US Patent 7197680 Communication interface for diagnostic circuits of an integrated circuit

An integrated circuit including diagnostic circuitry having serial scan chains or debug bus access circuits for establishing communication using an interface circuit coupled with a bi-directional serial link to an external diagnostic device. The bi-directional serial link carries both data and control signals. The serial protocol provides for a pacing signal for indicating to the external diagnostic device when it is ready to receive more data and/or when it has completed a particular diagnostic operation. This provides a self-pacing ability. A training signal generated by the external diagnostic device is detected by the interface circuit on initialization and used to derive sampling point timings.