papous
Posts: 72
Joined: Fri Jan 05, 2018 5:50 am

Weird pi happenings

Wed Jan 23, 2019 10:41 am

I hope that this forum is the appropriate one.
I am using a pi 3B+
Weird thing 1:
While thepi is off, an LED connected to pin 2 or 3 is swiched ON!
Weird thing 2:
With the pi off, a connection to pin 2 or 3 switches the pi ON (it boots)!
Any idea why?
Is this normal?

drgeoff
Posts: 9613
Joined: Wed Jan 25, 2012 6:39 pm

Re: Weird pi happenings

Wed Jan 23, 2019 10:48 am

papous wrote:
Wed Jan 23, 2019 10:41 am
Weird thing 1:
While thepi is off, an LED connected to pin 2 or 3 is swiched ON!
Weird indeed. A LED with only one leg connected lights up. :o .


Shutting down does not switch off a RPi. If a PSU is plugged in to the RPi It is still powered up. The CPU is not running but the SoC is powered.

LTolledo
Posts: 1655
Joined: Sat Mar 17, 2018 7:29 am
Location: Anime Heartland

Re: Weird pi happenings

Wed Jan 23, 2019 11:00 am

Pin 2 is 5V and Pin 3 is GPIO2 [SDA1 I2C]

If you are counting from the Left side top to bottom your pin 2 is GPIO2 and Pin 3 is GPIO 3 [SCL1 I2C]
see the diagram below
GPIO.png
GPIO.png (85.25 KiB) Viewed 273 times
If it is GPIO 3 its called "Wake From Halt", some say its an undocumented feature that remained on NOOBS installed Raspbian, but removed from image installed raspbian (since March 2014)
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rpdom
Posts: 14756
Joined: Sun May 06, 2012 5:17 am
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Re: Weird pi happenings

Wed Jan 23, 2019 11:11 am

LTolledo wrote:
Wed Jan 23, 2019 11:00 am
but removed from image installed raspbian (since March 2014)
I don't recall the wake from halt feature being removed from anything. It still works, as far as I know.
The thing that was removed from the bootcode was the ability to boot to a recovery system by grounding that pin when the Pi was booting up. I found that feature quite handy at times, but it would get triggered unintentional by some hardware connected to that pin, so was removed.

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