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I have some 3.3V Arduino Pro Mini boards and they require either an FTDI cable or breakout board to be programmed from a USB port.
So far I've been using a board, but it's annoying to have to either solder headers in place or hold the boards carefully every time I make a modification to the code. Also, switching between 3.3V boards and 5V requires resoldering traces on the FTDI board.

The process goes like this:
- ensure FTDI board is soldered for the correct voltage, resolder if necessary.
- connect FTDI board to arduino pro mini.
- plug mini usb cable into FTDI board.
- carefully hold arduino and FTDI board in place.
- click upload in the Arduino IDE.

Then everything works.

I am aware that FTDI cables don't always support 'auto-reset', but I've never had to manually reset my Arduino in order to load new code.
When does one have to press the reset button if using a cable?
before uploading?
afterwards?
What's the deal here?

Also, can I use a USB FTDI cable to load 3.3V and 5V boards?

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  • \$\begingroup\$ The board has the circuit on the board with a generic cable and FTDI cables have the circuit board built into the cable on a connector. \$\endgroup\$ – kenny Jun 26 '12 at 4:14
  • \$\begingroup\$ @kenny: Thanks for taking the time to reply, but your comment doesn't answer the question which is what is the practical difference? Namely, how or when do I have to reset the board when using a cable, and which board voltages does a cable support? \$\endgroup\$ – M_M Jun 26 '12 at 11:41
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I keep one FTDI cable around that I've terminated with breadboard pins for quickly connecting to, well, a breadboard! so I don't have a cable always hanging off my prototypes. I haven't bothered to implement auto-reset; I just use a reset button on the board. The timing that almost always works for me (in Eclipse) is to hold the reset button and release it just as I click Eclipse's download button. (Maybe someday I'll make a tiny PC boards to arrange all four pins into a pseudo BB-plug....)

This cable came with NerdKit AVR kit that was 5v based. So are my BB designs, currently. The FTDI head is on the USB-A male end; I just measured the BB end at 5.09v with no load (except, obviously, the meter). I think you only need to provide the appropriate regulator - 3.3v or 5v - as long as the input to it is higher than that (within reason). I power BB's with a 9v battery when I'm not using the cable.

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  • \$\begingroup\$ So am I to understand that I could plug a 3.3V Arduino pro mini into a 5V FTDI board without frying it? (in which case what's the point of the 3.3V board?) Is the V+ of the FTDI not wired directly to the Vin of the board? I didn't think it goes through the regulator. Obviously I'm reluctant to test this for fear of frying the board. \$\endgroup\$ – M_M Jul 2 '12 at 15:48
  • \$\begingroup\$ No, you wouldn't want to put 5V on the Vcc rail of a 3.3V Arduino; only put that on its RAW input. The Arduino site has this description of the ProMini - see "Power" halfway down the page. Consider your FTDI's 5V output to be unregulated as far as a 3.3V chip is concerned. \$\endgroup\$ – JRobert Jul 2 '12 at 21:00
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If you have a cable without auto-reset (or a older board) you need to hit the reset button before uploading the new sketch.

It will depend on the cable what voltage they support, sparkfun for instance has some that claims can do both. And this is from sparkfun's page

These have 5V VCC, and 3.3V I/O. The 5V Vcc output shouldn't be a problem unless this cable is being used to power a sensitive circuit such as a sensor.

There are pros and cons to the FTDI Cable vs the FTDI Basic. The FTDI Basic has great LED indicators, but requires a Mini-B cable. The FTDI Cable is well protected against the elements, but is large and cannot be embedded into a project as easily. The FTDI Basic uses DTR to cause a hardware reset where the FTDI cable uses the RTS signal.

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  • \$\begingroup\$ So am I to understand that I could plug a 3.3V Arduino pro mini into a 5V FTDI board without frying it? (in which case what's the point of the 3.3V board?) Is the V+ of the FTDI not wired directly to the Vin of the board? I didn't think it goes through the regulator. Obviously I'm reluctant to test this for fear of frying the board. \$\endgroup\$ – M_M Jul 2 '12 at 15:48
  • \$\begingroup\$ No, don't do it;) That cable in the link works with both, but not all does. \$\endgroup\$ – Jontas Jul 3 '12 at 7:28

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