Voltage from USB when powered off? (Proto Board w/ Prop Plug and USB Proto Boar

DogP

Hey,

I was messing around w/ my USB proto board from UPENE, and noticed when I had the switch off, but the USB cable plugged in, the 3.3V line is about 0.7V, and the 5V line is about .2V. I thought maybe it was just the USB board, but when I checked my regular proto board w/ Prop Plug it gave me the same readings... has anyone else seen this, is this normal, and is it okay?

DogP

Phil Pilgrim (PhiPi)

It's "normal", I guess. It happens because the serial output from the FTDI chip is powering the Vdd bus through the Prop's port protection diodes. There are ways at the original design level to avoid it, but it's harmless; so no harm, no foul.

-Phil

YorkBW

I don't know that I'd go so far as "harmless." As far as I can tell, leaving the USB plugged in, with normal power to my breadboard off, is what caused me to have some head-scratching failures with uSD card reads using fsrw, as I mentioned here. Once I got in the habit of unplugging the USB when I cycled power on the board, my problems went away.

DogP

Cool... thanks for the info guys!

DogP

Drone

I agree with YorkBW. I've been staring at my regulator output LED being on from the prop-plug for some time and meant to get to the bottom of this, but it's on the back-burner. Having the 5V via the FTDI chip might be a bad thing for certain peripherals. But then again maybe it can be exploited to trickle charge a battery or supercap at V3.3 when the prop is unplugged(?) Dunno - Just tossing this out. As I said I haven't really studied this enough.

Rgds, David