Is the USB connector useless if I use a wall wart?

I got a new Arduino Deminueve. If I have already loaded my program into the Arduino through the USB port, then disconnected the Arduino from my computer and power the Arduino with a wallwart or battery, then can I use the USB connector/port for anything else? for example to communicate with some other non-USB device using pin 2 and 3 in the USB port (according to the Arduino schematic)? or for some other control purpose?

Thanks in advance for any comments or suggestions.

You could use pins 0 and 1, which are on the UART. They are wired to the USB port, but you could use them for TTL level serial communication to some other peripheral, or just use 'em for digital pins.

Of course, whatever you plug in to 0 and 1 may have to be unplugged to allow you to upload a new sketch.


Thanks kg4wsv, for your response.

If I’m reading the Arduino schematic correctly, the USB connector pins are labelled 1,2,3,4. There is no pin 0, so I’m not sure which one you are referring to. I’m not very familiar with the FT232RL chip either. If I wanted to control the pins that you mentioned from a program, how would I do that? Or do these pins simply have the serial data signals already on them so that the usual TX and RX program commands work ? I do notice that RX/TX appear to be on pins 1 and 5 of the FT232RL. (Sorry if my questions don’t make sense - I am at the limit of my knowledge.)

Thanks again for any further comments.

Once you disconnect from the PC usb port the FT232RL chip can no longer be used by the Adunio. However you should still be able to use digital pins 0 & 1 which are the ones that wire from the AVR chip to the FT232RL. That make sense?


The two power pins of the USB are connected to supply power to the Arduino and its peripheral electronics.

The two data pins of the USB are essentially the same data as the Digital I/O Pin 0 and Pin 1, also labeled RX and TX on your Arduino board.

If you use Serial.print() function in your sketch, the printout goes to the TX pin and also through the USB data connection to your PC.

If you use the function in your sketch, it's expecting a fast-toggling signal on the RX pin, and will give you a whole byte of data at a time.

(Of course, if you wanted to rip apart the USB cable, you could send those data lines to any device, not just USB ports.)

Thanks for those who responded. Maybe what I'm thinking doesn't make any sense, but the reason I asked the question is because I'm trying to use the Arduino with a wall wart as input to a MAX233 TTL/RS232 converter. My thought was that I could just unplug the USB cable from the computer after loading the program and then plug the USB cable into the converter (which I haven't made yet - I'm just thinking about it) and use just the data lines in the USB connector, then I wouldn't have to make an additional connection to the serial lines 0/1. Of course, my converter would have to have a USB connector, but it wouldn't be a conventional USB device. I am sure that what I want to do CAN be done with the serial lines TXD and RXD (on the schematic) and there are a lot of tutorials about that. But I was thinking that I might be able to avoid making a connection to those pins if I could just use the data lines of the USB connector. Or does that make no sense for some reason?

Thanks again for your comments.

My thought was that I could just unplug the USB cable from the computer after loading the program and then plug the USB cable into the converter

Nope: the USB data only goes to the FT232 chip, which acts as a "translator" between the rather complex USB protocol and simple serial interface of the AVR chip.

That's why I have boards like the MaxSerial and RBBB, instead of the Duemillanove: I don't use them much as "PC peripherals", so the USB interface is only useful for downloading to me. It's far more likely that I'll want to connect to some serial device, and the FT232 would just be in the way.


Or does that make no sense for some reason?

As RT posted this won't work. The FT232R is a 'magic' chip that is UART compatable serial data on the Arduino side but a USB device on the PC side, so you can't use the data pins on the USB connector as a serial data stream because it's not serial data there but rather a USB client chip that can only wire to and talk a USB host controller.


Thanks to all - that answers my question. I will plan on using the conventional serial pins.