5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
or be supplied by USB or another regulated 5V supply
Maybe some of my bricked Megas can load sketches from their ICSP ports.
I'm glad that I saw this thread before I hooked up my Mega, it saved me a ruined board. I'm running off of a 80's vintage backplane, with a typical 5v supply. The supply runs hot, about 5.25 volts. Thanks to seeing these posts I disconnect the Mega2560 from the external supply while programing, then disconect the USB when installed. Works so far. The one time I left the USB conneced when attached to external supply, the Mega USB IC got hot fast.I also use a modified USB cable with the 5v line cut for when I need to get data while running.Since the problem is known and can break boards, how about a sticky, or a warning.Get the word out.ThanksBob Prescott
.25 is a hugh differance, I know, and old power supplies designed for TTL usually had some kick.