Question: The JTAG interface does not respond on a self-powered FX3 board unless the upstream connection is made. It works well on a bus-powered board.
The JTAG interface of FX3 does not work if the VBATT pin is not connected to a power supply. Although the JTAG interface is powered by the VIO5 supply domain, the internal architecture of the chip requires that the VBATT pin also be supplied with a valid power source.The underlying requirement here is that the USB domain of FX3 be powered. In bus-powered designs, it will be powered via VBUS; in self-powered designs, it needs to be powered via VBATT.