[TriEmbed] TriEmbed Digest, Vol 31, Issue 2

Shane Trent shanedtrent at gmail.com
Wed Dec 2 10:41:15 CST 2015


Chip,

The Vcc rail is already powered if the Arduino is running (unless you are
powering the Arduino from something other than Vcc?). The Arduino cannot
drive the output pin above Vcc. If you have voltage sags you may need more
or better bypass capacitors and or heavier traces on the pcb. But I expect
that a voltage sag on Vcc would drive down Vcc of the Arduino which would
then lower the output voltage on the I/O pin. The output pin voltage is
ideally a high percentage of the Arduino Vcc but must trace with Vcc.

I believe it would take almost catastrophic conditions to cause an Arduino
output pin to reach a full-diode drop above the Arduino's Vcc. I believe
your best bet would be a dead short from Vcc to GND at the diode output and
then it it is a footrace on discharging the bypass cap via the pcb trace to
the short vs the I/O pin trace and diode.

Shane



On Wed, Dec 2, 2015 at 11:28 AM, Chip McClelland <chip at mcclellands.org>
wrote:

> 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> 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> wrote:
>>>
>>>> Send TriEmbed mailing list submissions to
>>>>         triembed at triembed.org
>>>>
>>>> To subscribe or unsubscribe via the World Wide Web, visit
>>>>         http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>> or, via email, send a message with subject or body 'help' to
>>>>         triembed-request at triembed.org
>>>>
>>>> You can reach the person managing the list at
>>>>         triembed-owner at triembed.org
>>>>
>>>> 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
>>>> -------------- next part --------------
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>>>> http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20151202/f51de483/attachment-0001.html
>>>> >
>>>>
>>>> ------------------------------
>>>>
>>>> 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
>>>> -------------- next part --------------
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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
>>>> -------------- next part --------------
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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
>>>>
>>>> _______________________________________________
>>>> TriEmbed mailing list
>>>> TriEmbed at triembed.org
>>>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>>
>>>>
>>>> ------------------------------
>>>>
>>>> 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
>>> 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
>



-- 

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