Simple Question: Connecting Mac with 3.3V FTDI USB Cable to 5V Breadboard
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:


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.