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The posts I found online show conflicting information on how the J2 header should be used, and seem to be derived from trial and error. None of the answers I managed to find seem authoritative; i.e. given by engineers at the Raspberry Pi Foundation or sourced from component documentation.

The only information I was able to confirm is that

  • Power for the RBP4 is managed by the MXL7704 PMIC.
  • The state of the PMIC can be controlled by the J2 header.
  • The RBP4 is designed to to start up after being halted.

Given this information, I would like to have answers for the following questions:

  1. What is the proper way to power on (i.e. turn on) a halted RBP4 using the J2 header?
  2. What is the proper way to reset power using the J2 header?

Definitive answers to both questions would help me and, evidently, many other people.

Update: From a Raspberry Pi engineer, the answers to the questions are as follows:

  1. Momentarily short GLOBAL_EN to ground to restart a halted RBP4.
  2. Momentarily short GLOBAL_EN to ground to restart a running RBP4. Usual caveats about suddenly powering off your system apply.
Tenders McChiken
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Despite your unreferenced comment about online posts (you can find lots of flat earth posts if you look) there is no mystery - it is well documented. I am surprised you find engineers at the Raspberry Pi Foundation not authoritative.

See https://raspberrypi.stackexchange.com/a/100234/8697

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.

Despite this pulling the GLOBAL_EN pin LOW will reset the Pi4.

The GLOBAL_EN pin is connected to the MxL7704 power management chip, and pulling this low is similar to cycling power.

The RUN pin on the J2 header is connected to the SOC RUN line, so should RESET the SOC, although in the original firmware there was no power on the SOC, so it did nothing.

Milliways
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    I don't know what to say other than I spent about two and a half hours googling and reading posts, and I never came across that forum post (or perhaps I mistook it for discussing earlier versions of the rbp). For those also looking for the authoritative answer: https://www.raspberrypi.org/forums/viewtopic.php?p=1491661#p1491661 – Tenders McChiken Feb 24 '20 at 13:19
  • Since GLOBAL_EN is used to power on and reset the RBP4, could you also clarify the purpose of the RUN pin (if you can)? – Tenders McChiken Feb 24 '20 at 13:35
  • @TendersMcChiken that link is included in my linked Answer. I always try to include source in my Answers. – Milliways Feb 24 '20 at 23:03