, as an integral part of a programmable logic device. The switch is controlled by an enable signal in the programmable logic itself. By employing the onboard switch, a signal from a device can be isolated without requiring directional signal control and with a single part—the programmable logic device itself.
In one embodiment, the programmable logic device decodes the signal from the errant device on the signal line and provides the correct signal for the time in which the errant device is improperly functioning.
In another embodiment, bidirectional switches are again used to isolate a signal, but at the same time maintaining appropriate signal line termination. When a signal line to be isolated is split in two by a bidirectional switch, two other switches simultaneously switch in terminations, such as pull up resistors, to the new “ends” of each of the two segments. In still another embodiment, one bidirectional switch terminates the stub connected to the errant device and another bidirectional switch connects the other stub to the programmable logic for providing the correct signal.
Bidirectional switches are high-speed signal line connect devices having low impedance transmission gates which connect two signals. Unlike transceivers, the transmission gates allow current to flow in either direction without directional control. A signal driven by the programmable logic controls the bidirectional switch to isolate the errant transmitting device from the receiving device and drive the line with the appropriate logic level for the appropriate period. The new technique preferably implements the programmable logic and the bidirectional switch on a monolithic integrated circuit. Integration achieves a significant advantage by reducing the time required for the programmable logic to respond to a signal and then turn on or off an individual bidirectional switch element. Further, trace lengths are reduced.
Bidirectional switches provide an advantage over tristateable buffers, or transceivers, because the buffers must be continually controlled when the signal line is bidirectional. The logic to control a tristateable buffer or transceiver is generally more complex than can easily be modeled in a standard programmable logic chip.
REFERENCES:
patent: Re. 34363 (1993-08-01), Freeman
patent: 4713557 (1987-12-01), Carter
patent: 4758985 (1988-07-01), Carter
patent: 4835418 (1989-05-01), Hsieh
patent: 4987319 (1991-01-01), Kawana
patent: 5705937 (1998-01-01), Yap
patent: 5736870 (1998-04-01), Greason et al.
Basile Barry S.
Heinrich David F.
Tavallaei Saimak
Akin Gump Strauss Hauer & Feld & LLP
Compaq Computer Corporation
Le Don Pau
Santamauro Jon
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