Arduino Mega Analog Power Problem

Other Hardware 3rd Party Boards
1

Hello there,

For a project I am working on I am taking in analog signals with an arduino mega (ATMEGA 2560) analog pins (A1-11), but for some reason these inputted analog voltages are pseudo powering my board (no on-board LEDs are turning on, I reset the device so it should have no code installed) and causing all 54 digital pins to output 5 volts.
The board is not hooked up to any power supply, and is only connected to these analog input pins, each inputting at max a 5 volt analog signal. Voltage out of digital pins climbs to 5 volts after there is about 4 analog pins at 5v present, otherwise it follows a nonlinear escalation to 5 volts.

Is this normal for an arduino Mega? Or is my board compromised/defective? It is an off-brand Sunfounder model but seems to have the same architecture and uses the ATMEGA 2560.

Thanks for any help.

2

It’s not defective, it’s the way the the chips work. You must power the board if you apply voltages to input pins. You can damage the microprocessor without power on the board. There are ways to avoid damage if you cannot remove input voltages but there are some restrictions/limitations.

It’s a well known and understood behavior, google ”avr phantom power”.

A video if you’re so inclined: EEVblog #831 - Power A Micro With No Power Pin! - YouTube