Hey Bob,
My understanding of the reason they target 5.1v nominal instead of 5v is to give themselves a little extra voltage to compensate for voltage droop under heavy load, while still being USB compliant.
Actually, I just took a bit of a dive into the USB specs. USB Type C voltage requirements are defined in the USB 2.0 specification.
Interestingly, there was an ECN (Engineering Change Notice) issued in August 2014 which increased the maximum allowable USB voltage from 5.25V to 5.50V - and they’ve even stated cvompensating for voltage droop as the reason:
I’d be willing to put money on the 5.1V specification being a direct result of that ECN. The original USB spec was 5.0 ±0.25V, but now that it’s 5.0V (-0.25V +0.50V), it’s equivalent to 5.125V ±0.375V - hence the 5.1V of the official Pi supplies.
Percentage wise, that’s 5.0V +10%, -5% or 5.125V ±7.32%
All this is to say that a 5.0V ± 0.25V supply capable of 3A isn’t strictly USB spec compliant and hasn’t been since 2014 - though if the cable and connectors are low enough impedance it’d work fine.
This post says the voltage input requirement of the on-board supply is 4.00V to 5.50V - which is actually the same as the USB 3.2 spec:
Ah, but don’t forget to account for the impedance of the connector and traces on the board! If you’re pulling 1A and measuring 4.79V at the supply, as little as 90 milliohms would be enough to trip the Power warning on a 3B+.
FYI, here’s the datasheet for the voltage monitoring chip on the Pi 3 boards:
https://www.mouser.com/datasheet/2/115/APX803-82466.pdf
And here’s the one for the Pi 4s:
