Powering Teensy 3.0 from an analog read pin?

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jkern

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I've got a little project set-up where I'm reading battery voltage (3.2v nominal). I'm powering the Teensy 3.0 through the USB port. There's no voltage divider and the battery is connected to ground and pin 14 (A0). The program works as intended.

I notice that when I disconnect the USB, and the battery being measured has a large enough voltage, the program continues to run - that is the LED indicating a measurement being taken continues to flash.

Is this normal? Do I have something hooked-up wrong? Thanks!
 
It’s normal but insane. All input pins are internally protected through revers Schottky diodes towards VCC. Thus, if the battery voltage minus the junction voltage of that internal diode (ca. 0.3V) is above VCC, powering through that protection path can happen. But since that internal diode is for protection against transient overvoltage only, powering the Teensy through that path for a longer time might damage it and make it unusable.
 
Many thanks! So, maybe I'll just throw a voltage divider in there, to be on the safe side. As long as I'm doing that, I may just set it up to use the internal reference voltage.

Thanks again!
 
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