Even more fun is when it is powered via an input pin that goes thru some=20 resistance to the rails (like a 1K). Then not only is it mysteriously=20 powered, it will be really erratic as well. Or when an output on a PIC=20 that toggles goes to an input pin that then powers the other chip on/off=20 unwittingly. Good times. This comes up in design when doing things like having a hybrid self and=20 bus-powered USB device. If the FTDI chip handling USB is powered by=20 USB, and the rest of the design isn't, you can get situations where USB=20 accidentally powers the rest of it and vice-versa because of the UART=20 serial link between the FTDI chip and the PIC. As if USB isn't a=20 nuisance enough in general, this can lead to the FTDI chip not powering=20 up right so USB fails erratically, on and on. The obvious answer, Don't Design It Like That, is not always practical=20 or possible. The answer ... 10K in series on RX/TX lines to-from the FTDI and don't=20 use really fast serial rates (because of the RC rise/fall times the 10K=20 creates), which fortunately hasn't been needed in the designs when this=20 has come up (115200 was just fine as I recall). There are better ways=20 that require more parts. J David VanHorn wrote: > Unless it's in the data sheet, don't try to use it for real. You are > powering the chip through the protection diodes, so the voltage is lower > than you think, and those diodes aren't meant to be turned on in normal > use. It's an old trick, but it's not a good idea. > > On Mon, Feb 2, 2015 at 7:24 PM, David Meiklejohn > wrote: > >> Allen Mulvey wrote: >>> Sent: Tuesday, 3 February 2015 1:06 PM >>> To: 'Microcontroller discussion list - Public.' >>> Subject: RE: [EE] ICs working with Vdd/Vcc disconnected? >>> >>> This is a documented feature of the DS18B20 digital temperature sensor. >>> They call it "parasite power." >> >> Yeah, I knew about it from 1-wire devices - but didn't know that it was = an >> "undocumented feature" of chips with separate power supply pins! >> >> >> Cheers, >> David >> >> >> >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .