After reading more of this thread, I found the strong suggestion to not power the teensy with USB power and switched over to a buck transformer that I had in a parts bin: Teensy 4.0 100mA Max SN65HVD230 17mA max 100mA + 2 x 17mA = 134mA Max. Unless you got a real old USB 1.X. V-USB has enough power...

Try this one. I did a blind grep session on the disassembled code and I think I found the routine. It looks a bit strange. https://a.d-cd.net/yjtJjuiK1Fi6fdJjKAz0Vfg45BM-960.jpg CEM compares if the PIN stored in its flash is all zeroes or the last byte is 0xff and jumps over/skips the XOR sequence....