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Arduino UART communication quandary

I'm trying to hook up a 5v Arduino to a 3.3v datalogger. I just realized that in all the time I have been messing around with micro controllers, I have never communicated serially to a device that wasn't 5V. I was about to just wire the two devices together when I thought better of it and figured I should look into the matter a little further. Can I just directly connect the RX pin on the 3.3V device to the TX pin on the Arduino? Or do I need to add a resistor in-between? Or a voltage divider? Actually, that wasn't all that many questions. Basically, I'm just wondering what is good form in communicating between devices of varying voltages?

randofo (author) 9 years ago
Thanks for the Sparkfun link. I think I may try their resistor setup and see how that goes. I don't think the Arduino itself can be run at 3v, but I think the chip might be able to. I'd program it, remove it from the Arduino and then run it at 3v, but I want to try to keep this project as simple as possible so that other people can easily replicate it.
westfw randofo9 years ago
There isn't much to Arduino other than the chip, but you'd need to change regulators and such, and probably run at a lower clock rate. Last time I looked at the data sheet, the ATmegaXXX wouldn't quite run 16MHz at 3V. (IIRC, Arduino lilypad does 8MHz at 3V ?)
gmoon9 years ago
Any possiblity that an Arduino can be run @ 3.3V? I've always used AVRs that would work at the voltage of the other device...

Failing that, the standard way is to use a mosfet to translate the voltage. It's in the i2c specs (maybe even SPI, too.) You can find the full specs on the i2c PDF, or here's a link with just the level shifter. (i2c is what Atmel calls a "2-wire Serial Interface")

Sparkfun has a page with some simpler current-limiting resistors (as well as the mosfet shifter), but I've never tried just the resistors. That page is here....

Good luck.