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I'm trying to power on a RPi 4 using the RUN pin. The RPi is runing LibreELEC (Leia) 9.2 ALPHA1. I tried shutting down the RPi from LibreELEC's GUI, then grounding the RUN pin for a second by connecting it to the pin next to it, but nothing happens, the RPi doesn't boot. Previous models of the board could be powered on this way, did something change?

I should add that the board correctly resets when it's powered on. I just can't power it on when it's in a halted state, which I expect from a reset pin.

When looking at the schematics, I don't think I'm doing anything wrong.

RPi4 RUN and GLOBAL_EN pins schematics

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The Pi4B has additional circuitry which puts the board in a very low power state when shut down. This effectively turns the 3.3V (and other voltages) OFF, so the SoC has no supply voltage, and can't be reset. GLOBAL_EN pin causes the power management chip to restart voltage supply.

There is an updated bootloader which changes the behaviour to be the same as earlier models where both 3V3 and 5V pins remain powered on following 'sudo halt'

The RUN pin on the J2 header of the Pi4B is shown on the schematic as connected to the PG2 pin of the MxL7704 power management chip (which AFAIK is an open drain output). This is, in turn, connected to the SOC RUN line, so still should be able RESET the SOC, but I have not yet tested this.

The GLOBAL_EN pin is connected to the MxL7704 power management chip, and pulling this low should be similar to cycling power. I would expect that it would need to be pulled LOW for a few seconds, to give capacitors time to discharge.

Checking with hardware people, on Pi4 shorting GLOBAL_EN to ground will reset a Pi4 from halted state

  • I'm sure I'm using the right pin, but shouldn't it RESET the board (switch it on after it's been halted) when grounded? – MFlop Jul 3 at 8:27
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Question

grounding the RUN pin for a second by connecting it to the pin next to it, but nothing happens, the RPi doesn't boot.

Answer

There seems to be another pin called "Global Reset" connecting to a N channel MOSFET, with a gate threshold 1V. Should this pin set High to reset Rpi? It is a bit weird to have left the gate pin floating. Perhaps I have forgotten the MOSFET theory of operation! :)

rpi4 reset

Update 2019jun03hkt1306

I found Rpi4B reset circuit different from Rpi3B+. Perhaps that is the cause of confusion! rpi3b+ reset cct

References

DMG1012T N-CHANNEL ENHANCEMENT MODE MOSFET - Diodes

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    I don't think it's such a big difference, there's only a 10K pull-up resistor between PG2 and ground, which should work fine when grounded. Perhaps PG2 isn't meant to reset the Pi but only shut it down. – MFlop Jul 3 at 7:17
  • @MFlop, I am sorry that I have confused everybody by mixing up the "Run" and "Global Reset". the Run circuit in Rpi3B+ and Rpi4 are the same. It in only the "Global Reset" circuit is different. I am guessing that for Rpi3B+, Global Reset pin, is Low active, but for Rpi4, it appears High active. I have not thought about it thoroughly, and I have forgotten the MOSFET operation. I need to think harder again. And my Rpi4 is arriving this weekend, so I will verify my guess using my hands then! – tlfong01 Jul 3 at 7:49

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