Fail safe bias system for a tri-state bus
OwnerMinhui Wang, Dean A. Wallaceinventors
Life of this patent
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Abstract
A Norton equivalent implementation of a fail safe bias circuit drives a bus to provide a stable bias voltage which is not affected by an output impedance of a current source. The Norton equivalent implementation has first current source, a bias resistor and a current sink in series. The bias resistor is connected across a pair of differential bus lines so that the current flowing through the bias resistor causes a bias voltage to be generated between the bus lines. The current to the bias resistor is selectively switched on and off to control the bias voltage so that the bias voltage can be turned off during high speed data transfers on the bus.
Claims42
A fail safe bias circuit for selectively providing a bias voltage to a bus having a first signal line and a second signal line, said bias circuit comprising:
- at least one resistor connected between said first signal line and said second signal line;
- a current source;
- a first switch interposed between said current source and said first signal line;
- a current sink;
- and
- a second switch connected between said current sink and said second signal line, said first switch and said second switch operable in respective first states to enable current to flow from said current source via said at least one resistor to said current sink to generate a bias voltage between said first signal line and said second signal line and operable in respective second states to interrupt current flow from said current source through said at least one resistor.
A fail safe bias circuit which generates a desired bias voltage between a pair of differential signal lines of a bus, said fail safe bias circuit comprising:
- at lease one resistance connected between the two signal lines;
- a current source which generates a current having a magnitude selected to be substantially equal to the desired bias voltage divided by the at least one resistance;
- a current sink which receives the current from said current source via said at icast one resistance;
- a first switch disposed between said current source and said at least one resistance;
- and
- a second switch disposed between said current sink and said at least one resistance, the first and second switches switching between an on state in which current flows from said current source to said current sink through said at least one resistance and an off state in which said current source and said current sink are disconnected from said at least one resistance, said first switch and said second switch changing from said on state to said off state and from said off state to said on state in response to a control signal.
An apparatus for selectively providing a bias voltage to a bus having a first signal line and a second signal line, said apparatus comprising:
- at least a first switch interposed between a current source and a first signal line;
- and
- at least a second switch connected between a current sink and a second signal line, wherein said first and second switches are selectively switched to generate a bias voltage between said first signal line and said second signal line.
A method of selectively providing a bias voltage to a bus comprising the acts of:
- selectively switching a current source to a first signal line;
- selectively switching a current sink to a second signal line, wherein said first and second signal lines operate in at least two transfer modes;
- and
- controlling said acts of selectively switching to generate said bias voltage between said first and said second signal lines when no signals exist on said first and second signal lines.
A fail safe bias circuit which generates a desired bias voltage between a pair of differential signal lines of a bus, said fail safe bias circuit comprising:
- at least one resistance connected between the two signal lines;
- a current source which generates a current having a magnitude selected to be substantially equal to the desired bias voltage divided by the at least one resistance;
- a current sink which receives the current from said current source via said at least one resistance;
- a first switch disposed between said current source and said at least one resistance;
- and
- a second switch disposed between said current sink and said at least one resistance, the first and second switches switching between an on state in which current flows from said current source to said current sink through said at least one resistance and an off state in which said current source and said current sink are disconnected from said at least one resistance.
Dependencies are inferred from the claim text.