Method of using shadow registers in designing an on-chip debug unit of a microprocessor

A method to enhance non-intrusive debug capability in microprocessor design is presented. This method works by introducing shadow registers in the debug circuitry. By using the registers, designers no longer need to insert scan chain in the critical path of CPU. Also, using shadow registers to assist debug is extensible when the CPU micro-architecture is becoming more and more complex. In this paper, we present the method of inserting shadow registers and special datapath in the CPU debug module, and illustrate the steps of using these registers during chip debug. This method has also been implemented in our CK520 core and is proved to be successful.