avr - TTL RS-232 current limiting

I'm making software UART (TTL levels) on ATtiny85. Need I connect current-limiting resistor to output (TX) port? If "yes", what value should I use?

I plan to connect my device to a UART-USB converter (on PL2303, FT232, etc.).

Answer

No, you can connect the ATmega directly to the FT232 etc.

In general, a TTL or CMOS level output does not need a current limiting resistor when connected to an equivalent input. Reasons to include a current limiting resistor can be

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  damage limitation when the output is accidentally connected to ground, power, or another output

  
  


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  damping of oscillations & limiting edge speed, both mainly for relatively long connections (not likely to be needed for the design of average PCBs)