I'm making my own board and using an ATmega 328 with the Arduino bootloader. I have a DIP switch to select either an FTDI chip (for programming) to be connected to the ATMega's Rx and Tx, or a GPS that outputs serial to be connected. I was looking at this schematic for reference: http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf
Why are there 2 resistors on Rx and Tx coming from the ATMega? Do I need those just for the connection to the FTDI chip, or do they need to be there for the GPS too?
Answer
One of them is there to prevent damage that could occur if the AVR has RxD programmed as an output, pins on both devices could be damaged if that happened as AVR pins can source and sink quite a lot of current. I don't think that the other resistor is necessary.
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