Q on PCB routing - Page 1
Forum section: electronics / projects

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: July 05, 2021, 12:57:32 am
content: I guess this is not an EDA question, but I am torn.First, a little about me. I have very little experience with PCB routing. I've done a couple of boards as a student a long time ago. I'm a CS guy who has done firmware and drivers for several decades, but now have a project. I am designing a small keyboard (*why* is a long story). This is a proof of concept for a larger design. I don't feel there is much value in building a blinky board. I'm pretty sure that I know what this MCU needs.I am using a AT90USB1286-AU mainly because that is one of the few suitable parts I was able to source recently. I can find a lot of information on similar Atmel processors.I have the Teensy++ 2.0 schematic which is a good minimal design. (I don't feel good about how there was no protection on the USB port, and they have a pull-down resistor on ~HWB). But that is the arrogance of the clueless talking.I need USB for power and data, so I have a differential pair coming off the edge of the board. The MCU also has a large number of Vcc pins. I cannot figure out how to route this board so that they don't cross. I don't think that it is possible, but I haven't sat down and written up a proof. I assume that it is a really bad thing to send either of these over a via, but clearly have no choice. My current thought is to send Vcc over the via and not risk messing with the differential pair.I am curious how someone who knows what they are doing would approach this.

author: ataradov
author_rank: Super Contributor
timestamp: July 05, 2021, 02:29:30 am
content: Full Speed USB you can route any way you like. There is no need to overthink differential pairs here.I don't put protection on my boards. I don't ship millions of devices, of course, but I have never seen any issue in real life.Those Teensy boards are shipped in huge quantities, and you don't hear about them massively being killed via USB.I'm not advocating to remove protection, but just think if it is really necessary in your case, don't do it just because "everyone seems to do it".

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: July 05, 2021, 02:32:03 am
content: Thanks, that is a lot to think about.5 2-layer boards is a bit over $15, and going to 4L takes it to $48. This is a prototype for a larger board, and 4L is going to be way outside of my price range.As this is a smallish keyboard, I am constrained by the physical dimensions of the switches.One might wonder why I don't do something smaller. As long as I'm going to the trouble of laying out a board, I might as well have something useful. And I have a use for it.There is a huge ground plane on the bottom. The board is mostly empty space. I have been leaving at least 5x the trace width between traces.There are plenty of (5) decoupling capacitors on the MCU.For a keyboard, slow speed is fine. I have been trying to act as if I was going to run at a higher speed as a good practice. Most designs use 22-ohm resistors on D+/D-. I guess that is close enough to 28-ohms that the traces make up the difference.The third signalintegrity.com link asks for $10K for the material! Maybe I missed something. I don't need to know that bad.PS: Teensy doesn't even have any Zener diodes!

author: ataradov
author_rank: Super Contributor
timestamp: July 05, 2021, 02:40:22 am
content: You don't need 4L, I can pretty much guarantee you that. Just route your stuff so it is not a total mess, and don't overthink vias. Your MCU can't go faster than FS USB anyway.Look at how your front panel USB is connected to the motherboard inside the PC - it goes though 0.1" connectors and regular wires. Your one via on the path can't make things much worse.Again, what makes you think that you absolutely need protection in your design? There are legitimate cases for that, but for most consumer and hobby stuff it does not matter.