The paper discusses debugging methods, that is, identifying and correcting errors in projects of digital systems specified by the hardware diagram and the text of software or firmware. Debugging is considered on a fabricated layout of hardware and on a software model of a digital system, as well as on a combination of the layout of individual blocks and a software model of the rest of the system. The methods of organizing debugging modes for all considered methods are analyzed. It is indicated that for digital systems on a chip, the most effective method is to debug projects using a software model. Keywords: design of digital systems, design debugging, organization of debugging modes