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.).

  • \$\begingroup\$ You don't need a current limiting restistor UNLESS there is any specific reason for it. Do tell us more about what is (possibly) connected to your output pin. \$\endgroup\$ Commented Nov 18, 2012 at 13:08
  • \$\begingroup\$ @Wouter van Ooijen: I edited my question. \$\endgroup\$
    – user54579
    Commented Nov 18, 2012 at 13:24

1 Answer 1


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

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

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


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