Abstract: An asynchronous debug interface is disclosed that allows TAG agents, JTAG-based debuggers, firmware, and software to debug, access, and override any functional registers, interrupt registers, power/clock gating enables, etc., of core logic being tested. The asynchronous debug interface works at a wide range of clock frequencies and allows read and write transactions to take place on a side channel, as well as within the on chip processor fabric without switching into a debug or test mode. The asynchronous debug interface works with two-wire and four-wire JTAG controller configurations, and is compliant with IEEE standards, such as 1149.1, 1149.7, etc., and provides an efficient and seamless way to debug complex system-on-chip states and system-on-chip products.