Dual use of an integrated circuit pin and the switching of signals at said pin

A signal processing system comprising:

• means for providing a first signal intrinsic to an integrated circuit;
• means for providing a first operational function to the first signal at a pin of the integrated circuit,
• means for providing a second operational function to the first signal by the integrated circuit, and
• means for coupling a second signal extrinsic of the integrated circuit to the pin of the integrated circuit so that the means for providing the second operational function operates on the second signal,
• the second signal being switchable “in” and “out”, and when switched “in” the first signal is switched “off” at the pin.

A signal processing system comprising:

• means for providing a first signal intrinsic to an integrated circuit;
• means for providing a first operational function to the first signal at a pin of the integrated circuit,
• means for providing a second operational function for the first signal by the integrated circuit, and
• means for coupling a second signal extrinsic of the integrated circuit at the pin so that the means for providing the second operational function operates on the second signal,
• the pin serving as a bi-directional conduit for signals,
• the second signal being switchable “in” and “out”, and when switched “in”, the first signal is switched “off” at the pin.

A circuit for switchably coupling a signal comprising:

• first and second transistors each having respective base/gate electrode, emitter/source and collector/drain electrodes, the two transistors being of the same conductivity type and connected in series between a voltage supply and a reference point,
• in a signal coupling mode, the signal output coupling being from the first emitter/source electrode, the second collector/drain electrode being coupled to the first emitter/source electrode through a resistor, and
• the first base/gate having bias means and receiving the input signal with the bias means placing the first transistor in a linear signal transmission state, and the output signal being coupled out by the first emitter/source, the bias means also biasing the second base/gate electrode so that the second transistor is in a saturated state,
• the circuit being switchable from the signal coupling mode to a non-signal coupling mode by the bias means placing the first transistor in a cutoff state so that the first transistor is rendered non-coupling for the signal, the bias means also placing the second transistor in a cutoff state so that via action of the second collector/drain electrode, the first emitter/source electrode is switched to be at a high impedance with respect to the reference point thus removing impedance loading by the first emitter/source electrode.