Question: Can the 8051 dynamically decide who handles the standard USB requests or is this set on power-up?
Answer: An EZ-USB control bit called ‘RENUM’ (ReNumerated) determines whether USB device requests over endpoint zero are handled by the Default USB Device or by EZ-USB firmware. At power-on reset, the RENUM bit (USBCS.1) is zero, indicating that the Default USB Device automatically handles USB device requests. Once firmware has been downloaded to the EZ-USB and the CPU is running, it can set RENUM=1 so that subsequent device requests(except Set-Address) are handled by the downloaded firmware and descriptor tables.
