5 ms·

> "The PCB shape had to be defined early on so that the industrial designers could get on with the housing design, and I then stared endlessly at the PCB layout

by a4444f 6y ago

> "The PCB shape had to be defined early on so that the industrial designers could get on with the housing design, and I then stared endlessly at the PCB layout, trying to get one of the USB ports to route to the right side without wrecking the signal integrity of the memory or the HDMI; I could not find a way to do this. " I'm not an electronics designer, but why would some kind of a symmetry of the PCB not work?

pjc50 6y ago
"I have no expertise in the relevant area and no specific knowledge of the problem, but I propose a random under-described solution that the experts actually doing the thing definitely won't have thought of" (Hint: you can mirror the board but that doesn't mirror the pinout of components)
- singemonkey 6y ago
  The person you responded to is asking a question. I don't get why you need to be so weirdly sarcastic. (I have theories, but they're quite denigrating towards you, so I'll refrain from airing them) Anyway, I look forward to your explanation, and perhaps we can find a way to help you interact more successfully with human beings?
- NLips 6y ago
  Please don't be snarky. A better response would have been just your hint: "You can mirror the board but that doesn't mirror the pinout of components"
geerlingguy 6y ago
See this image from my tear down [1]: https://www.jeffgeerling.com/sites/default/files/images/4-pi400-back.jpeg https://www.jeffgeerling.com/sites/default/files/images/4-pi... You can see it's hard to find a good path from one side of the board (where the usb controller is located) to the other (where the HDMI ports are located) without using some vias or some crazy routing. The HDMI ports could be relocated probably but at the expense of more layers on the PCB (higher cost) or rotating the SoC, which would then necessitate even more of a redesign. [1] https://www.jeffgeerling.com/blog/2020/raspberry-pi-400-teardown-and-review https://www.jeffgeerling.com/blog/2020/raspberry-pi-400-tear...
- a4444f 6y ago
  This seems to be an interesting and common problem (is there is a name for that?). If ribbons or additional pcbs are out of question, maybe a specialized / dedicated IC (I would describe its function as topological-only / electrically null) would work, I wonder if such ICs exist.
  - pjc50 6y ago
    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.
- cesarb 6y ago
  If you look closely at that image, and at the image of the other side of the board (https://www.jeffgeerling.com/sites/default/files/images/3-pi400-in-shell.jpeg https://www.jeffgeerling.com/sites/default/files/images/3-pi...), it's fascinating. The keyboard is USB 2 (as mentioned in the article, "one of the USB2.0 ports is dedicated to the keyboard") with its connector is on the right side, and the designer did find a good path from one side of the board to the other, using some crazy routing. It goes between the two USB 3 ports, and then below all the other ports until it reaches the right side, where it passes right next to the antenna and below the wireless chip to reach the keyboard connector. It's a tight fit, and there's no space for another pair.
ape4 6y ago
Perhaps they could have added some grooves or small plastic hooks outside to route USB cables.

by a4444f 6y ago

pjc50 6y ago

"I have no expertise in the relevant area and no specific knowledge of the problem, but I propose a random under-described solution that the experts actually doing the thing definitely won't have thought of" (Hint: you can mirror the board but that doesn't mirror the pinout of components)

singemonkey 6y ago
The person you responded to is asking a question. I don't get why you need to be so weirdly sarcastic. (I have theories, but they're quite denigrating towards you, so I'll refrain from airing them) Anyway, I look forward to your explanation, and perhaps we can find a way to help you interact more successfully with human beings?
NLips 6y ago
Please don't be snarky. A better response would have been just your hint: "You can mirror the board but that doesn't mirror the pinout of components"

geerlingguy 6y ago

See this image from my tear down [1]: https://www.jeffgeerling.com/sites/default/files/images/4-pi400-back.jpeg https://www.jeffgeerling.com/sites/default/files/images/4-pi... You can see it's hard to find a good path from one side of the board (where the usb controller is located) to the other (where the HDMI ports are located) without using some vias or some crazy routing. The HDMI ports could be relocated probably but at the expense of more layers on the PCB (higher cost) or rotating the SoC, which would then necessitate even more of a redesign. [1] https://www.jeffgeerling.com/blog/2020/raspberry-pi-400-teardown-and-review https://www.jeffgeerling.com/blog/2020/raspberry-pi-400-tear...

a4444f 6y ago
This seems to be an interesting and common problem (is there is a name for that?). If ribbons or additional pcbs are out of question, maybe a specialized / dedicated IC (I would describe its function as topological-only / electrically null) would work, I wonder if such ICs exist.
- pjc50 6y ago
  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.
cesarb 6y ago
If you look closely at that image, and at the image of the other side of the board (https://www.jeffgeerling.com/sites/default/files/images/3-pi400-in-shell.jpeg https://www.jeffgeerling.com/sites/default/files/images/3-pi...), it's fascinating. The keyboard is USB 2 (as mentioned in the article, "one of the USB2.0 ports is dedicated to the keyboard") with its connector is on the right side, and the designer did find a good path from one side of the board to the other, using some crazy routing. It goes between the two USB 3 ports, and then below all the other ports until it reaches the right side, where it passes right next to the antenna and below the wireless chip to reach the keyboard connector. It's a tight fit, and there's no space for another pair.

ape4 6y ago

Perhaps they could have added some grooves or small plastic hooks outside to route USB cables.