Flashing Frsky R9 Receivers
Note: This will be the same wiring you'll use for flying and the next firmware updates (via Passthrough). Forget the factory wiring guide!
Passthrough Flashing (works with Betaflight, iNav)¶
Make sure the correct Bootloader has been flashed to the receiver prior to wiring it up to your flight controller. Using the wiring guide above, find a free, uninverted UART in your FC. You can use your FC's wiring guide for a Crossfire or Ghost receiver.
Once wired to your FC, connect USB. Did your receiver powered up too (with both LEDs lit)? If so, disconnect USB, hold the bind button on your receiver, and reconnect to USB. The LED should start alternating between the Green and Red LEDs. Once it's alternating, you can then let go of the Bind Button.
If your receiver didn't get powered from USB, have a lipo ready and continue with the next steps. On the ExpressLRS Configurator, with your Firmware Options set, click on Build & Flash. Like on the TX module, it will take a while on the first time. Watch out for the
Passthrough Init stage. This stage will check your FC Configuration for the Serial RX UART (Software Inversion via "set serialrx_inverted" and Half Duplex mode via "set serialrx_halfduplex" will be checked; both should be off.)
Retry... lines appear, connect a LiPo if your receiver isn't powered by the USB (i.e. power up your FC and receiver). On subsequent flash, you can have the LiPo plugged in and receiver powered up from the start prior to connecting the USB.
Wait for this process to finish. It's done once the "Success" prompt is shown.
Flashing via STLink (only recommended as last resort)¶
This method is irreversible. It will remove the ability to reflash back to Frsky firmware. You have been warned! Make sure your STLink dongle is properly recognized by your System as such (Drivers are installed, etc.).
After doing so, Disconnect from STLink Utility.
Using the correct target specific for your receiver, set your Firmware Options and hit Build & Flash on the ExpressLRS Configurator.
Once done, wire your receiver to your Flight Controller. Passthrough flashing can now be used for updating the receiver.