Arduino powered via serial interface?

Using Arduino Project Guidance
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Folks,

I was have a atmega just listening to the serial port and printing this to a lcd, its powered by some external 5V. Now in order to test this
I programmed the arduino to output text to the serial board, then connected the RX/TX lines to the atmega and GND to the GND of the
atmega. To my surprise the arduino started up and issued the text to the serial port. Note that the arduino itself had no external power
and was also not connected to USB!

So does where does the arduino get his power from? Only possibility are the RX/TX lines. I am puzzled.

Thanks for any explanation,
Heiko

2

I think that devices that use very little power sometimes can be powered via a serial port rts/dtr pins. The X10 "firecracker" is such a device.

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When the IO pins have signal on them and VCC is not powered, the input clamp diodes can be forward biased and leak power into the chip to power it somewhat.

4

CrossRoads:
When the IO pins have signal on them and VCC is not powered, the input clamp diodes can be forward biased and leak power into the chip to power it somewhat.

I assume this is bad and we should avoid it? I kind of sort of left mine in this state for several hours... the AVR has no "apparent" damage, so maybe I got lucky?

5

"this is bad and we should avoid it" Yes - data sheet says IO pins should not exceed Vcc by more than 0.5V and not go below Gnd by the same amount. If you had had some LEDs or something connected up, you could have damaged those diodes.
So maybe lucky indeed.

6

Technical term: Phantom power

Good when used by design, not good at all in the case of powering digital chips through the clamp diodes. Wikipedia has a good writeup for both analog/digital phantom currents.

Ray

7

"Phantom power" 48V used to power condensor microphones:

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