Also the RP1 on the RPi5 has ADC capability, but is of no use measuring 'from external'. Various Rockchips can have their SARADC multpliexed/mapped on one of the GPIO pins, but also quite often the SARADC is already used for doing something on the board itself. I have not looked into detail because of that. Other reason is that those ADCs cannot work when going through sleep-modi.The PMIC on the 4B/CM4 I believe had two ADC inputs but cannot recall how usable or subvertable they were so, yes, it needs an external ADC, appropriate circuitry and reference voltage.
But I am not sure what good having it would do. The low-voltage detector on a Pi SBC indicates the voltage has dropped such that a critical fault condition exists, so what more is there to know, what would an ADC and actively monitoring provide for ?
I suppose you could adjust clock speeds and whatnot when voltage drops in an attempt to stop it becoming a critical fault condition but it would be simpler to use a PSU which avoids that happening.
I do not know the characteristics of the voltage sensing logic, it can be non-lineair I think. Other option is to just reset the board. I think I prefer that actually, there is no risk of partial crash of something or strange kernel messages while the SoC keeps running, but who knows what data is or will be corrupted then.
Lower clock is simple workaround, I did that once on my RPi4 v1.1 with TV-HAT to workaround a SW/driver issue. In other role and with only a 2A 5V PSU, that let undervoltage messages disappear.
W.r.t. RPi3: I have no RPL produced PSU for those. What worked well for me are 100Mbps active PoE splitters, microUSB cable is only 30cm or so. ON/OFF is then done via SW in PoE switch, so no mechanic actions. I can also see the power used there by the way.
For RPi3B+ 1Gbps, all 8 wires are needed else it fails on autonegotiation if standard setting in switch. But for magic mirror I guess I would derive 5V from that, but I have no clue what it needs actually.
Statistics: Posted by redvli — Sun Nov 23, 2025 10:41 am