I have soldered wires to "HappyModel EP2 2.4G ExpressLRS Receiver". There is one pad I don't know and can't find what it is for. See the pic "EP2 RX connection diagram". The problem is it was my first time I soldered something real (let alone test boards for practice).
As you can see on the picture GND pad and "unknown" pad are really close.
Maybe my solder leaked from GND to "unknown". I cannot see with my eyes. But what I can do is to make a continuity test with my multimeter. I can hear a buzzer sound. So two pads are connected. But I don't know if it is my bad soldering or it was designed.
BTW, I have seen an ELRS receiver from BetaFPV where is the pad very similar to "unknown".
(BETAFPV ELRS Lite Receiver 2.4GHz) However there is a documentation about this:
"Firmware upgrade button (Use a metal object to short-circuit the GND while powering on)"
When I soldered wires to the board ( connected the receiver ) , sometimes when I touch or move the receiver, the drone disconnects from the computer (USB connection) and even all lights stop blinking. Looks like a shortage or some joints not soldered reliably by me. Anyway I accurately stuck the receiver with mounting tape, closed the drone. And it flies. It was the first fly test - take off, hover a little bit, forward, turn and landing. However, I am afraid that with more aggressive flight the receiver could disconnect.
Long story short, if somebody knows what the hell the "unknown" pad is and if it should be connected to GND by default.
P.S. Actually it is very easy to check with a new receiver, just make a continuity test. I would do it by myself, but I am a beginner in soldering and really afraid of re-soldering everything.