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Powering FPGA

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--- Quote from: cyberfish on November 12, 2010, 10:36:52 pm ---
The DDR interface is the only high speed signal on the board level. Everything else will be <10MHz (well, maybe except the configuration signals).

--- End quote ---

It is not the frequency of the signal that matters - it is the speed of the edge transition. I was in the position a few years ago using a CPLD. We had a low frequency signal (3Hz) that caused problems when it happened as the edge transiition was so fast (and the routing so poor) it caused a voltage spike on the rail. The result was the signal was corrupt and I got random transitions instead of a nice wave.

If you are aim to sell the product in Europe it will have to pass EMC regulations, what ones will depend on the product you are making. when these changed for my industry a few years ago I was charged with going trhough those products that were non-compliant with the new standard.

I have seen many examples of people trying to save money by comprimising on the BOM cost. Even if you did manage to get the thing working with 2 layers you might find that the money you saved by going that route will be lost by having to retest, redesign or actually pushing up the price as you add ferrites into the unit to get it to pass.

I have done some design with Xilinx chips - the applications notes are quite comprehensive (and a bit confusing at first) but they are very useful and if you follow them you should not have a problem.

If you need additional resouce on board layout I recommend http://www.compliance-club.com/. Look for articals from Keith Armstrong. I have followed these on several occasions and have not had many problems, especially with high speed signals. When I read them it highlighted several things that I was not aware of, or had not realised that they could make as much differance as they do.



Thanks for the good information.

This is a hobbyist project, so I'm not really worried about EMI and such.

The problem with 4 layer is, at prototyping quantities, it's very expensive, or have insane lead time, or both. Most fab houses won't even do 4 layers for low quantities.

For a hobby project, if you ran out of power line space on the ground plane, you could probably bring the power pins through on vias and decoupling caps, and wire up the power with good old tinned copper equipment wire. I think sometimes we get so distracted with doing things neatly and "correctly" by routing lines all over the place, that we forget that a wire link is often a perfectly valid solution.


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