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?
Connecting a 5V signal to an input that expects VCC + 0.6V at most, is not a good idea. You will likely kill it.
- Level translation IC or voltage divider or transistors, to bring the ATMega's 5V TX down to 3.3V.
- Power the ATMega at 3.3V. Keep in mind that this will require a slower clock speed.
- Get a USB to 5V UART cable.
- 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.