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I'd like to connect my iMac to a breadboard with a 3.3V TTL FTDI USB cable. The breadboard has an Atmega328 AVR microcontroller powered by a 5V regulated power supply. Sorry, if this is a naive question, but is it okay to have the cable powered by the iMac (I guess) at 3.3V and the breadboard running at 5V? Is that going to fry something? Do I have to have the breadboard running at 3.3V, too?

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  • \$\begingroup\$ Why did you choose a 3.3V TTL connection if you're going into a 5V microcontroller? But to answer your question: I have used similar FTDI cables, and you don't need to connect the power cable as both sides of the interface will be powered (one from your PC, the other from the regulator), just make sure the ground is connected. The FTDI cables are quite rugged. Whether the Atmega can communicate straight with a 3.3V TTL link is a different question for someone who has read the datasheet. \$\endgroup\$ – Puffafish Mar 15 '16 at 17:15
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Connecting a 5V signal to an input that expects VCC + 0.6V at most, is not a good idea. You will likely kill it.

Options are:

  1. Level translation IC or voltage divider or transistors, to bring the ATMega's 5V TX down to 3.3V.
  2. Power the ATMega at 3.3V. Keep in mind that this will require a slower clock speed.
  3. Get a USB to 5V UART cable.
  4. Hack your cable to make it 5V. You need it's schematic and an understanding of how it works.

This is also assuming you actually have a FTDI based cable and not a similar cable with a different IC, or a fake FTDI IC, which is very very common in the last few years.

There is some issue with a genuine FTDI powered at 5V VCC, with its UART at 3.3V VCCIO being okay with 5V in, but there are too many ifs and buts to give a simple answer.

Easiest thing, just get a cable meant for 5V Uart.

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    \$\begingroup\$ This would be correct if the FT232 is actually powered by 3.3v. Much more common is for the supply to be the USB 5v, but VCCIO to be set to 3.3v. The voltage tolerance is relative to VCC (which would in that case be 5v) and not to VCCIO - in other words, it's not random that people get away with using the same "3.3v" cable on both 3.3v and 5v systems. However if VCC (and not merely just VCCIO) is at 3.3v, then yes you are right. \$\endgroup\$ – Chris Stratton Mar 15 '16 at 17:48
  • \$\begingroup\$ @ChrisStratton yea, the data sheet confirms that. Didn't know that before, but I also haven't seen many people saying they used a 3.3V cable on a 5V UART. Also can't be sure OP has a genuine FTDI. It's simpler just to get a cable made for 5V. \$\endgroup\$ – Passerby Mar 15 '16 at 17:56

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