1. An apparatus, comprising: a first input/output (I/O) pad; a second I/O pad; a programmable output circuit coupled to the first I/O pad and the second I/O pad, the programmable output circuit configurable to drive an output signal from a processing system to the first I/O pad at a plurality of voltages, wherein at least one of the plurality of voltages is based on a voltage supplied by an external circuit coupled to the apparatus, wherein the programmable output circuit comprises a programmable output buffer configured to receive a first programmable reference voltage signal as an output high level reference, wherein the first programmable reference voltage signal is received from the external circuit via the second I/O pad; and a programmable input circuit coupled to the I/O pad, the programmable input circuit configurable to detect an input signal to the processing system from the first I/O pad at the plurality of voltages.