What is the purpose of the VAUX pin on a CoolRunner-II device?
The VAUX pin powers the buffers to the JTAG pins (TCK, TMS, TDI, and TDO). The voltage limits to the VAUX pin are described in the data sheet under "Recommended Operating Conditions." The CoolRunner-II device data sheets are available at:
The VAUX pin must always be powered, even if the JTAG pins are not being used. The TCK, TDI and TMS pins have weak pull-ups connected to VAUX; therefore, if the VAUX pin is not powered, these pins could be left floating and the TAP controller may change state.