Passing internal bus data external to a completed system

Abstract

An apparatus and technique to allow internal bus activity of a system on a chip to be monitored external to the integrated circuit, but without requiring additional external pins. A snooping pass through device on the internal bus, e.g., a snooping external memory interface (EMI) includes operability to directly pass through activity on the internal bus to the external memory bus. One or more snoop cycles are inserted into a memory access of an internal bus of a system on a chip. The snooping pass through device preferably includes an external bus already having pins routed external to the integrated circuit. The external bus leading from the snooping pass through device (e.g., from the EMI) may be multiplexed for use both for its otherwise conventional function while not in a snoop cycle, and for use in directly observing activity on the internal bus when during a snoop cycle. Additional signals may be multiplexed into the EMI for pass through during snoop cycles. One or more snoop cycles may be inserted under software and/or hardware control. Moreover, snooping functionality may be used to replace idle cycles in a memory access operation. Virtually any internal signal within a system on a chip may be passed through a snooping pass through device during a snoop cycle, allowing step-by-step cycle sensitive debugging by system designers.