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Archive for December 18th, 2009

Arduino Pro: Power Adaptation for FTDI Basic USB

Arduino FTDI Basic on modified Arduino Pro

Arduino FTDI Basic on modified Arduino Pro

Some time ago, I bought a 5 V Arduino Pro board (about which you read earlier there) and a nominally compatible FTDI Basic USB-to-serial adapter. Turns out that they’re not quite a perfect match, although they do play nicely together in normal use.

The FTDI Basic board produces a 3.3 V regulated output voltage that’s connected directly to the output of the Pro’s 5 V regulator. This doesn’t cause any particular problem, but one side effect is that you can’t shut the board’s power off: the USB power will keep the CPU alive, more or less.

You should, of course, use a 3.3 V FTDI Basic board with a 3.3 V Pro, which would at least put two similar voltage sources head-to-head.

The Pro is using a backup power supply that, for reasons that make perfectly good sense, backfeeds the Pro’s 5 V regulator: when the +12 V main supply Goes Away, the backup power supports VCC directly, rather than through the regulator. The regulator can take a joke like that, as witness the FTDI vs Pro situation; in my case, a diode isolates the two supplies in normal operation.

For reasons that I don’t completely understand, some combination of voltage to the Pro regulator and the (diode isolated!) backup support voltage caused the FTDI chip to lock up with both TX and RX LEDs on solid.

I suspect the FTDI chip’s internal 3.3 V regulator, in combination with the USB +5 V supply, in combination with the Pro board power, drove something outside its normal operation range. So I simply removed the 3.3 V pin from the connector, disconnecting that supply from the Pro’s overvoltage, and the thing now works fine.

Side effects:

  • The FTDI board remains powered when the Pro board gets turned off, thus preventing Linux from changing the serial port device when the power comes back up again
  • I can actually turn the Pro power off, without having the FTDI supply keep it alive. Handy for soldering!

The Pro pin labeled GND connects to the FTDI CTS line, an input that floats high when not connected. I yanked that pin and shorted CTS to GND on the FTDI board: one less pin to worry about, for reasons that you’ll see tomorrow.

There are many different versions of the boards and USB adapters, so current production probably doesn’t match what I have. Pay attention to what you have, though…

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