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Topic: power supply problem on Arduino NG (Read 654 times) previous topic - next topic


I'm working with Arduino NG (ATMEGA168) and it works fine when suplying power via USB cable. But when I give external power supply the POWER led is on but nothing happens (the power selector jumper is changed to EXTernal, logically). Then, if I connect the USB cable (not power, just for data) it starts the program. I can unplug the USB at any moment and the program continues perfectly, but if I press the reset button it won't start until I plug the USB again.

┬┐Why is this happening and more important, how can I make it work without the use of a USB cable?


Triffid Hunter

sounds like the ft232r is holding reset low. I think that's why they replaced the resistor from DTR to reset with a capacitor on the diecimila, and completely removed the one from RTS.

Adilson Akashi

Apr 07, 2008, 04:23 am Last Edit: Apr 07, 2008, 04:28 am by adilson Reason: 1
I'm not sure, but I think it's an old problem, with many posts.

Try to put a 10k resistor from RX (pin0) to +5v. or to GND when not using USB cable.

Atmega RX pin keeps "floating" when not using USB, that makes Atmega think there is incoming data.

PCB Designer


I've tried connecting RX to TX because I can't find a 10K resistor right now and it's working.  :)

Thank you very much.

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