Choosing the supply pin according to the regulated source, anticipating radio current spikes, and avoiding conflicts when USB stays plugged in parallel.
Cutting the microcontroller free of the USB cord is mandatory for a portable device, but the choice of power injection point determines stability. Wiring to the wrong pin means shorting the board regulator or starving the module of the voltage it needs.
Choosing the pin by source
If the source is already regulated to the core voltage, inject directly on the regulated rail, bypassing the board regulator. If the source is higher and unregulated, go through the raw input that feeds the onboard regulator. Confusing the two destroys the regulator or leaves the module under-powered.
The voltage margin must cover the regulator dropout under load, not just the idle value.
Radio spikes and dual source
On every radio transmission, current can jump by several hundred milliamperes in a few microseconds. A poorly decoupled source then collapses and triggers a reset. You need a reservoir capacitor close to the module. Finally, leaving USB plugged in parallel with an external supply creates a source conflict: two regulators push against each other.
The takeaway: one source at a time, on the right rail, with a reserve for radio spikes. The rest is just symptoms.