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