This paper presents a technique of re-using DFT logic for system functional and silicon debugging. By re-configuring the existing DFT logic implemented on an ASIC, we are able to 1) test each part of an ASIC in a system environment separately and thus locate manufacturing defects, 2) control and observe any state elements of an ASIC to facilitate system function and silicon debugging, and 3) use structural tests to cover device and their interconnect tests on a board. Therefore, we can achieve debugging and test at both device level and system board level.