Re: Differential terminations in Virtex2 Pro.

Symon,

Hmmmm.  I was thinking of both input and output.  You are right.

Perhaps the software thinks like I do?

Austin

Symon wrote:

> "Austin Lesea" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> > Symon,
> >
> > V2P does not support 3.3V Vcco powered LVDS (at least, that is what the
> data
> > sheet says).
> >
> > Austin
> >
> > Symon wrote:
> >
> > > Hi All,
> > >     I'll open a webcase too, but I'm posting in hope of a super quick
> > > answer! Here's my question:-
> > >
> > > V2P has on-chip differential terminations for LVDS signals, e.g.
> LVDS_25_DT.
> > > See answer #17244. However, although 3.3V banks can support LVDS
> receivers,
> > > the terminated mode is not allowed. I quote:-
> > >
> > > "Requirement to Turn on the On-chip Input Differential Termination
> > > The VCCO of the I/O bank must be connected to 2.5V to provide 100 ohms
> of
> > > effective termination.
> > > NOTE: Starting ISE 6.1i, this requirement is implemented in the
> software. "
> > >
> > > So, this sounds like it's just the new 6.1 software that stops you
> turning
> > > on the termination in 3.3V banks. What happens if you use old software
> and
> > > turn on the termination in a 3.3V bank? Why is it not allowed? Is it
> just
> > > that the termination impedance is different? If so, what is it? It's not
> > > hard to change the characteristic impedance of my traces to match a
> > > different termination. Is the problem that it's not tested when the
> parts
> > > are produced?
> > >     My problem is in banks 4 and 5 where a 3.3V 8 bit data bus
> configures
> > > the device. These banks also contain some dedicated clock input pins
> that I
> > > want to use internal terminations on.
> > >
> > >     Thanks for reading, Syms.
> >
>
> Austin,
>      (With tongue in cheek) I think you'll find that
> "The differential input buffers are powered by VCCAUX and are not
> VCCO-dependent. For this reason, you can put LVDS_25 and LVPECL_25 input
> buffers in a 3.3V bank; the software does not report errors and the device
> is not damaged. In this case, the input specifications are as specified for
> LVDS_25 and LVPECL_25."
>       Or at least that's the gospel according to answer 16830!!
>
>     So, I'm allowed LVDS inputs on a 3.3V bank, so why can't I terminate
> them on-chip?
>
>                 thanks, Syms.