Arduino Nano USB to 3.3V TTL (serial)

Using Arduino Project Guidance
1

My understanding is to tie the reset pin to ground, to make it into a USB to Serial adapter. What's required to make it a 3.3V signal level. I'm trying to re-flash a Gotek Floppy Emulator to work in an Amiga 500.

2

A 5V input will generally work with a 3.3V output so RX is covered. You can use a resistor voltage divider for the TX pin: Pin to 5K resistor to 10K resistor to Ground. Take your output from the point between the two resistors. You can just use two 10k resistors (instead of 5k and 10k) to get a 2.5V output which should work fine on a 3.3V input.

3

A few questions:

  1. Can I get away with only using the voltage (5V) from the USB port (PC USB port) or should I supply the nano with an external 5V power supply?

  2. How does connecting the reset to ground turn it into a USB to 3.3V TTL (serial) converter/adapter?

  3. In making a USB to 3.3V TTL adapter shouldn't there be software loaded into it? When it's connected to a PC (USB port) how does Windows (Windows XP) detect it and assign a com port?

  4. Are there two way to make a USB to TTL adapter (reset to ground with no software or with no reset to ground jumper with software)?

  5. By default it's at 5V signal level?

  6. What is the signal level on a PC serial port?

4

  1. Can I get away with only using the voltage (5V) from the USB port (PC USB port) or should I supply the nano with an external 5V power supply?
    As long as you don't need more than 500 mA the USB power should be plenty.
  1. How does connecting the reset to ground turn it into a USB to 3.3V TTL (serial) converter/adapter?
    That only works with ATmega328P-based Arduinos that have a separate on-board USB to Serial chip. That trick won't work on the Micro where that main processor connects to USB directly. I was not familiar enough with the Micro to recognize that your original information was false.
  1. In making a USB to 3.3V TTL adapter shouldn't there be software loaded into it? When it's connected to a PC (USB port) how does Windows (Windows XP) detect it and assign a com port?
    Yes, for the Micro you need a sketch that copies characters from Serial (USB) to Serial1 (TTL Serial) and vice-versa.
void setup() {
    Serial.begin(19200);
    Serial1.begin(19200);
}
void loop() {
    if (Serial.available())
        Serial1.write(Serial.read());
    if (Serial1.available())
        Serial.write(Serial1.read());
}
  1. Are there two way to make a USB to TTL adapter (reset to ground with no software or with no reset to ground jumper with software)?
    Use Reset to Ground on the UNO and other ATmega328P Arduinos. Use the bridge sketch for Micro, Leonardo, and other ATmega16U2 Arduinos.
  1. By default it's at 5V signal level?
    Yes. The Arduino Micro runs on 5V. The USB input provides 5V. The VIN power input of 7 to 12V is regulated down to 5V. If you connect an external 3.3V supply to the +5V pin (and disconnect USB) you can run the Micro at 3.3V and thus use 3.3V signal levels BUT the chip is not rated for 16 MHz on less than about 3.8V. Technically you should run it at 8 MHz when providing only 3.3V but in most cases it will still run at 16 MHz.
  1. What is the signal level on a PC serial port?
    The 9-pin (or older 25-pin) PC serial port uses RS232 signaling levels: -12 to +12V. The logic is also inverted: -12 is HIGH and +12 is LOW. Look at the schematic for the Single Sided Serial rev3 (SSS3) Arduino board for how to connect a 5V ATmega chip to a PC serial port.