3.3v or 5v usb to serial for breadboard arduino?

Hi all, I've read the threads about the breadboard arduinos and usb to serial adapters, I couldn't find a definitive answer to the topic.

Throughout all of the threads and all of the tutorials for building a breadboard arduino the part quoted for usb to serial is always a 3.3v whatever the chipset is. I have at least 3 different usb to serial adapters here, an ftdi 232R (5V) cable, a couple of prolific 2103's? and a moschips 7720 based dual serial port adapter. I expect the ftdi cable is most likely to work, I haven't tried it yet, I did try the moschips cable and got sync errors when trying to upload, I can confirm that the breadboard arduino is working ok though as I can re-program it in my diecimila.

As I understand things, the TX/RX from the atmega 168 pins are just re-purposed GPIO pins, so as such aren't they at 5V? which means the ftdi cable at least should work, the moschips hopefully, but seems to be a slightly more obscure chipset, not sure how well its been implemented and I know from experience that usb to serial adapters are flaky.

If 3.3V is essential then can I use a logic level converter between the TX/RX pins on the atmega 168 and my target usb to serial device?

Regards, Reggie.

The TX and RX pins are at the VCC level of the '168.

You can use 5V tolerant logic (LVC or AHC series) to perform the level shifting. Some people just use a resistor divider to perform the attenuation.

On a side-note -- if you buy more FTDI cables only buy the 3V3 versions. The 3V3 cables will work with either 5V or 3V3 logic without level shifting.

(* jcl *)


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I am feeding the arduino via a 7805 5v 1A voltage regulator, as such I expect VCC to be at 5V so I should be fine using any 5V cable on 168 chip.

As for future ftdi cables I'm trying to avoid buying any more, the ones I have were for other specific purposes before i started with the arduino, the ftdi in particular had to be 5 volts.