I need a help regarding troubleshooting of the USB issue.
I made 2 custom PCBs (MK22FN1M0VLH12), both have USB connector, routing is almost the same. Both PCBs are working fine, the firmware is absolutely the same inside both MCUs.
BUT, as soon as I connect a USB cable to PCB A, it stops working, generates hard fault (could be bus or usage fault), while PCB B is working fine and USB functionality works as expected.
Measures taken to solve the issue:
- At the beginning I put 51Ohms terminator resistors on both PCBs. In this stage PCB A does not work, PCB B works fine.
- Then I changed resistors to 33Ohms (as recommended) on PCB A, did not help.
- Then I tried to remove resistors and short cut on PCB A, did not help, so I moved back to 33 Ohms.
- Then I enabled MPU on PCB A and saw generates MPU errors, this is just an additional information
I attached pictures that show how PCBs are routed.
I need some ideas what to try more? Should I add pull-down 15kOhm resistors to data lines?