Arduino Board Gets powered up from GPIO pin connected to another Arduino

We are trying to connect two Freeduino’s with ATMEGA328P chipsets.
Please see the attachment for pictorial representation.

Connections
Device 1 is powered by USB.
Device 2 is not connected to any power source.

A0 of device1 is INPUT_PULLUP, and connected to A0 of device 2.
A1 of device 1 is OUTPUT,set to HIGH and connected to A1 of device 2.
GND of device 1 is connected to GND of device 2.

Issue
Device 2 is getting powered up as soon as Device 1 is powered up.

Queries.

  • Is this normal, if not what are the solutions to avoid this.
  • Will Device 2 get damaged due to this.

Is this normal,

Yes

Will Device 2 get damaged due to this.

Yes It is called parasitic powering and can cause latch up and damage the unpowered device.

what are the solutions to avoid this

1) Do not apply active signals to an unpowered chip.

If you must then an opto isolator will prevent the parasitic powering but it will not work with analog signals. A cheaper but less effective solution is to have something like a 10K resistor in series with the signal lines.

Or keep A0, A1 low until the 2nd Arduino is powered up. Or connect 5V header pins together.

Thanks Grumpy_Mike, CrossRoads for the quick response.

My understanding is all the devices should be powered up together using single power source.
We tried to connect 4 devices, one of which will take inputs from the other 3 devices, all 4 connected to a single power source in parallel, powersupply capable of providing enough power(2A, 5V)

Device getting input from other 3 devices was seen to be rebooting randomly.

We will try the 10k resistor method/opto isolator.