uUSB-PA5-II Interferes with running program

So I have a Arduino design that has been working perfectly for about a year and a half and decided that I wanted to upgrade it to include a USB port for program updates etc. Added a uUSB-PA5-II to the design permanently, which by the way is the interface I used to program the design before I decided to attach it permanently, and now with the uUSB-PA5-II connected the design doesn’t work. Well actually it’s more complicated than that. When I run the design powered up by the USB port everything works fine, but when I disconnect the USB connection and run the design on its own power, nothing!!!

After a lot of painful troubleshooting I found that if I disconnect the transmit pin on the processor everything works fine when powered up on it’s own. So any ideas on what I’m missing?

I’ve included the schematic to help visualize how everything is connected.

Thanks in Advance,

Ken Dabek
West End Recording

PDF Schematic.pdf (48.4 KB)

Is the USB serial adapter being powered if USB is not connected? If not you may be using it outside of spec - most devices do not like voltages applied to data pins that exceed the supply voltage (which is nominally zero if the device isn't powered...)

What does your sketch look like? My guess would be that it's trying to do something with that pin, and having trouble because it's fighting against the dead weight of a back-powered USB serial adapter...

The USB serial adapter is powered when the unit is not connected to a USB cable. I'm now suspecting that it might be that in the original design, before adding the USB Serial converter, I did nothing to initialize the Rx and Tx ports because they didn't matter, but now that they are connected they may be influencing the behavior of the sketch.

Is there a common method of initializing the ports to ignore them until needed? That's what I'm exploring now.

Also you asked to see the sketch but I'm not sure how best to do that.