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:
- Mega2560 R2 powered from external 5V regulated (+-2%) works fine.
- Plug and unplug USB cable over a few hours.
- 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.
- Buy new Mega R2. Repeat steps 1-3.
- Repeat step 4 a few more times.
- Buy Mega R3, but have second thoughts about the 5V rail.
- 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?)