[TriEmbed] TriEmbed Digest, Vol 31, Issue 2

Chip McClelland chip at mcclellands.org
Wed Dec 2 10:28:07 CST 2015


Shane,

But then the Arduino pin is driven High can directly connected to Vcc,  how
can I make sure it does not try to power the whole board even if briefly
due to temporary voltage drops?

Chip

On Wednesday, December 2, 2015, Shane Trent <shanedtrent at gmail.com> wrote:

> Chip,
>
> About your point 2).
>
> The Bluetooth dongle would not see the diode drop, the dongle will see as
> close to Vcc as the Arduino I/O pin can drive it. The diode drop only shows
> up in your Vcc when you are powering the board from the FTDI cable through
> the diode. With the Bluetooth dongle connected, it will be like the diode
> is not there at all, unless something really strange happens.
>
> Shane
>
> On Wed, Dec 2, 2015 at 11:01 AM, Chip McClelland via TriEmbed <
> triembed at triembed.org
> <javascript:_e(%7B%7D,'cvml','triembed at triembed.org');>> wrote:
>
>> Brian and Shane,
>>
>> Thank you for your responses. Originally, I rejected the diode solution
>> for two reasons:
>>
>> 1) I thought the Arduino pin would end up connected to the Board's Vcc
>> rail exceeding the 40mA capacity but, as you pointed out, the voltage drop
>> across the diode will prevent this.
>>
>> 2) I worried the voltage drop across the diode would not provide the
>> Bluetooth dongle the voltage it needed. However, it sounds like I might be
>> able to find one with a small enough drop to fix this.
>>
>> I will give this a try and thank you for your advice.
>>
>> Chip
>>
>> On Wednesday, December 2, 2015, <triembed-request at triembed.org
>> <javascript:_e(%7B%7D,'cvml','triembed-request at triembed.org');>> wrote:
>>
>>> Send TriEmbed mailing list submissions to
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>>>
>>> When replying, please edit your Subject line so it is more specific
>>> than "Re: Contents of TriEmbed digest..."
>>>
>>>
>>> Today's Topics:
>>>
>>>    1. IEEE Internet of Things meeting tonight (Wednesday)
>>>       (Rodney Radford)
>>>    2. Is there a FET trick for this? (Chip McClelland)
>>>    3. Laser cutter/engraver (Craig Cook)
>>>    4. Re: Is there a FET trick for this? (Brian)
>>>    5. Re: Is there a FET trick for this? (Shane Trent)
>>>
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Wed, 2 Dec 2015 05:42:49 -0500
>>> From: Rodney Radford <ncgadgetry at gmail.com>
>>> To: trianglerobotics <trianglerobotics at yahoogroups.com>,        TriEmbed
>>>         Discussion <triembed at triembed.org>,  Triangle Linux Users Group
>>>         General Discussion <trilug at trilug.org>
>>> Subject: [TriEmbed] IEEE Internet of Things meeting tonight
>>>         (Wednesday)
>>> Message-ID:
>>>         <
>>> CA+kfaxFKNQLqdvYnsNqmAvbic0U7FA+yg9ffQkgOtH4ffxy5UQ at mail.gmail.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> Don't forget tonight to attend tonight's innagural meeting of the new
>>> IEEE
>>> Internet of Things IoT group on NCSU Centennial campus.
>>>
>>> The meeting begins at 6:30pm with food and drink:
>>>
>>> https://meetings.vtools.ieee.org/m/36760
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>>>
>>> ------------------------------
>>>
>>> Message: 2
>>> Date: Wed, 2 Dec 2015 18:31:29 +0530
>>> From: Chip McClelland <chip at mcclellands.org>
>>> To: "triembed at triembed.org" <triembed at triembed.org>
>>> Subject: [TriEmbed] Is there a FET trick for this?
>>> Message-ID:
>>>         <CABQO2+nHLX8x1_q9hQCL73L0bjsBW7jSm=
>>> cccrrTiZS3HZFSXg at mail.gmail.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> 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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>>>
>>> ------------------------------
>>>
>>> Message: 3
>>> Date: Wed, 2 Dec 2015 13:53:15 +0000 (UTC)
>>> From: Craig Cook <cncook001 at yahoo.com>
>>> To: "triembed at triembed.org" <triembed at triembed.org>
>>> Subject: [TriEmbed] Laser cutter/engraver
>>> Message-ID:
>>>         <364134559.2216081.1449064395581.JavaMail.yahoo at mail.yahoo.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> Just heard about this on the Embedded.fm podcast.
>>> https://glowforge.com/
