On Thu, 6 Sep 2007 17:23:39 -0500, "Dan K" <[email protected]>
wrote:

>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
>
>

Try adding a "dump" resistor from Vccaux to ground, and see if that
pulls the voltage down.

John