[TriEmbed] Is there a FET trick for this?

Shane Trent shanedtrent at gmail.com
Wed Dec 2 09:31:02 CST 2015

I typed so slow I missed Brian's response! What he said!


On Wed, Dec 2, 2015 at 10:29 AM, Shane Trent <shanedtrent at gmail.com> wrote:

> Chip,
> 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.
> Shane
> 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/


A blog about some of my projects.  http://fettricks.blogspot.com/
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