Does anyone know where the RPi3B+ detects undervoltage?
Does anyone have a schematic that includes the 5V lines?
David Higton <dave@davehigton.me.uk> wrote:
Does anyone know where the RPi3B+ detects undervoltage?
There's an APX803 chip which detects the voltage falling below 4.63 +/- 0.07V. That chip then asserts PWR_LOW_N, a GPIO the GPU firmware can
detect. It's also used to drive the PWR LED (ie the LED going off is the same as PWR_LOW_N being asserted).
https://forums.raspberrypi.com/viewtopic.php?t=158777
Does anyone have a schematic that includes the 5V lines?
https://datasheets.raspberrypi.com/rpi3/raspberry-pi-3-b-reduced-schematics.pdf
has the power supply and detection circuit.
The upshot is that I'm going to have to reconsider power supply and distribution for this one, which is used as NAS, so it needs to power
some spinning drives. The incoming supply is rated at 8 amps, but its
output voltage is just 5V, and it appears that its transient response
may leave something to be desired. One thing I can try is whacking
several 1000uF capacitors, via short leads, on the 5V line on the GPIO connector. I can also see where I would need to interfere with it so
as to increase the output voltage to about 5.1V. I've cut the output
cabling as short as it can reasonably be made so that cable resistance genuinely can be neglected.
David Higton <dave@davehigton.me.uk> wrote:
The upshot is that I'm going to have to reconsider power supply and distribution for this one, which is used as NAS, so it needs to power
some spinning drives. The incoming supply is rated at 8 amps, but its output voltage is just 5V, and it appears that its transient response may leave something to be desired. One thing I can try is whacking several 1000uF capacitors, via short leads, on the 5V line on the GPIO
connector. I can also see where I would need to interfere with it so
as to increase the output voltage to about 5.1V. I've cut the output cabling as short as it can reasonably be made so that cable resistance genuinely can be neglected.
FWIW I've just installed a 3B in my 3D printer to run Klipper. It's
powered off the main 24V PSU via a Chinese '5V 5A' DC-DC module, which outputs 5.2V under no load. Klipper was running fine but reporting a
number of power supply dropouts, despite no USB devices connected.
I fitted a 330uF electrolytic and a 22uF ceramic (for low ESR) across the power rails at the power input to the Pi (here via GPIO) and that resolved the dropout problem. I'd guess the DC-DC just has insufficient capacitance to hold up the rail when the Pi takes a gulp of current, and so needs a bit of help.
I don't know the frequency response of the Pi's PDN (whether the gulps of current are seconds, milliseconds or microseconds) but maybe try a range of capacitors to handle them - not just big electolytics, which can have high ESR and so a slow frequency response.
I've just tried 9000uF (9 1000 uF capacitors in parallel) this afternoon.
I didn't see a restart in the logs when Clonezilla did a backup to OMV,
but the backup failed anyway, and now I don't know why.
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