Interface system for bus line control
First Claim
1. An interface system comprising:
- a bus means including at least one bus line;
at least one bus driver means for driving each of said bus lines to a selected one of two logic voltages thereby generating a predetermined logic state on said bus means when said bus driver means is actuated and for isolating each of said bus lines from each of said logic voltages by a high impedance when said bus driver means is inactivated; and
a default state latch means coupled to said bus means for continuously sensing the voltages on each of said bus lines and for automatically maintaining the last logic voltage on each bus line when all of said bus driver means are inactuated without external control, thereby preserving said predetermined logic state on said bus means.
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Abstract
An external I/O pull down latch and a system embodiment thereof. The interface system has a bus line including of a first means for providing an output at a fixed voltage level on said bus line for a first time interval, and a second means coupled to said bus line for maintaining the bus line at the fixed voltage level subsequent to the first time interval. In the preferred embodiment, the second means is comprised of said external I/O pull down latch. The present invention is a replacement for traditional pull up or pull down resistors in controlling I/O bus lines coupling main processor circuits with external memory circuits. In the preferred embodiment, the external I/O pull down latch is comprised of a read-write memory bit cell, sized such that it may be overdriven by any driver attached to the bus line.
64 Citations
11 Claims
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1. An interface system comprising:
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a bus means including at least one bus line; at least one bus driver means for driving each of said bus lines to a selected one of two logic voltages thereby generating a predetermined logic state on said bus means when said bus driver means is actuated and for isolating each of said bus lines from each of said logic voltages by a high impedance when said bus driver means is inactivated; and a default state latch means coupled to said bus means for continuously sensing the voltages on each of said bus lines and for automatically maintaining the last logic voltage on each bus line when all of said bus driver means are inactuated without external control, thereby preserving said predetermined logic state on said bus means. - View Dependent Claims (2, 3)
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4. An interface system comprising:
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a bus means including at least one bus line; a first integrated circuit coupled to said bus means having (a) a bus driver means for driving each bus line to a selected one of two logic voltages thereby generating a predetermined logic state on said bus means when said bus driver means is actuated and for isolating each of said bus lines from each of said logic voltages by a high impedance when said bus driver means is inactuated, and (b) a default latch means for continuously sensing the voltages on each of said bus lines and for automatically maintaining the last logic voltage on each bus line when said bus driver means is inactuated without external control, thereby preserving said predetermined logic state on said bus means; and at least one second integrated circuit coupled to said bus means having a bus responsive means for sensing the predetermined logic state of said bus means and generating a control signal corresponding to said sensed predetermined logic state. - View Dependent Claims (5, 6, 7, 8)
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9. An interface circuit comprising:
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a bus coupling means for coupling to a bus having at least one bus line; a bus driver means coupled to said bus coupling means for driving each of said bus lines to a selected one of two logic voltages thereby generating a predetermined logic state on said bus when said bus driver means is actuated and for isolating each bus line from each of said logic voltages by a high impedance when said bus driver means is inactuated; and a default state latch means coupled to said bus coupling means for continuously sensing the voltages on each of said bus lines and for automatically maintaining the last logic voltage on each bus line when said bus driver is inactuated without, external control, thereby preserving said predetermined logic state on said bus. - View Dependent Claims (10, 11)
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Specification