AVR-Chat

My #1 rule for EMI:  Put the current back where you got it from.

Notice how I didn't say "let it find it's own way back there"... :)


#2 rule: Keep high frequency loops short, and small.

If you can, route out and back either side by side, or top and bottom.
Minimizing loop area is a primary method to reduce both radiation and pickup.  Take the same length of track, and make it a circle or a square, and it's a very effective antenna. Stretch it till the two legs are side by side, and there's almost no loop area, and now you have a very ineffective antenna, also known as a transmission line. 

Crystal caps are a good example. These should have a separate DEDICATED track back to the uP's ground pin. DO NOT dump them into a plane, or worse, onto a ground track that runs off to other places (This is also known as an ANTENNA). The crystal tracks should run close parallel, by the shortest possible track length, to the uP's xtal pins. Try to flood an area of ground around them if possible. Your dedicated cap return track can go down the middle of the two xtal tracks.

#3 rule: control where noise goes, by giving it fat tracks to where you want it to go, and skinny ones to where you don't want it to go, but know that it will go both ways, according to the relative impedances.

#4 rule: Bypassing: Put the bypass at the client's ground pin. Make this the connection point to system ground. Route a fat track from the client's VCC pin(s) to the cap. Route a skinny track from the cap to system VCC.  If you can, follow this same procedure with the ground, back to system ground (normally the big cap on the power supply output).