Help needed: APM 2.5 - no communication, neither USB nor telemetry. Otherwise works ok.


the BEC of my ESC burned out for some reason (really, with smoke) and since then my APM doesn't talk anymore, neither via telemetry, nor via USB. It is recognized by the PC as Arduino Mega 2560 and the com LEDs flicker but no data is coming through from the APM. Establishing a link also triggers a reset of the APM.

Besides the lack of communcations, the APM otherwise seems to work fine. It shows GPS lock after a while and I can arm it and spin up the motors. I didn't dare to test more, but ARM / Disarm and GPS works all ok, as it seems.

It just doesn't communicate anymore.

What could be the problem? A visual inspection of the PCB didn't show any obvious damages on the APM board, while one IC in the SBEC of the ESC bursted...


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Yep, checked everything double. When I go to the UART0 pins on the side of the board directly, I can talk to the APM @57k6 baud, but telemetry port and USB are dead. USB still shows com LEDs and those also flash when I send something from my PC. But from the APM nothing comes inbound.

I did some googling and it seems, a part called "automux" is dead. Seems to be no uncommon failure. Question is, can it be repaired?

I have the same problem on a 2.5 here. Can talk by using uart0 pins @ 57k but usb and radio don't work. Appears to be a common problem. I hope 3DR comes up with a fix. I could not load new firmware on also. Couldn't see the 2860 with 57k setup.


Nope, doesn't help. Flashed the latest version via ICP headers...

I agree with Earl... As often as this problem occurs, it seems, the APM's design might be improvable.

JTAG? Do you mean ICSP?


So, if I understand it right, the automux function is also handled by the 32U2?

Is it possible that a surge just kills the 32U2 UART which is connected to the APM? Or would the 32U2 be dead then alltogether?

I have read a couple of cases from the forums by now. Practically all contain some kind of surge, like e.g. a MinimOSD's MAX burning out and probably sending 12V to the APM or similar. Unfortunately, my understanding of the electronics of the APM board is too limited to do hardware-debugging.

I received a new MUX IC today from 3DR - free of charge I might add! Talking about great customer service!!! Thanks Rogelio!

After replacing the MUX (which is devilish hard if you don't have specialized SMD soldering equipment!) my APM works perfectly again and talks to me - via USB as well as via radio!

Again, thanks 3DR for great service!

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