I hope you can help review my design-
I'm working on a keyboard PCB design. I thought I was nearly done, but after happening across discussions on USB impedance matching of the data traces (and differential pair routing), I thought I'd better at least try and address this.
Due to design constraints, I'm not able to place the USB plug near the MCU (Atmega32u4) - so I have to route the data traces a loooong way across the board.
From what I read, to attempt to design to match an impedance value, you need to put a copper fill (GND) on the back of the PCB, have no interruptions to that ground plane under the traces, and set your data line traces to be a specific width, and a specific width apart - and - also try and keep the traces the same length.
I've managed to get the ground plane in, uninterrupted from USB plug to MCU. I've also set my D+/D- trace widths and separation, as per this calculation - trying to reach 90 ohms on differential impedance.
(Values picked from the manufacturer: https://jlcpcb.com/capabilities/Capabilities)
Since this is only a keyboard, I don't need high transfer speeds, so I guess that means my design does have to be 110% perfect in this regard... Due to the spacing of the USB connector pins and MCU pins, I had to compromise on trace width and taper it in..
Here are a few close-ups:
Here are the Termination Resistors: In the keyboard design world, it common convention to put in 22 ohm resisters in the D+/D- lines:
These are 0402 package resistors (so, tiny).
Here is another chat about these termination resistors for USB: https://www.eevblog.com/forum/projects/why-usb-data-series-resistors/
- Do you think I've done a reasonable job at routing those D+/D- traces to try and match impedance? Any obvious things I could improve?
- I'm worried about the Termination Resistors - they cause the traces to diverge a bit... I think that will mess up the impedance matching efforts? I don't have JLCPCB specs for min distance for SMT assembly... So I don't know if I can move them closer... Any thoughts on this? Should I just try and make them as close as possible together?
- The very value of Termination Resistors is a question... 22 ohm is the usual value cited (also in the atmega32u4 datasheet) but I don't know how that factors into the overall impedance of the D+/D- lines? (I'm still learning...)
Thank you very much :)