On 2/4/2010 8:23 AM, David Brown wrote:
>
> You have a lot more experience at this sort of thing than me, Rick, so
> I'm a little wary of disagreeing with you. But I'm sure you'll tell me
> if I get something wrong!
>
> I don't see that you have to worry about any termination here. With fast
> enough signal edges, you can get ringing - but that typically will not
> matter because you don't sample the signals until the ringing has
> subsided.
>
> Ringing can cause a few problems - the overshoot/undershoot can go
> outside the voltage range of the pins on the line, it can cause
> interference for neighbouring signals, you can't read the signal while
> it is ringing, and it can cause big trouble when connected to
> edge-sensitive inputs. But most of these are not going to be a problem
> in your case, I think.
>

Dear David, Steve,

Going "outside the voltage range of the pins on the line" can break the
device. IIRC there are Xilinx appnotes which go into this problem in
some detail; powering 3.3V with 3V was something I think they suggested!
(See XAPP653)
Also, the thing will probably fail any sort on electromagnetic
compliance test that you would need to do before you sell this. And you
are unlikely to be able to listen to 'The Archers' while this thing is
in the room.


To the OP, in the absence of micro-vias, I would recommend a 6 layer
board. Maybe like this:-

signal
signal
ground
ground
power/signal
signal

Keep all the layers as close together as your PCB manufacturer allows
and make up the board thickness with the core between the two ground
layers. Xilinx on the top. Route the powers, or use copper pours. Try to
make room for bypass caps on the back of the board from the FPGA. This
stack up will make it very difficult for a beginner to go wrong from an
SI point of view, as ground is always near. This is particularly true if
you have a nice spread of ground vias tying the two ground planes
together. That doesn't mean you shouldn't simulate it with Hyperlynx,
but I bet that won't happen! Always examine your ground plane pair at
the end of the routing process to make sure you haven't cut any big
slots in it with string of vias.

Finally, _real_ engineers use DRAMs! ;-)

HTH., Syms.

p.s. Did I make it clear that the ground is important?