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by ianburrell 15 days ago
USB-C uses resistors to signal the power level for 5V. There is 5.1k for USB legacy, and I forget for 7.5W and 15W. Lots of cheap devices leave out the 5.1k resistors and depend on the same resistors in the USB-C to USB-A cable.

It is perfectly fine for USB-C device to signal USB Legacy since every charger needs to support it for microUSB to USB-C cables. The other way is for device to dynamically negotiate power level with the charger, but this is different than USB PD for higher voltage.
1 comments
seba_dos1 10 days ago
The device only signals that it's a power sink with its 5.1k resistors, there's only one way to put them there without implementing the full-blown PD protocol. It can't signal power levels (USB legacy / 7.5W / 15W) and doesn't need to, it's the power source that signals its capability.

You're probably confusing it with the spec requiring that the device does not attempt to consume more power than USB legacy from a source that doesn't advertise such capability - but even then there's no "negotiation" happening, it's just a matter of measuring the voltage on CC lines.

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