For a specific application reason, I need to prevent the following thing from happening:
Rather than requiring a physical press of the reset button before an upload, the Arduino Uno is designed in a way that allows it to be reset by software running on a connected computer. (...) This setup has other implications. When the Uno is connected to either a computer running Mac OS X or Linux, it resets each time a connection is made to it from software (via USB).
The reason is that my particular project will require the Arduino USB cable connected to a PC at all times, and there is also a particular piece of software on the PC which scans the serial ports for GPS's periodically. Each time this scanner software hits the Arduino, it resets. I need the Arduino not to reset in this case.
I am aware that I can scrape away this trace:
The Uno contains a trace that can be cut to disable the auto-reset. The pads on either side of the trace can be soldered together to re-enable it. It's labeled "RESET-EN".
I have successfully been able to scrape away this trace, and it works. However, this is a very fiddly thing to do that is not a pleasant procedure to undertake. And it is my intention to build several identical devices, each with their own Arudino Uno inside, that need this auto-reset feature disabled.
I could scrape away the pad on every one of the Arduinos, but, since I'm in the middle of the design of the PCB that will attach to the Arduino, I thought, maybe there is a way to do this much easier in my PCB design without having to scrape away traces on the Uno board. If I could just incorporate this into the design of the PCB, then the assembly of several of these devices would be much much easier, without any trace-scraping required.
Sure enough, there is this quote here:
You may also be able to disable the auto-reset by connecting a 110 ohm resistor from 5V to the reset line; see this forum thread for details.
Aha! So I follow the link to that forum thread, and it says it should be as simple as connecting a 110 ohm resistor between the 5v pin and the reset pin. I can only assume it means the header connectors labeled "5v" and "Reset" on the board, the ones just above the Vin and the two Gnd pins.
Now, as mentioned in the thread, 110 ohm is a fairly rare value, and the local Radio Shack didn't have any on hand. So I got a pair of 220's and twisted them together, side by side in parallel, which, if I remember my theory correctly, should halve their value, thus making the resistance 110. I connected those to the 5v and Reset pins, and....
... it still resets when a piece of software connects to the USB port.
What am I doing wrong?