Dangerous Prototypes Open Source Hardware

Why large plane/polygon for 1.2V versus supplies?

May 21, 2010, 09:39:57 am

This certainly isn't critical, but I am wondering why the 1.2V supply ended up with a half-plane/polygon.  The 3.3V supply has a tiny polygon under the FPGA, and the 2.5V supply has an even smaller polygon on the bottom copper.  I'm just curious whether there is a particular reason for 1.2V to be brought to the edges of the PCB.  Seems like the 2.5V or 3.3V should have the bigger polygon since they actually connect to two or three header pins.

On a related note, does anyone have a good idea of whether a 4-layer or 6-layer board would perform better?  I realize that the costs would increase, but at the same time it would be educational to know exactly what differences there would be.

Re: Why large plane/polygon for 1.2V versus supplies?

Reply #1 – May 21, 2010, 09:45:30 am

I'm not sure about the 1v2 plane.

I'd like to design a 4 or 6 layer board some day, but we'd need to move to a CAD program that supports that in a free version like kicad. That's my main motivator.

Re: Why large plane/polygon for 1.2V versus supplies?

Reply #2 – May 21, 2010, 10:16:24 am

I understand about the Eagle license.  I guess I feel spoiled, because the cost of an Eagle license came out of the profits for my first commercial design, and I haven't looked back.

If you want, I could take an existing Eagle 2-layer design and run the autorouter on a 4-layer or 6-layer board and provide the Gerbers.  I might have to tweak the autorouter a bit, so this would depend upon my having some spare time.  But I believe that you could still look at the resulting files in the free version of Eagle, so you wouldn't be totally stuck.

This could be helpful if you get stuck with unexplained 2-layer issues and want to see whether 4-layer or 6-layer would solve the problems.  I'm thinking it would only be a stopgap measure during the design phase to eliminate variables.