Abstract: On-chip debug is essential for design and development of embedded systems and software.UART based in-circuit debug system is designed for our PKU-DSP II system-on-a-chip.With the assist of debug data channels and pipeline control unit within the processor,the on-chip debug module can support functions such as hardware/software breakpoint,single-stepping,system reset as well as online program downloading and debug software with GUI is developed for host computer.Accurate pipeline control technique and proper handling of pipeline mis-suspending provide the debug system the advantage of zero timing overhead.The FPGA prototyping results show that the proposed design is correct and the hardware cost increases by no more than 3% without reducing the performance of the system.