Resetting the Pro Micro’s, in particular, can be tricky, because there’s no
reset button. The ‘RST’ pins needs to be connected to ground to initiate a
reset. This can be done with a small piece of wire, or an externally
connected button.
Why would you need to enter bootloader mode in the first place. Glad you
asked…
How to Revive a “Bricked” Pro Micro
Incorporating all of the USB tasks on a single chip is an awesome feature
that makes the Pro Micro and boards like it truly unique. But it also places
more stress on a single chip, and if anything goes wrong with that chip, the
board becomes nearly unusable. It’s not uncommon for Pro Micro’s to
become “bricked” and unprogrammable. But, in most cases, the bricking is
reversible!
The
most common source of Pro Micro “bricking” is uploading code to it
with an
incorrectly set board
(e.g. programming a 16MHz/5V Pro Micro
with the board set to 8MHz/3.3V). Also, make sure your sketch doesn’t
mess with the ATmega32U4’s PLLCSR register, or any other register that
sets up USB functionality on the ATmega32U4. The Pro Micro will actually
take code compiled for the wrong operating speed, but when it tries to re-
enumerate, you’ll be greeted with a notification like this:
To revive the Pro Micro, you’ll need to find a way to upload a sketch to it
with the board option correctly set. We can do this with a little help from the
bootloader.
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