Pi 400 not working with Pi-Supply PoE Switch Hat

Question

The "official" PoE/PoE+ hats will not work with the Pi 400 for a couple of reasons: The main reason being that there is no access to any of PoE pins on the Pi 400 PCB, as there are on the Pi 4 board (just behind the Ethernet port) - indeed, ignoring the fact that the case of the Pi 400 prevents any access to the PCB of the Pi 400, the PCB of the Pi 400 doesn't even have these pins.

However, the Pi-Supply PoE Switch Hat (which supports 802.3af) does not rely on these pins as it has its own input Ethernet port, from which the PoE is extracted, and as such is also compatible with the Pi 2.

Being the optimist, I tried one of these PoE hats with the Pi 400 but it didn't work - it doesn't power the board - not even if you hit the power button on the Pi 400, nor click the switch on the PoE PCB.

Fortunately, it doesn't blow either the Pi 400 nor the PoE hat - I have checked that both the Pi 400 and the PoE Switch hat continue to function afterwards (the Hat on a Pi 4, and the Pi 400 powered via the usual USB-C connection).

So, my question is, what is different about the Pi 400 (or rather its PCB) that means it can not be powered by directly applying power to the power pins on the GPIO?

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Note I have not tried just applying power to the power pins of the Pi 400 directly using a bench power supply (yet)... I should probably do that. However, before potentially blowing the Pi 400 by doing so, I thought it judicious to check here first to see if anyone knows more about the power circuitry of the Pi 400.

Answer 1

TL;DR: The Pi 400 will not work with a PoE board.

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The Pi 400 differs from all other Pi devices, w.r.t. the 5 V pins on the GPIO connector.

There is a design change on the Pi 400 - an additional IC, marked 0F=11R, on the board, which is a Richtek RT9742:

This IC, according to this post by a Pi engineer, prevents a PoE board (or any 5 V line) from powering up a Pi 400:

powering via GPIO is not possible on the pi400 due to stuff in there to stop nasty back powering issues happening.

More info is available on these, most informative, blogs, related to LiFePO4wered/Pi400 - that I am not affiliated with in any way.

In particular, from Raspberry Pi 400 analysis - with reference to the two ICs in the above photo - take note of the last line:

These are identical parts, presumably the load switch talked about. Two of them: one to switch 5V and the other to switch 3.3V. The one in the center definitely drives the GPIO 5V pins, the other one switches 3.3V but oddly is not connected to the GPIO 3.3V! I think it switches the SD card slot power. Protect the one GPIO power rail but not the other? I don't get it.

They are marked "0F=11R". It took some digging to find what exactly they are, but eventually I figured out that they are almost certainly Richtek RT9742 load switches. The connections match the datasheet, as does the "0F=" marking. Oddly, two variants are both marked "0F=": the 3A RT9742ANGJ5F and the 1A RT9742GGJ5. The only difference to tell them apart is that one of them is in the "TSOT-23-5" package and the other in the "TSOT-23-5 (FC)" package. But if you check the mechanical outline specification for these packages, they seem to be identical according to the spec. I don't know what the supposed difference is then.

So I decided to just test which one it is. I hooked up my electronic load, and increased load from 1A up in 0.1A steps. It tripped at 1.4A, so it's the RT9742GGJ5 1A version.

The datasheet has this info about the pass transistor: "Unlike a normal MOSFET, there is no parasitic body diode between drain and source of the MOSFET, the RT9742 prevents reverse current flow if VOUT is externally forced to a higher voltage than VIN when the chip is disabled." So indeed no hope of turning the Pi 400 on from GPIO power.