A custom designed on-chip event debugger (OCeD) has been proposed for the automation of the user-defined code flow detection. Conventional on-chip code debugging is performed by controlling the instruction execution with line-by-line break- points. An event-driven breakpoint concept is adopted to trace the collection of multiple triggered breakpoints using a built-in hardware-in-the-loop methodology. The tracer unit in the OCeD saves abstract trees of code access flow of interest in real time to search multiple conditions as an event breakpoint. The OCeD unit acquires the system control in detecting the user-defined event as multiple matched breakpoints through runtime execution of an entire firmware code and reports the traced flow graph to the host side. The OCD-to-event translation converter is inserted between the original OCD and the debugger software to control the code debugging flow. The proposed OCeD hardware is integrated in the 8051-based microcontroller and evaluated with FPGA-based target system. The event-breakpoint description for Dhrystone ^TM benchmark enables that the verification of the code execution flow to be fully automated without any conventional line breakpoint.