Transmitter configured for test signal injection to test AC-coupled interconnect

An integrated circuit (IC), comprising:

• a transmitter having a differential output configured for alternating current (AC)-coupling to interconnect;
• test logic configured to generate a test signal and a test enable signal;
• serial-to-parallel logic in the transmitter having a plurality of flip-flops and a plurality of multiplexing stages configured to serialize parallel data signals into a serial data signal based on a plurality of clock signals;
• a driver in the transmitter having a first input receiving the test signal, a second input receiving the enable signal, a third input receiving the serial data signal, and a plurality of current-mode logic (CML) stages, a CML stage of the plurality of CML stages comprising:
• a differential transistor pair configured to be biased by a current source, the differential transistor pair comprising an differential input coupled to receive the serial data signal and a differential output coupled to the interconnect;
• a resistor pair coupled to the differential output;
• a transistor pair coupled between the resistor pair and a voltage supply and receiving a gate voltage derived from the test enable signal;
• a bridge transistor coupled between the resistor pair and receiving a gate voltage derived from the test enable signal;
• and
• a pair of three-state circuit elements coupled to differential output through the resistor pair, the pair of three-state circuit elements receiving a differential input voltage derived from the test signal and a control voltage derived from the test enable signal.

A method of controlling a driver circuit in a transmitter for testing interconnect AC-coupled to the transmitter, the method comprising:

• generating a serial data signal from a plurality of parallel data signals using serial-to-parallel logic based on a plurality of clock signals;
• coupling the serial data signal to a differential transistor pair, which is coupled to a differential output of the driver circuit and is biased by a current source, for transmission over the interconnect;
• controlling a voltage applied between gates of the differential transistor pair to isolate the current source from the differential transistor pair;
• generating a differential test voltage between inputs of a pair of three-state circuit elements coupled to a node pair, the node pair coupled to the differential output of the driver circuit through a resistor pair;
• generating a control voltage to be coupled to control terminals of the three-state circuit elements, to gates of a transistor pair coupled between a voltage supply and the node pair, and a gate of a bridge transistor coupled between the node pair;
• and
• controlling the control voltage to enable the three-state circuit elements, to isolate the voltage supply from the node pair, and to isolate the node pair.