So @s0 has been reviewing the board files from Rev 0.2, where we moved to a demountable motherboard scheme. She asked about the P channel MOSFET that's used as a reverse polarity protection element and it made me realise that the P channel MOSFET will do nothing to help if we have VBAT connected to the spare pin on our motherboard headers. This got me thinking about how to protect all of the motherboard power pins. I wondered about a low side version of the same circuit, using an N channel MOSFET. Why don't we see that arrangement in articles/app notes about reverse polarity protection?

I think the answer given in a Stack Overflow/Exchange post is the most plausible - it's better practice to connect the low side of a load to GND, rather than something slightly above GND potential on the high side of an N channel MOSFET, so that different loads can share common GND. 

But in our application, having GND as the high side of the N channel MOSFET is fine, and we have a separate net for BATT- at a potential slightly below that. Now, if we want to connect USB or TRRS cables to a wired version of Aerodox, it doesn't matter that the battery's negative terminal is slightly lower potential than GND. There is nothing other than the MOSFET which needs to reference BATT-, so I think we're fine in this instance. 

One additional benefit of moving the battery reverse polarity protection from the high to the low side is that we can add a battery charging circuit and easily read battery voltage (taking in to account the small - perhaps negligible - voltage drop across the MOSFET).

Here's a summary of the changes. First, old Rev 0.2 schematic:

And now, new Rev 0.2a schematic:

and the resulting changes to the PCB

Anyone see any dramas with this?

The Outemu key switches, kepcaps, Keilh PCB hot swap connectors have all arrived, so I need to get on and design a first motherboard or two and get these PCB's ordered soon.