[TriEmbed] Is there a FET trick for this?

Shane Trent shanedtrent at gmail.com
Wed Dec 2 09:29:44 CST 2015


If I follow, I believe you can achieve your goal by connecting the power
terminal on your incoming FTDI connector directly to your Digital I/O pin
and then connect this node to your PCB Vcc via a diode (or a Schottky diode
for a slightly lower voltage drop).

This would power your board from the FTDI through the diode when connected,
so your board is operating at one diode drop below the FTDI's Vcc. But the
UART Friend would only see power when you drive the FTDI pin's Vcc pin high
via the digital I/O pin. When powered via the I/O pin the extra diode will
do nothing since the chip is powered from VCC, the I/O pin will be
incapable of rising a diode drop above Vcc so the diode will never be
forward biased. You would want to ensure you never try to drive the I/O pin
low when FTDI cable is connected.


On Wed, Dec 2, 2015 at 8:01 AM, Chip McClelland via TriEmbed <
triembed at triembed.org> wrote:

> All,
> Down to the final stretch on my new board - just need one more piece of
> advice.
> I am trying to see if there is a way to accomplish something without
> resorting to a physical jumper. I would like to use the FTDI header on my
> new board in two ways with respect to the power pin.
> 1) when I connect a FTDI connector, it could power the board. The power
> pin is connected directly to the board's power rail. This is how it is
> today.
> 2) when I connect the Adafruit UART friend, I can turn it on and off using
> one of the Arduino's digital pins.  In this scenario, I only want the power
> pin connected to the digital pin on the Arduino.
> Is there some way to automatically switch between these two modes based on
> whether there is power supplied by the FTDI adapter?
> Thanks,
> Chip
> --
> Sent from my iPhone - please excuse the typos
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A blog about some of my projects.  http://fettricks.blogspot.com/
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