Flashing split keyboards works a little differently than your typical keyboard since you obviously have to flash each half.
But what do you do to flash them an in what order? What about when you update your keymap or want to enable some special features of the firmware like QMK’s tap dance or CAPS WORD for example?
Typically a split keyboard will communicate with the host device (host meaning the device your keyboard is connected to–typically your computer) via the left half. Depending on the keyboard this is configurable so you may choose to connect the right half instead but basically only the one half is connected to the host itself. The rest of this post will assume the left half is the one connected to the host; assume the reverse if the right half is connected.
So the main half is connected to the host device, and the right half communicates with it and simply monitors what keys are being pressed. This means when flashing an updated keymap you flash it only to the left half.
Some things to remember: flash the left half only when updating the keymap
Flash both sides if you’re making changes to things like the OLED, etc. When in doubt, flash both halves.
Remember to unplug the keyboard from the host device before you disconnect the TRRS cable connecting both halves. This is done to prevent the possibility of shorting the halves and rendering them inoperable. Once the keyboards are unplugged from the host device, then unplug the TRRS cable.
Once you’re finished flashing the keyboard, whether that’s one or both, make sure you’re plugging in the TRRS cable first and connecting both to each other first. Only then should you connect the keyboard to the host.
Don’t try to test the non-main half otherwise your host will treat it as that side. Say your left half is the main half and you flash both sides. If you then connect the right half and try to test it the host will assume it’s the left half and you might assume it didn’t flash the firmware properly.
Connect both to each other and the main half to the host before you test out your revised keymap and whatever other firmware functions you updated.