Apparatus and methods are described for a background microcontroller debugger. A method to debug a microcontroller includes sending a three level signal from a debug module, receiving the three level signal at a single pin on an MCU, sending a second three level signal from the single pin on the MCU, and receiving the second three level signal at the debug module. An apparatus to debug a microcontroller includes a tri-statable pad driver to transmit a three level signal, a reference voltage divider coupled to the tri-statable pad diver, a plurality of voltage comparators to receive the three level signal, a resistive voltage divider to maintain thresholds for the plurality of voltage comparators, and a plurality of logic elements coupled to the plurality of voltage comparators to receive the three level signal.