>>> Looks like a great christmas present ;)
>>> Craig
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>>> ------------------------------
>>>
>>> Message: 4
>>> Date: Wed, 2 Dec 2015 10:15:53 -0500
>>> From: Brian <triembed at undecidedgames.net>
>>> To: triembed at triembed.org
>>> Subject: Re: [TriEmbed] Is there a FET trick for this?
>>> Message-ID: <565F0B29.7020501 at undecidedgames.net>
>>> Content-Type: text/plain; charset=windows-1252; format=flowed
>>>
>>> Let me see if I understand correctly:
>>>
>>> When the FTDI is connected, you want power to flow FROM the FTDI adapter
>>> TO your board.
>>>
>>> When the Adafruit doohickey is connected, you want power to flow FROM
>>> your board TO the adapter (the opposite direction) by the same pin.
>>>
>>> If that's it, a simple diode between the pin and your board's power rail
>>> is all you need.  Oriented to allow current to flow from the connector
>>> to the power rail and not the other way.  Connect both the connector pin
>>> and the Arduino pin to the anode of the diode, and connect the power
>>> rail to the diode's cathode.  If the connected board is supplying power,
>>> the diode will be forward-biased and will conduct.  If the board is
>>> receiving power, the diode will be reverse-biased and will not conduct.
>>>
>>> Make sure the diode is rated for the amount of current that will flow in
>>> from the FTDI adapter.
>>>
>>> Crude attempt at ascii art:
>>>
>>> [FTDI?] ----+---[>|---- +V rail
>>>              |
>>>              -
>>>              ^ [optional]
>>>              |
>>>           arduino pin
>>>
>>> [FTDI?] represents the connector pin.
>>>
>>> The optional diode is if you want to prevent current flowing into the
>>> arduino pin from the FTDI adapter, but is unnecessary unless the FTDI
>>> adapter supply voltage is too high for the pin.
>>>
>>> Ordinary diodes do impose a nominally 0.7V voltage drop when in
>>> conduction, but you can shop for a Schottky diode with a lower voltage
>>> drop if that will be a problem.
>>>
>>> Hope this helps!
>>> -B
>>>
>>>
>>>
>>> On 12/2/2015 8:01 AM, Chip McClelland via TriEmbed 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
>>> >
>>> >
>>> > _______________________________________________
>>> > Triangle, NC Embedded Computing mailing list
>>> > TriEmbed at triembed.org
>>> > http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>> > TriEmbed web site: http://TriEmbed.org
>>> >
>>>
>>>
>>>
>>>
>>> ------------------------------
>>>
>>> Message: 5
>>> Date: Wed, 2 Dec 2015 10:29:44 -0500
>>> From: Shane Trent <shanedtrent at gmail.com>
>>> To: Chip McClelland <chip at mcclellands.org>
>>> Cc: "triembed at triembed.org" <triembed at triembed.org>
>>> Subject: Re: [TriEmbed] Is there a FET trick for this?
>>> Message-ID:
>>>         <
>>> CAAFjd3rLUnu7yEoCyOjVsbejMFUmAucAfH0FkNZuTwGCHXOoLQ at mail.gmail.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> 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
>>> >
>>> > _______________________________________________
>>> > Triangle, NC Embedded Computing mailing list
>>> > TriEmbed at triembed.org
>>> > http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>> > TriEmbed web site: http://TriEmbed.org
>>> >
>>> >
>>>
>>>
>>> --
>>>
>>> A blog about some of my projects.  http://fettricks.blogspot.com/
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>>>
>>> Subject: Digest Footer
>>>
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>>> TriEmbed mailing list
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>>> ------------------------------
>>>
>>> End of TriEmbed Digest, Vol 31, Issue 2
>>> ***************************************
>>>
>>
>>
>> --
>> Sent from my iPhone - please excuse the typos
>>
>> _______________________________________________
>> Triangle, NC Embedded Computing mailing list
>> TriEmbed at triembed.org
>> <javascript:_e(%7B%7D,'cvml','TriEmbed at triembed.org');>
>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>> TriEmbed web site: http://TriEmbed.org
>>
>>
>
>
> --
>
> A blog about some of my projects.  http://fettricks.blogspot.com/
>


-- 
Sent from my iPhone - please excuse the typos
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