Hi John.
That's what I suggested in my posting yesterday.
"Great minds think the same way" :-)
Peter Alfke

On Sep 7, 9:29 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 6 Sep 2007 17:23:39 -0500, "Dan K" <danielgkNOS...@visi.com>
> 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