I'm having problems with COM ports and my Itsy/Feather
32u4/M0
Theres two COM ports you can have with the 32u4/M0, one is the user port and
one is the bootloader port. They are not the same COM port number!
When you upload a new user program it will come up with a user com port,
particularly if you use Serial in your user program.
If you crash your user program, or have a program that halts or otherwise fails, the
user COM port can disappear.
When the user COM port disappears, Arduino will not be able to automatically start
the bootloader and upload new software.
So you will need to help it by performing the click-during upload procedure to re-
start the bootloader, and upload something that is known working like "Blink"
I don't understand why the COM port disappears, this
does not happen on my Arduino UNO!
UNO-type Arduinos have a seperate serial port chip (aka "FTDI chip" or "Prolific
PL2303" etc etc) which handles all serial port capability seperately than the main
chip. This way if the main chip fails, you can always use the COM port.
M0 and 32u4-based Arduinos do not have a seperate chip, instead the main
processor performs this task for you. It allows for a lower cost, higher power
setup...but requires a little more effort since you will need to 'kick' into the
bootloader manually once in a while
I'm trying to upload to my 32u4, getting "avrdude:
butterfly_recv(): programmer is not responding" errors
This is likely because the bootloader is not kicking in and you are accidentally
trying to upload to the wrong COM port
The best solution is what is detailed above: manually upload Blink or a similar
working sketch by hand by manually launching the bootloader
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