Tx Rx resistors

Looking at the schematic for the arduino nano, I’m wondering why they have placed two resistors in series with Rx and Tx? I checked the atmega328 datasheet and it seems there’s no need for resistors on USART. Could someone please explain why they have been placed there?


It just adds some protection, without it I could program the AVR to self-destruct by pulling the AVR's UART RX pin low when the TX from UART on USB bridge pulled high.

without it I could program the AVR to self-destruct

Uh, no. They protect the FT232 if a user improperly connects those pins to external I/O. The AVR is unprotected as the I/O points D0/RX and D1/TX do not go through resistors, they are direct connections to the atmega328.

The same resistors appear on the Uno schematic between the atmega328 processor and the atmega168 used as the USB convertor.

When the FTDI TX is high and a user program tells the ATmega pin (e.g. D0/RX) to pull low then it is a good idea to have some way to limit the current, even if there are no other connections to off-board stuff.