Hi, I hope you guys can help review my design-
I'm working on a keyboard PCB design. I thought I was nearly done, but after happening accoss 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.
(appologies, I can't get the inline attachments working... images are visible below post)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.
![PDB Design Review: Impedance Matching & Termination Resistors 1120686-1](https://www.eevblog.com/forum/index.php?action=dlattach;topic=261986.0;attach=1120688;image)
(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:
![PDB Design Review: Impedance Matching & Termination Resistors 1120690-2](https://www.eevblog.com/forum/index.php?action=dlattach;topic=261986.0;attach=1120692;image)
![PDB Design Review: Impedance Matching & Termination Resistors 1120698-3](https://www.eevblog.com/forum/index.php?action=dlattach;topic=261986.0;attach=1120700;image)
Here are the
Termination Resistors:
In the keyboard design world, it
common convention to put in 22 ohm resisters in the D+/D- lines:
![PDB Design Review: Impedance Matching & Termination Resistors 1120694-4](https://www.eevblog.com/forum/index.php?action=dlattach;topic=261986.0;attach=1120696;image)
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/Questions:1) 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?
2) 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?
3) The very
value of Termination Resistors is a question... 22 ohm is the usual value cited... 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
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)