Voltage on AnalogIn pin makes power light come on

Hi all. I just blew an Arduino somehow. It was working until I switched computers. But when I got a new one to test, I noticed something strange. When I connect GND to the Arduino, and a measurement voltage to AnalogIn, pin zero, the power and TX lights come on. This seems wrong... I don't want to try it this way, for fear that I'll blow this one.

Thanks,
Andrew

Does anyone agree that this is weird?

No, not particularly.
If it had flashed at 14.00Hz, that would've been weird.

When I connect GND to the Arduino, and a measurement voltage to AnalogIn, pin zero, the power and TX lights come on. This seems wrong... I don't want to try it this way, for fear that I'll blow this one.

You board setup condition when seeing this symptom is not clear to me. Is the arduino not being powered by anything when you apply this 'measurement voltage' to the analog input pin ? If that is the case you need to not do that, because it's not safe to apply active voltage sources to a unpowered arduino board.

Lefty

Thanks Lefty. That was the case, and that's great to know.

Look up "back powering AVR" on the google.

Common problem people run into: they want to use Analog-In to measure a battery's voltage, so they connect a wire directly from the battery to an analog-in pin. When power is cut to the Arduino, the board is still active through that connection.

More generally pretty much all CMOS chips have input protection diodes on all the signal lines: these are meant to protect against static electricity. Applying a voltage to a signal pin on a powered-down CMOS chip will cause the power rail to be powered via the diode until it burns out... Early 4000 series CMOS chips had large enough diodes that they would appear to work OK "most of the time" if the Vdd line was open-circuit. Modern processes are so small that this is no longer the case and a milliamp or so can be enough to trash the pin's protection circuit if applied continually.

One way to reduce the chance of such accidental damage while prototyping is to connect resistors in series with the inputs, a few kOhm or so.
External protection diodes that can take much more current help - schottky diodes are best as they have a lower forward voltage than the on-chip ones.