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TheNextGuy
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Location: Washington State
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How to keep Pi 3 off when power is applied

Thu Nov 28, 2019 6:35 pm

Is there a pin somewhere on the Pi 3B+ where you can hold low or high to prevent the bootup sequence from starting? I understand that when you turn the Pi "off" it really just puts it into this standby mode, which is fine. That's what I want.

hippy
Posts: 6720
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: How to keep Pi 3 off when power is applied

Fri Nov 29, 2019 2:09 pm

If you keep the RUN header pin shorted to 0V while turning the Pi on I believe that should do what you want, delay it from booting until the RUN pin is disconnected from 0V.

You can also short the PEN pin header to either 0V/3V3/5V - you'll have to check which first - which should stop the PMIC from enabling, supplying power to the board, until that is disconnected.

I have not tried either myself.

MarkDH102
Posts: 377
Joined: Fri Feb 13, 2015 3:18 pm

Re: How to keep Pi 3 off when power is applied

Fri Nov 29, 2019 4:57 pm

This is interesting. Does it work with a Pi0W?
My intention will be to boot manually, then throw a switch to short the "RUN" pin to 0V.
So if we have a (frequent) power cut, my Pi will not re-boot (I presume no access to the SD card is performed?) when the power comes back up.
And I will then go to the device and "unthrow" the switch then when it has booted, re-throw it...

hippy
Posts: 6720
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: How to keep Pi 3 off when power is applied

Fri Nov 29, 2019 6:59 pm

MarkDH102 wrote:
Fri Nov 29, 2019 4:57 pm
This is interesting. Does it work with a Pi0W?
Shorting RUN to 0V should hold the Zero in its reset, won'e boot, state, and same with all Pi's.
MarkDH102 wrote:
Fri Nov 29, 2019 4:57 pm
My intention will be to boot manually, then throw a switch to short the "RUN" pin to 0V.
That won't work. As soon as you throw the switch you'll put the Pi into its reset state. It will stop immediately and your data may be lost, your SD Card corrupted or damaged.
MarkDH102 wrote:
Fri Nov 29, 2019 4:57 pm
So if we have a (frequent) power cut, my Pi will not re-boot (I presume no access to the SD card is performed?) when the power comes back up.
And I will then go to the device and "unthrow" the switch then when it has booted, re-throw it...
You could create a self-latching relay configuration. Plug it in, press the button, power comes on, Pi boots, the relay holds itself on. If power then disappears the relay contacts opens and no power power goes to the Pi until you press the button again -

Code: Select all

                 \
5V >------.-----O O---------> 5V Pi
          |_     :
            ) ...: 
           _)    :
          |      \
          }-----O O---.
          |           |
       _| O           |
      |_|             |
        | O           |
          |           |
0V >------^-----------^-----> 0V Pi
there's still the risk of SD Card corruption or damage with power suddenly being pulled. You would be better off using some kind of UPS which can gracefully shut the Pi down and wait for a button push to re-boot it.

MarkDH102
Posts: 377
Joined: Fri Feb 13, 2015 3:18 pm

Re: How to keep Pi 3 off when power is applied

Sat Nov 30, 2019 6:58 am

hippy

Thanks for the clarification.
I do use UPS (of a kind) but would be happier not to.
Plus I just have too many Pi's dotted around the house to buy UPS's for every one of them...

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