I had a very peculiar situation on a PCB I am working on, and the only possibility I can think of is that my assembler installed the wrong chip on my board. I'd like to present it to the forum to see if anyone has another idea.
I had pin 1 tied to D-, pin 4 tied to D+, pin 2 & 3 to GND, pin 6 & 5 to Vbus. Pins 3 and 6 are NC, but I've used this same wiring previously and it worked no problem, and I'd prefer not to leave them floating.
When I went to test the USB communication I noticed a short from D- to VBUS and also from D+ to GND. After probing a couple copies of the board and noticing the same thing, I desoldered the chip from the board. The short on the board & the USB cable was removed, and USB worked flawlessly.
I probed the removed chip itself and saw shorts from pins 1 (IO) - 6 (NC) and 3 (NC) - 4 (IO). This explained the shorts on the board. But I can't understand why there are shorts here given the "not internally connected" statement in the function column of the table.
Is there any reasonable guess for what happened here, besides the manufacturer installing the wrong chip?