Reset Problem when USB is disconnected

Hi all,

a newbie question:

i have a problem using my Arduino NG without my computer : i can put my programs onto Arduino and it works fine while the USB is plugged.

However if i connect the Arduino alone to a separate power source, - jumper switch to external, - power led is ok, - the led pin 13 blinks for a start, but then waits forever and never blinks again, (and the program never starts), unless i plug my computer (even if not sending any data!!).

I can make the board work without the computer only if i it started with the USB (but powered separately) and then i can remove the USB cable.

it should be possible to start the board without the computer plugged as i read on the bootloader page in arduino website.

any suggestions ?


Yea, this is the unfortunate result of the way the circuit is laid out. When the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground or +5V with a 10K resistor (or connecting it to the TX pin).

Yes! that was it.

It works now when connecting the Rx to Tx or to the ground. Thanks!

Was this written somewhere that i missed in the Arduino booklet or in some other source you recommend reading ?

It's on the troubleshooting page:, though admittedly not so easy to find.

Oh, that's something to make sure people know or put a jumper and resistor on for the next revision. I had the same issue and was worried I'd not be able to use my Arduino standalone.

I think this will be fixed in the Diecimila (the next revision of the hardware - coming soon). The circuit will be modified to correct this, and the bootloader will probably also activate the internal pull-up on the RX pin.