Raspberry Pi PoE+ HAT: Responding to Pi Foundation

On 17th June 2021, Jeff Geerling released a video showing his experiences with the new PoE+ HAT. His experiences are much the same as my own and we’re both left wondering how the issues we’re seeing weren’t seen and fixed earlier (before release).

At the end of the Video and over on Jeff’s blog post he shares an initial response he received from The Raspberry Pi Foundation where they attempted to address some of the questions he put them. The quoted text below is taken directly from Jeffs blog post.

USB-C Back-Powering the Raspberry Pi PoE+ HAT

This should not do any damage, but according to the Pi folks, this is not a supported mode of operation and the HAT should be removed if using USB-C to power the Pi.”

Back Powering protection circuit from https://github.com/raspberrypi/hats/blob/master/backpowering-diagram.png

The Raspberry Pi team publish a specification to follow when designing HATs. In the high-level overview, there is a paragraph that reads: “If back-powering via the 5V GPIO header pins you must make sure that it is safe to do so even if the Pi 5V supply is also connected. Adding an ideal ‘safety’ diode as per the relevant section of the design guide is the recommended way to do this.

Now a PoE/PoE+ HAT does exactly this and powers the Pi via the GPIO header. So it should include this protection. However, the designers may have wanted to still allow the fan to function, allowing part of the HAT to be powered by the USB-C connection, that being the case the protection should still have been put in place to avoid energising the high voltage side.

So the if having the HAT installed and powered by USB-C, is unsupported, does that mean that this new HAT doesn’t meet the HAT specification, and so isn’t actually a HAT?


Higher Power Consumption

This is to be expected with the new power circuit, especially if the PoE voltage is at the upper end of the allowed range (48V), like I have from my PoE+ switch. If lower power consumption at idle is important, you should stick with the original PoE HAT.

According to the IEEE 802.3at-2009 specification for 802.3at Type 2 “PoE+” devices, they quote voltages at the source as 50-57V with voltage at the powered device of 42.5-57V. This means Jeff’s 48V PoE voltage is lower than the average voltage in the specification, at only 38% in the allowable range. Stating that his voltage is at the upper end of the allowed range, looks to be just plain wrong.

Louder ADDA Fan

The new fan is also rated for higher temperatures and a longer lifetime, and the bearings used are a little noisier at high speeds.

Only time will tell if this statement is true. Given the new HAT does run hotter it may have needed to have a better performance at this increased temperature.

Finally

The response from the Raspberry Pi foundation so far has failed to answer many of the problems already identified with the Raspberry Pi PoE+ HAT and where they have responded, their answers don’t appear to make a whole lot of sense.

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