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As mentioned in a comment to https://raspberrypi.stackexchange.com/a/19701/8697 and also in a few other places on this Forum "when the pi is shut down, pulling pin 5 low will make it boot up again"

I can find no reference to this supposed behaviour, although holding this pin low was supposed to boot in safe mode in older versions of Raspbian.

Indeed it does not work on my Pi Model B Revision 2. I can find no mechanism by which it could possibly work. The P6 connector makes the reset function accessible.

Have those proposing this tested this behaviour?

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Milliways
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  • pulling +5V pin low will definitely make RPi boot up again, though this may put a lot of strain on your power supply. I2C pins generally have pull-up resistors connected to them, and pulling one down might results in the same effect like pulling +5V down, hence the probable reboot. personally, I would not rely on this nor use in any reasonable design. – lenik Jun 28 '14 at 04:36
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    This is called the `wake-from-halt` function. This is implemented in the firmware. So you might need to update your Pi. – Gerben Jun 28 '14 at 09:38
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    Where is this documented? When was it implemented? I last updated my pi `rpi-update` on 2014-05-17 – Milliways Jun 28 '14 at 11:05

1 Answers1

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When the pi is halted using sudo halt or similar, pulling pin 5 low will make it boot up again.

This is implemented in bootcode.bin as documented
http://elinux.org/RPI_safe_mode#Wake_from_Halt.5B1.5D

NOTE This will not reboot the Pi if the power has been interrupted.

The original Pi4 put the MxL7704 power management chip into a low power state when off - turning off the 3.3V supply and effectively totally shutting down the SoC. (I verified this by measuring the current.)
Unfortunately due to lobbying by a lot of luddites who had designed HATs that assumed 3.3V power was always available the EPROM is now configured by default to enable the MXL7704 PMIC when shut down. So you can now reboot by pulling pin 5 low.
It is possible to change this behaviour by modifying the Pi4 EEPROM. See Pi 4 Bootloader Configuration.

A reset button can be attached to the P6 header on Pi Model B Revision 2, which performs a similar reboot, and also works if the power has been interrupted.

EDIT The B+ and A+ have a Run header to reboot.

Milliways
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    Anyone reading this answer may also be interested in the details as documented here: https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README Just search for "poweroff" or "shutdown". – Rik Renich Feb 27 '21 at 18:47