"Dan K" <[email protected]> wrote in message
news:K5%Di.100073$[email protected] ..
> Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and
> VCCAUX (2.5v). VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v,
> and 2.92v on the 3 boards I grabbed and measured. All boards seem to work
> just fine.
>
> The regulator output voltage is controlled by just 2 resistors. When I
> changed one of the resistors to lover the voltage a bit, VCCAUX did not
> change. This leads me to believe that VCCAUX is somehow being "back"
> powered from the Xilinx chip. These voltages are present like this before
> the Xilinx chip has been programmed. I have not removed the regulator to
> measure current yet. Another thought was to put a shotkey diode between
> the regulator output and the load to see if the Xilinx really is powering
> VCCAUX, but I thought I'd post and see if anyone else has come across this
> issue. Half the I/O banks are 2.5v and half are 3.3v if that makes any
> difference.
>
> Thanks - Dan
>
>

It sounds like you're driving one of the dedicated configuration pins with a
3V3 signal. They should all be 2V5.

There's a Xilinx app note about 3V3 configration which suggests series
resistors to limit current through the ESD diodes, and provision of
current-sink capability on the 2V5 supply!