Re:Designing Raspberry Pi 400
by pjc50 on Wednesday November 04, 2020 @11:12AM
(#24990300)
The two-pin version certainly exists and is sold as "zero ohm resistors" in surface mount packages. The multi-pin version probably exists but I can't find it with 30 seconds of searching.
But then there are two problems: controlled impedance and buried layers. Both the signals in question, HDMI and USB, are high-speed, and design recommendation will be that they cross as few vias as possible. They will be on "microstrip" paired line arrangements carried inside the board on one of the inner layers, while the top and bottom layers are ground planes. This won't be visible from a simple photograph of the board, you need an xray or the original Gerber files.
So while it's possible if you have enough space to punch a signal up to the surface of the board, through a package, and back down again in a different place, that will usually have an effect on signal integrity.
The normal solution is more layers. I don't know how many are there at the moment but I suspect it's 4. Increasing that to 6 costs more - not quite 50% more, but heading that way.
But then there are two problems: controlled impedance and buried layers. Both the signals in question, HDMI and USB, are high-speed, and design recommendation will be that they cross as few vias as possible. They will be on "microstrip" paired line arrangements carried inside the board on one of the inner layers, while the top and bottom layers are ground planes. This won't be visible from a simple photograph of the board, you need an xray or the original Gerber files.
So while it's possible if you have enough space to punch a signal up to the surface of the board, through a package, and back down again in a different place, that will usually have an effect on signal integrity.
The normal solution is more layers. I don't know how many are there at the moment but I suspect it's 4. Increasing that to 6 costs more - not quite 50% more, but heading that way.