A digital system provides a serial bus diagnostic port such as, for example, a universal serial bus (USB) diagnostic port. The USB diagnostic port can include a USB slave device operable to interface to a universal serial bus, a flash memory controller interface coupled to the USB slave device and operable to interface to a flash memory controller, and a test access port controller coupled to the USB slave device and operable to interface to a test access port. The diagnostic port can further include a system interface coupled to the USB slave device and operable to interface to an integrated controller independently of an operating system. Via the diagnostic port, a USB testing tool with testing software can access and debug internal logic of the digital system. As an alternative to a USB diagnostic port, the serial bus diagnostic port can be implemented in a variety of wired or wireless implementations such as, for example, an I.E.E.E. 1394 diagnostic port, an Ethernet diagnostic port, or a wireless Internet diagnostic port. Further, the serial bus diagnostic port can, for example, be used for software development, in-factory testing or in-field diagnostics of the digital system.