I started using a 1284 xplained board yesterday to provide a USART test signal to a new project. While getting it going, I discovered that it's powered by USB. Having no PC USB ports handy, I decided to plug the USB cord into a USB port on one of those power strips plugged into mains power. That worked fine, and I set to troubleshooting the new circuit. After a few minutes I noticed strange behavior in the circuit, and I looked at it with my scope. What I saw was 57VAC at 60 HZ on the ground radiating all through the xplained and my circuit under test.
I felt like I had been violated. I pulled the USB cord out of the wall strip and the voltage disappeared. My question for you guys is how are those things isolated (or supposed to be anyway)? There's no transformer. Is it just a little regulator? Also, any guesses on how much current that connection could carry? The AVR on my new circuit is still working and I saw no smoke, but the DSP may be inop now. How will stray AC on the ground plane affect these modern IC's, any idea?
I'm not going to do that again.