My UNO board appears to have a fairly strong pullup on the PD0 pin. I configure the pin as an input, no pullup enabled, and put a 4.7k resistor to ground. The voltage only drops to 4.12v from the 5.0v when open. From the schematic this pin is shown connected through a 1k resistor to the TXD pin of the interface processor. Is this being driven even if I have not enabled the serial? Any way to shut it off?
It's being driven high by the serial adapter. Can t get rid of that without modifying the hardware.
I recommend using a pro mini if you need the serial pins for something else - pro mini doesnt have serial adapter built in, so you just disconnect it from serial adapter when not programming it
Thanks for the help. The serial adapter is an atmega processor using gpio pins to drive the serial link. It would be be possible to shut off the driver, but would need firmware support to do it,and some way to command it. Oh well, I'll work around it.