DisablingAutoResetOnSerialConnection for Duemilanove

Hi I am having auto reset problems when connecting to a Mac using Windows 7. I found this way(s) to disable auto reset on connecting the USB cable. This appears to only deal with the ATmega168 and Diecimila.


Will any of these methods work on a Duemilanove? and could this create any problems. My system works fine on a PC with Windows XP and also Windows 7. Thanks Martin

Yep. The only difference between the two boards (I think) is a new atmega and some other small stuff- basic idea is the same.

Be adventurous! Try it! You can't hurt anything!

An alternative to a resistor of a fairly specific value, is a small electrolytic capacitor in a wide value range (1uF - 47uF) between Gnd & Reset. So long as its bigger than the autoreset capacitor its value doesn't really matter. If its too big, it will delay the startup of the arduino when powering up due to the capacitor charging through the pull-up resistor in the reset circuit.

Thanks Pluggy I'll give your idea a try Martin

The hardware solution did not work on a MAC + Win 7 running a VB6 App. I found a solution propsed by "jumpjack" at (old Forum) http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1213719666/60

Using this code in VB6 solved the problem of the MAC OS resetting my Arduino on opening the USB port: MSComm1.Settings = "9600,n,8,1" MSComm1.CommPort = 4 MSComm1.InBufferCount = 0 MSComm1.DTREnable = False MSComm1.PortOpen = True

The “MSComm1.DTREnable = False” does the trick

Or use a Seeeduino board, they have a manual switch to turn on or off the auto-reset feature.


Unfortunately Seeeduino is not readily available in the Southern tip of Africa. Arduinos are stocked here.

As pluggy suggested, even as less as 0.1uF between RESET and GND worked for my boards, though I haven't tested the values less than that. I've found also that plugging between RESET and +5V do the same trick.