Input voltage range (under volt)

#1

quick question, related to running 4 boards (and PI/other board) off a 7 port USB powered hub. (rated at 2A)

so when I power everything up, boards are all at 4.93-4.98V, so this seems fine.
sometimes I see one start lower, around 4.85V, but seems to then rise up to 4.9+v, perhaps related to SDCard... anyway no matters, it seems ok.

what I want to know is, is there any danger if you run them under voltage (short period) e.g. around 3.5V
Im not doing this on purpose!
BUT Ive noticed, if I 'forget' and unplug the USB hubs supply, before I disconnect the host USB cable, it powers the boards off the host (another powered hub in this case), and boards all continue to run, but of course drop to 3.5V, as it can only supply 500ma (or 1a I think on charging port)

could this damage anything?
(Im not worried about the hub that ends up powering it, only the axoloti boards smile )

again, this is not 'normal' running, just if its 'accidental'... so do I have to be extremely careful about this?
(or are the boards, just not going to work correctly etc.)

#2

Another, only slightly related question

Assuming I wrote some code, is it possible to remotely power down a board?
(ie is there an api in chibios or stm libs which can shut it down ?)

#3

4.75V is needed to comply with the USB spec for the host port.
Below that, say 4.5V, the DIN Midi output may get less reliable, and the "headroom" is not enough for the 3.3V regulator anymore. The processor may crash/reset. But I really don't expect any damage.

Powering down: really powering down is not possible. The RAM, audio converter and microcontroller could go into low power sleep mode. But I have not seen an application that motivates implementing a low power mode.

#4

Cool, means I don't need to get paranoid about it smile

Low power, yeah was more after a 'power off', really was just so I could reduce power if I ran from another hub or as a powering down procedure. but not a priority.

I would like a 'restart' though,I really want to avoid having to power cycling to restart board
( it's a pain if you have external power and a host, both have to be unplugged)
And sometimes firmware updates don't seem to reset the board into a proper state.
(Or perhaps that is because it's not a proper restart?)