01-10-2020, 10:39 AM
The nRF5's SDK has a bootloader with a few ways to enter it (button on startup, set a bit and force a restart over a normal BLE). Almost every commercial sports tech sensor is using the Nordic Reference or a modified derivative. The down side is that the reference is really setup for "no backsies", as in firmware is given a version, and you can't put on an older one. There was an open bootloader in 15.3.0 (haven't checked 16 yet) but it's nrF52840 USB and not BLE.