DCI question

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Hi all,

I have a very peculiar problem with some
digitally controlled impedance (DCI) inputs
on a Virtex2 device: whenever I enable the
DCI on the inputs the signal levels go way
down, without DCI I see about 800 mV and with
the DCI on about 200 mV. The IO standard is
HSTL_I_18, this is a QDR Sram interface design.
Another strange thing is that I only see this
behavior on the XC2V8000 device, the XC2V4000
and XC2V6000 devices work fine on the same
Any suggestions?

Re: DCI question

Check that the Vrp and Vrn reference resistors go to the right supply.
A common mistake is to have the Vrp (pmos) resistor go to Vcco (wrong),
instead of ground (right), and similar for the Vrn resistor which should
go to Vcco (right), not ground (wrong)

Be sure to actually measure the reference resistor value.  Really tiny
surface mount resistors are easy to get wrong:  instead of 51 ohms, they
may have placed 510 ohm resistors there!  Seeing the markings requires a
  loupe or magnifying glass!

Also check you have used the right Vrp and Vrn pins for that bank.  I
know that some devices have alternate Vrn, Vrp pins, which require that
the software know which ones you are using.  I do not think this affects
the 2V8000 (only smaller parts in smaller packages).

All the devices share the exact same cell for the DCI (layout), so there
is nothing magic about the 2V8000.


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