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powering Raspberry Pi from bench supply with USB

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metebalci:

I know I can power RPi from GPIO and using it like that, but I wonder if I can with USB. So I get a USB cable and connected +5V and GND to bench supply, and to comply with USB power spec. I shorted the D+ and D-. However, RPi is still not properly booting, I dont have a screen attached so not sure if it writes anything but I see some activity in LEDs then it seems like it is rebooting. What am I missing ?

Ian.M:
A screen?

Before the Pi 4, the Pi's power in port was very dumb, IIRC simply Vbus and Ground with no data pins connected.  The Pi 4 has some resistors to signal the charger its expecting 5V power (which was incorrectly implemented on early  board revisions).

Many cheap USB cables have too much resistance to reliably power even a Pi Zero!  Measure the 5V at the GPIO header, to check you are actually delivering the correct voltage to the board, and scope it to check it isn't dipping.  *DON'T* be tempted to crank up the voltage past 5.25V in an attempt to compensate for a crappy cable.  Instead, cut it down to 3" long and splice it onto much thicker wires back to your PSU.

metebalci:

--- Quote from: Ian.M on January 23, 2022, 09:20:47 pm ---A screen?

Before the Pi 4, the Pi's power in port was very dumb, IIRC simply Vbus and Ground with no data pins connected.  The Pi 4 has some resistors to signal the charger its expecting 5V power (which was incorrectly implemented on early  board revisions).

Many cheap USB cables have too much resistance to reliably power even a Pi Zero!  Measure the 5V at the GPIO header, to check you are actually delivering the correct voltage to the board, and scope it to check it isn't dipping.  *DON'T* be tempted to crank up the voltage past 5.25V in an attempt to compensate for a crappy cable.  Instead, cut it down to 3" long and splice it onto much thicker wires back to your PSU.

--- End quote ---

I checked with the scope and sometimes it seems it actually falls down, not sure to what extent it is OK. But it seems to be working now. I dont know what was the problem. I guess it might be because of the cables.

Ian.M:
I believe anything below approx 4.63V will trip the undervoltage detector.  USB connector contact resistance can be significant - I have one Pi with a SATA HDD (in a USB<=>SATA 'pod') that needs its power in USB jack and plug cleaned and reseated at regular intervals to prevent the Pi detecting undervoltage, and the HDD restarting.

rpiloverbd:
Personally I always recommend using an official RPI power supply to power up the RPI. You know, RPI needs 5v,2A. Is your power supply really giving this output?

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