I just purchased the C51 development kit, with uVision V4.20a. I had an older version that came with the Cypress EZUSB-FX development kit. The following code, which disconnects and then reconnects from the USB, used to work with the old debugger.
USBCS |= bmDISCON; EZUSB_Delay(1000); USBCS &= ~bmDISCON;
USBCS &= ~bmDISCON;