I have a brand new ATmega32U4 that I initially read and programmed using the DFU bootloader. All was OK. For reference, the HWB pin is tied to the deck via 1 kΩ.
Unplugging the device and plugging the device to the USB port doesn't do anything. MC Studio doesn't see it, and the FLIP programmer doesn't see it. It doesn't appear in the Device Manager on the PC either.
How do I get it back?
Does programming the device automatically overwrite the DFU? If so, how do I prevent that next time? It's not supposed to be an OTP device. Why doesn't it appear on a port in the Device Manager?
I've unplugged and replugged it in like a hundred times (I know, insanity... yes yes). Nothing. I don't have any idea what to try next. I don't have any other programming device for the chip. I haven't found anything in searches elsewhere for this specific problem.
It seems I forgot to set the Hardware Boot Enable fuse. By default, it is set to 1, which means that the hardware boot pin (!HWB) on PE2 is ignored and cannot force bootloader entry. It must be cleared to 0 or the bootloader only gets used the one time.
I'm still not entirely sure why it won't enumerate on the USB, but I can deal with that once I get the bootloader working reliably.
Anyway, one tiny little oversight/misunderstanding at one single bit is all it takes to bork the chip. Don't forget to set your fuses correctly, all of them, before you do anything else.