USB Track Width / Parallelism

[vince.cimo]

Hi guys, I just got my first custom teensy boards in the mail and unfortunately I'm not able to get the USB device to show up. I've pushed the program button and also floated the NMI pin but don't seem to have luck. I'm going to probe around, but one thing I'm very unsure about is the tolerences of the USB connector design. I've read this article (http://www.appfruits.com/2015/03/building-your-own-custom-teensy/), which indicates that USB is quite specific about differential impedance, and this article (http://www.focusembedded.com/blog/high-speed-usb-in-a-two-layer-pcb/), echoing the sentiment, but some of the forum posts here have mentioned that at short lengths (3" or so), these design considerations are less critical.

Unfortunately, I stumbled on this information after the boards were made, so the differential impedance is most likely not to recommended spec. How likely is it that -assuming everything else is kosher on the board (will post my probing results here to check that), the USB connection is failing because of ignoring this consideration?

 

[PaulStoffregen]

How likely is it that -assuming everything else is kosher on the board (will post my probing results here to check that), the USB connection is failing because of ignoring this consideration?

Rather unlikely. This is only 12 Mbit/sec USB. Especially if your connection to your PC is well under the worst case of 5 hubs plus 6 cables each 5 meters long, you have quite a lot of margin for varying from the ideal routing. Many people have designed boards with less than ideal USB routing and they work fine.

Really, the crystal is the only part that's usually layout sensitive. Routing digital signals underneath the crystal without a ground plane for shielding (eg, 4+ layer PCB) usually will cause problems. Even then, boards with terrible crystal layout usually are able to sometimes work, but act "flaky".