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Topic: Driving Mega2560 from external 5V bricks USB port (Read 10 times) previous topic - next topic


We should know in a couple of days if the particular board I removed T2 from dies. But this won't be a very definitive test in that I'm sticking with this one particular mega, running on external power from an OKI-78 dc-dc converter. If it dies, then the OKI-78 is probably doing it or, it's a bad batch of 16u2's....I'd suggest just starting your project on a board with T2 removed to be on the safe side.


Hi folks, I am the Original Poster for this thread. Appreciations to SurferTim and kurt6string for digging deeper into this.

I was told on several forums that there should be no problem powering the Mega from the 5V rail, but my drawer of bricked Megas kept getting fuller. They were all R2, none were R3. The only R3 I have has been exempted from drinking at the 5V trough because I was running out of $$, so I modified my backplane to power it from 7V in the Vin pin. It was a bother, but it has not bricked. Here's the algorithm:

1) Mega2560 R2 powered from external 5V regulated (+-2%) works fine.
2) Plug and unplug USB cable over a few hours.
3) Mega is no longer recognized as device by system. USB port disappears from IDE. Mega still runs last sketch but new one can't be loaded through USB port.
4) Buy new Mega R2. Repeat steps 1-3.
5) Repeat step 4 a few more times.
6) Buy Mega R3, but have second thoughts about the 5V rail.
7) Power Mega R3 by Vin pin. After 1 month of use, USB still works. Also Ruggeduino (Uno clone) works without problem in same circuit.

While this alone does not conclusively prove there is a design flaw or limitation in the Mega power circuit, the rest of this thread supports the idea. Can't prove R3 survives any better than the R2. I like kurt6string's idea about removing T2, but disappointed that he wasn't able to recover his bricked Megas. I am going to try his procedure when I get a chance.  Maybe some of my bricked Megas can load sketches from their ICSP ports.

Modifying the Megas by removing T2 is not a solution for me. I need a plug-and-play. At this point, I am going to keep external supplies away from the 5V pin and provide the power through Vin. Perhaps this problem will be addressed in R4.

(Now... what to do with these bricked Megas?)


Note that I've sent emails to both sales and team at arduino.cc asking them to look at this problem AND to fix the broken zip file containing the rev3 reference design. First I emailed sales, then a week later forwared to team, after 10 days or so I still haven't received a reply and the broken zip file is still un-openable....nobody's home....


Others have power supply problems related to a power supply (Vin in this case) in conjunction with the usb power.
Take a look at the o-scope displays in reply #6 here.  :smiley-eek:


(Now... what to do with these bricked Megas?)

I think they should be sent back to the vendor with a warranty replacement request. The vendors get away with a lot that wouldn't be tolerated in consumer products under the umbrella 'electronic hobbyists'. And in this case I think that replacements are justified. I hope to do this as soon as I have a talk with my customer, who purchased the boards I used in development. My usage of the boards falls within all the documented specifications, and there are no caveats mentioned about usb and external power. If distributors start getting boards back then that drives change at the manufacturing level.

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