On Sep 6, 4:03 pm, "Andrew Holme" <and...@nospam.com> wrote:
> "Dan K" <danielgkNOS...@visi.com> 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
I would load the Vccaux pin with a 100 Ohm resistor to ground. That
adds 25 mA to the regulated current.
Normall, this should hardly change the voltage, but if the regulator
is back-fed, there will be a big voltage drop.
Parallel resistors to ground are nice and easy, and do not cuase any
irrepairable tdamage.
Peter Alfke

> > 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!