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How do mobile phones (Samsung, in particular) distinguish between being charged and having to power the connected USB device (i.e. flash memory stick)?

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  • \$\begingroup\$ I may be divulging my lack of knowledge for all things cell phone, but please explain the setup in a bit more detail so dolts like me can better understand the issue that you are observing. We are common and ever present. For instance, a flash memory stick needs no power if it is idle. Also, why not charge... and do stuff simultaneously? Is the phone/host not capable of that? \$\endgroup\$ Jun 11, 2020 at 20:26
  • \$\begingroup\$ The setup is quite simple: A Samsung phone is connected through the standard data transfer usb cable to another device (that requires5V). The device is just a passive appliance, like an LED torch. \$\endgroup\$
    – Noideas
    Jun 18, 2020 at 16:11

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It is done with the USB OTG. Usb mini and micro have a OTG pin that the phone uses to know if it's supposed to be host or slave.

Usb C has a more complicated signaling system but the idea is the same.

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