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I have an Arduino UNO, and use the serial for debugging purposes. What happens if I run the arduino on batteries and try to write to serial? Will the code stop running until a serial connection is availible?

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    What will happen is the data will dribble out of the unconnected port and drip onto the floor. Data stains can be a real pain to get out of carpet, so put some newspaper down first. – Majenko Jun 6 '15 at 12:18
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No, that will not cause any problem.

What might hurt you is the fact that if you want to leave your Arduino in low power mode, but keep the serial potr disconnected and the serial port is a wakeup source, it might pick up sporadic (but frequent enough to cause be noticeable) noise that will trigger wakeups and spam with meaningless data your RX.

This, however, is not specific to the serial port, but it can happen to any wake up source that is enabled and left dangling.

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    It's not left dangling. The RX and TX are connected to the ATMega16u2 that does the USB-to-Serial conversion. – Gerben Jun 6 '15 at 12:17
  • ok. Actually there is a way to disable the USB to serial converter. If you short its reset pin, like in the procedure required to update its firmware with dfu utils, it should do in high impedance mode (I think). – Igor Stoppa Jun 6 '15 at 13:51
  • I seem to recall the Arduino core or bootloader code activating an internal pullup on the serial input line anyway. – Chris Stratton Jun 6 '15 at 16:37

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